GROWTH AND OPTICAL ABSORPTION OF SINGLE
CRYSTALS OF NICKEL SULFATE HEXAHYDRATE
by
Hsuchiao Yeh
Submitted in Partial Fulfillment of the Requirements
for the Degree of
Master of Science in Engineering
in the
Electrical Engineering
Program
Dean of the Gradu te School
YOUNGSTOWN STATE UNIVERSITY
August, 1972
ii
ABSTRACT
GROWTH AND OPTICAL ABSORPTION OF SINGLE
CRysrrALS 0.11' NICKEL SULFATE HEXAHYDRATE
Hsuchiao Yeh
Master of Science in Engineering
Youngstown State University, 1972
The crystalline alpha-hexahydrate of nickel sulfate
was obtained by growing crystals from solution by utilizing
Holden's Rotary Crystallizer within the temperature limits
of 35 d.eg C and 50 deg C. '.rhe crystals were then cut in two
different directions, one in parallel with and the other
perpendicular to the optic axis, and polished on abrasive
papers and powders to a thickness of 2.53 + 0.01 mm. The
dichroism was then studied with Perkin-Elmer Spectrophoto 
meters in ultraviolet, visible, and infrared regions. The
ranges of wavelength under study are from 190 to 700 milli 
microns in UV-VIS region and from 2.5 to 40 microns in IR
region. It is found that in the IR region, the material has
nearly total absorption, and in the UV-VIS region the mate 
rial has total absorption in the wavelength ranges of 190 to
195 millimicrons, 350 to 430 millimicrons, and 590 to 700
millimicrons, while the maximum transmittance occurs at 310
millimicrons and 490 millimicrons. It is also found that
iii
there is neither a detectable linear dichroism existing in
the alpha-hexahydrate of nickel sulfate crystals within the
UV, VIS, and IR wavelength regions, nor a shift in the edge
of the absorption bands with regard to changes in the crys 
tallographic direction.
iv
ACKNOWLEDGEMENTS
I would like to express my gratitude to Dr. Allan J.
Zuckerwar for his valuable suggestions and advisement. Also
I would like to express my appreciation to Prof. Raymond E.
Kramer, the department chairman, and Prof. Samuel J. Skarote
for their valuable time in reviewing my thesis.
v
TABLE OF CO~~ENTS
ABSTRACT. ???????????
ACKNOWLEDGEMENTS ? ? ? ? ? ? ? ? ?
TABLS OF COlffENTS ????????
CHAPrER
? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? 0 ? ?
PAGE
ii
iv
v
1
4
9
10
1.5
18
20
23
28
28
31
38
42
43? ?? ? ? ?? ? ?? ? ? ?? ? ?? ? ? ?
I.
II.
III.
aEFERENCES ? ?
INTRODUCTION ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
THE CRYSTALLIZATION PHOCESS ?????????
GROWING CRYSTALS ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
Preparing a Saturated Solution ? ? ? ? ? ? ?
Growing Crystal Seeds ???????????
Seeding the Solution in the Tank ? ? ? ? ? ?
Growing Crystals ? ? ? ? ? ? ? ? ? ? ? ? ? ?
Harvesting Crystals ????????????
IV. O?fICAL ABSORPTION MEASUREMENTS ???????
Preparing the Sample ? ? ? ? ? ? ? ? ? ? ? ?
Optical Measurements ? ? ? ? ? ? ? ? ? ? ? ?
V. SUMMARY AND DISCUSSION ????????????
BIBLIOGRAPHY ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
1
- - - - - -- - - ~ - -
CHAPTER I
INTRODUCTION
The purpose of this study is to grow, from liquid
solution, very pure crystals and seek linear dichroism in
the alpha-hexahydrate of nickel sulfate crystals in ultra-
violet, visible, and infrared regions.
There are many kinds of crystals been grown by va 
rious industrial companies. But most of these crystals are
contaminated by some other chemicals, while those which are
really pure are often very expensive. So we decided that we
would grow the pure crystals which we needed ourselves.
Since our intention is to find the dichroism of a crystal,
we found that the alpha-hexahydrate of nickel sulfate crys 
tals are very suitable for our purpose.
Nickel sulfate hexahydrate, NiS04.6H20, forms tetra 
gonal crystals. Their unit" containing four molecules, has
the dimensions: a ~ 6.80 A., c = 18.3 A. The space group
can be one or the other of the enantiomorphic pair Dft or Dft.
The stacking of the atoms is an equal number of Ni{HZO)6
octahedra and S04 tetrahedra; their shapes are, however, so
different one from the other that the resultant grouping
does not approximate a simple structure. Within the Ni{H20)6
octahedra Ni-H20 = 2.02 and 2.04 A., the sulfate tetrahedra
have their usual shape with S-O = 1.52 A. Contact between
2
the two groups is through O-H20 separations that range from
2.69 A. upwards. Also nickel (Ni) is in a lower symmetry
site with only two-fold symmetry while nickel sulfate hexa 
hydrate has a four-fold crystal symmetry. Thus it is ex 
pected that when the nickel sulfate hexahydrate crystal is
exposed to the light sources from two orthogonal directions,
of which one of them is along the optic axis of the crys 
tal, the nickel ions located at a low symmetry site, after
absorbing some light energy, may alter the light transmit 
ting characteristics of the crystal in the ultraviolet and
1 2visible wavelength ranges.' Furthermore, it is found that
crystalline nickel sulfate hexahydrate can be grown very
easily from liquid solution3 by utilizing Holden's Rotary
crystallizer.4
Nickel sulfate crystallizes from solution at room
temperature (30 deg c) as NiS04?7H20 in the orthorhombic
system. At about 35 deg C it crystallizes as NiS04.6H20 in
1R. rrrehin. "Etude Esperimentale de la Bande
d'Absorption de l'Ion Ni++ Dans Ie Proche Ultra-Violet pourles
Solutio~s Aqueuses de Sulfate de Nickel Pleoohroisme desCristaux
SO~Ni, 60HZ (Quadratique) et So4Ni, 70H2
(Orthorhombique)," Annalen Der PhlSik,XX (1945), 372-90.
2C. Furlani, "Absorptionsspektren Magnetisch
Normaler Ni (II)-Komplexe,"Zeitschrift Fur Physikalisch Chemie (Frankfurter),
X (1957>, 291-305.
3Alan Holden and Phylis Singer, Crlstals and Crlstal
Growing (Garden City, N. Y.: Anchor Books DOUbleday & Com pany, Inc., 1960) p. 112.
4R? A. Laudise, The Growth of Single Crystals
(Englewood Cliffs, N. J.: Prentice-Hall, Inc., 1970),
p. 264.
3
the tetragonal system, while at about 60 deg C it crystal 
lizes as NiS04.6H20 but in the monoclinic system. So far
circular dichroism and absorption of crystalline nickel sul-
5 6fate have been studied.' This thesis is to report the
details of an investigation with the tetragonal crystals
using Perkin-Elmer Model 202 Ultraviolet-Visible and Model
457 Infrared Spectrophotometers.
5L? R. Ingersoll, P. Rudnick, F. G. Slack, andN. Underwood, "Optical Activity, Circular Dichroism, and
Absorption of Crystalline Nickel Sulfate," Physical Review,
LVII (1940), 1145-53.
6C. Furlani, "Absorptionsspektren Magnetisch
Normaler Ni (II)-Complexe," Zeitschrift Fur Physikalisch 
Chemie (Frankfurter), X (1957), 291-305.
4
CHAPrER II
THE CRYSTALLIZATION PROCESS
Some salts grow from water solution into large crys 
tals relatively easily; others tend to form many samll crys 
tals. The paperback book, Crystals and Crystal Growing des 
cribes two simple methods for growing, without speciallized
equipment, single crystals of some salts that readily form
large cyrstals.7
In one of those methods, the salt to be crystallized
is dissolved in water in an amount sufficient to saturate
the water at a temperature higher than that of the room. A
small seed crystal is hung in the solution, the container is
sealed, and the solution is allowed to cool to room tempera 
ture. Since the solution was saturated at a higher tempera 
ture, it is holding at room temperature more salt than it
can stably retain. In other words, the solution that was
saturated with salt at the higher temperature becomes super 
saturated at the lower temperature. The extra salt slowly
deposits on the seed, growing it into a larger crystal.
In the other simple method, the solution is made sa 
turated at room temperature instead of at a higher tempera 
ture. A seed is hung in it, and the solution, instead of
7Holden and Singer, Crystal Growing, pp. 93-107.
5
being sealed, is left open to the surrounding atmosphere and
allowed to evaporate. Evaporation slowly reduces the amount
of solvent that is holding the salt, and the solution again
becomes supersaturated and deposits salt on the seed.
Notice that both methods provide the indispensable
condition for growing a crystal from solution: supersatura 
tion. In the first case, the supersaturation is provided
abruptly, when the solution is cooled; in the second case,
the supersaturation is provided gradually, as the solvent
evaporates. Clearly, the second method offers the advantage
that the material for crystallization is made available at
a rate more nearly approximating the rate at which the grow 
ing crystal withdraws that material. But evaporation takes
place at the surface of the solution, and thus the supersa 
turation is highest where spurious seeds, entering from out 
side, may gain a foothold.
Furthermore, both methods suffer from two other
difficulties: (1) Unless these methods are used in a room
whose temperature is controlled, variations in the tempera 
ture of the solution will cause uncontrolled variations in
its supersaturation. (2) Both methods leave the solution
uncirculated, and differences in supersaturation can accu 
mulate in it that disturb the regular growth of the crystal
more and more as the crystal becomes larger.
The crystal-growing apparatus used in this experi 
ment, the Rotary Crystallizer, avoids these two difficulties
by controlling the temperature of the solution directly, and
6
by moving the crystals through the solution continuously.
Furthermore, it combines the advantages of both of the sim 
plier methods, sealing the solution against evaporation and
the entry of spurious seeds, and by cooling the solution
slowly so that material for crystallization is made availa-
ble gradually.
As shown in Figure 1,8 if one plants a crystal seed
in the nickel sulfate solution before it is supersaturated
as shown at point A, salt will be deposited on the seed as
soon as the temperature of the solution is reduced to point
B so that the solution becomes supersaturated. This deposit
will continue until all excess salt is evenly deposited on
the seed and the solution again reaches its more stable
state of saturation. The seed, or crystal, will have grown
from the added deposit of salt upon its surface; the solu 
tion will have lost excess solute and assumed the state of
saturation at point C. Reducing the temperature further to
point D will again bring the solution to supersaturation,
and again the crystal will grow from the deposit of excess
solute by the supersaturated solution.
This process may sustained until the solution
reaches 35 deg C. This, as shown in Figure 1, will avoid
growth of the heptahydrate, which takes place more easily at
temperature below 35 deg C.
8Artherton Seidell, Solubilities of Inorganic and
Metal-Organic Compounds (4th ed.: Princeton, N. J. Van
Nostrand, 1958), p. 453.
7
60
0
C\J::r::
50
00
0
0
M 40
~
~
.:::T0 L
(/) 30
..-I 6H
2Ozr:r..
0
(/) 7H2Os?
~ 20
~0
10
60302010
O'--__..a..-__........__.....L.__~ ____JL..___.L__
o
DEGREES CENTIGRADE
Fig. 1.--So1ubility of nickel sulfate.
8
The rate of crystal growth depends upon the rate at
which the temperature of the solution is reduced. To grow
nearly perfect crystals the growth rate should be very slow.
After the crystal has grown to the desired size, it can be
removed from the apparatus for use in further experiments or
display. The solution may be reseeded for the growing of
more crystals, or stored for future use.
9
- ~ -- -~ - - ~ ~
CHAPTER III
GROWING CRYSTALS
The crystalline process is carried out in such a way
that the point on the solubility curve, which is tempera 
ture dependent, moves into the metastable solution region,
when the temperature is lowered, near the saturation curve
in the direction of lower solubility as shown in Figure 1,
The apparatus is Holden's Rotary Crystallizer which can be
purchased, completely assembled, from the Dependable Printed
Circuit Corp" Wayne, N, J, The chemical is fine crystals
reagent of NiS04,6H20, A. C. S. grade, whose maximum impuri 
ties are shown in Table 1, and can be purchased from
Matheson Coleman & Bell Manufacturing Chemists, Norwood
(Cincinnati), Ohio.
Since nickel sulfate will grow into crystals of two
distinctly different forms, nickel sulfate heptahydrate
(NiS04'7H20) and nickel sulfate hexahydrate (NiS04,6H20),
certain precautions must be taken to insure the growth of
the hexahydrate desired, These are: (1) The solution must
be saturated at the starting temperature for the hexahydrate
to be produced because the solUbility is different for the
two hydrates, (2) The solution must be seeded with crystals
of the same hydrate as the crystals to be produced, i,e.,
hexahydrate crystals. (3) The temperature range through
10
TABLE 1
IilAXIHUN IHPURITIES OF NiS04?6HZO USED
Impurities
Chloride (as Cl)
Cobalt & ~langanese (as Co)
Copper
Insoluble MatterIron (Fe)
Nitrogen Compounds (as N)
Substances not pptd by (NH4 )ZS
Maximum Contents
0.001 /;
0.003 J&0.005 ;b
0.005 i~
0.001 j;
O.OOZ %
0.100 %
which crystal growth of the hexahydrate takes place should
be confined to the limits of 35 deg C and 50 deg C. This
will help avoid growth of the heptahydrate, which takes
place more easily at temperature lower than 35 deg C.
Crystals of two types can be identified by their
shape, color, and properties. Nickel sulfate hexahydrate
belongs to the tetragonal system, is blue-green in color,
does not dehydrate easily, and has excellent cleavage pro 
perties. The plane of cleavage is perpendicular to its
axis of four-fold symmetry. Nickel sulfate heptahydrate
belongs to the orthorhombic system. It is yellow-green in
color, and dehydrates readily.
Preparing a Saturated Solution
The first step of growing crystals is to make a so 
lution that is saturated at the temperature at which the
crystals will be growing. To prepare a solution for the
crystallization of nickel sulfate hexahydrate use a propor-
11
tion of 257 grams (9 ounces) of nickel sulfate hexahydrate
for each cup (237 cc.) of distilled water. The saturation
temperature of a solution made from these quantities will be
approximately 45 deg C.
Heat three liters of distilled water in a 4-liter
beaker (or equivalent mixing container), which is covered
with a lid, over a hot plate or a burner to a temperature of
about 60 deg C. After the water has come to the temperature
specified, add, a little at a time, 3255.4 grams (approxi 
mately 7.2 pounds) of nickel sulfate hexahydrate. Agitate
the solute constantly with a stirring rod until all solute
has dissolved to prevent a temperature stress which will
occur at the walls of the tank and, consequently, to pre 
vent the container from breaking. Meanwhile keep the lid
on the beaker between stirrings to reduce loss of water by
evaporation from the solution.
After all the salt has dissolved in the water, the
solution is an unsaturated solution. As this solution
cools, it will become saturated and then supersaturated be 
cause of the change of salt solubility that accompanies the
change in temperature. The temperature at which the solu 
tion is saturated is then determined; it is near this point
that seeds will be planted in the solution and crystals
will start to grow. Errors in the published data on the
saturation temperature, as well as errors in weighing and
measuri~~ the amount of the solvent, the loss of the sol 
vent during heating are all factors which may alter the sa-
12
turation temperature of the solution at the moment of pour 
ing into the crystallizer from the expected value. It can
be tested if a solution is unsaturated, saturated, or su 
persaturated by hanging a test crystal in the solution and
observing whether it dissolves or grows. Test crystals are
obtained by allowing some of the solution to cool and crys 
tallize in a separate container. In this case, imperfec 
tions are not important.
If the test crystal dissolves, it loses material to
the solution immediately surrounding it, thus indicating
that the solution is unsaturated. The part of solution
that gains this salt is made more dense than the remaining
solution, and it therefore flows downward from the crystal
toward the bottom of the container. The density current can
be seen if it is looked into the region within the tank at
the lower edges of the crystal against some source of light.
It looks as though a heavy viscous fluid were flowing from
the crystal through the solution to the bottom of the tank.
If this density current is not seen, it probably means the
solution is saturated, since the test crystal will neither
dissolve nor grow in a solution at this state. And, if the
density current is rising from the crystal, it means the so 
lution is supersaturated. When the solution is in this
state, the test crystal will grow, because of deposits of
excess salt from the solution onto its surfaces. The part
of the solution thus depleted at the surface of the crys 
tal is less dense than the remainder of the solution in the
1)
tank and will rise visibly from the crystal.
The color of the salt solution of nickel sulfate
hexahydrate is deep blue-green in which these density cur 
rents are not so readily seen. In this case, the state of
the solution may be approximated if a test crystal is hung
in the solution and any change in its shape is observed.
Inspecting the corners of this crystal will be able to tell
whether it is dissolving or growing. If it is dissolving,
the corners will quickly get round; if it is growing, the
corners of the crystal will become very sharp.
After the saturation temperature has been deter 
mined (in our case, it is found to be about 45 deg C), warm
the solution to a few degrees above the assumed point of sa 
turation and pour it into the growing tank of the Rotary
Crystallizer, filling the tank to an inch or two below the
brim. Pour some of the remainder of the solution into the
seed-growing bowls or small glasses, and allow it to cool
slowly for the production of seed crystals. Place the tank
on its base support of the Rotary Crystallizer and cover it
with the lid. The thermoregulator is then electrically
connected to the temperature control circuit of the Rotary
Crystallizer. Next, turn on the apparatus, bring the solu 
tion to its saturation temperatur~ and set the thermoregu 
lator, with the control magnet, to the point at which the
incandescent heater of the Rotary Crystallizer just goes
off, then lock the thermoregulator in place. The motor of
the Rotary Crystallizer should not be turned on at this
14
stage.
The solution is left in the growing tank under tem 
perature control for a few days and observed whether or not
crystals form and grow in the bottom of the tank. If no
crystals form, add a few grains of the salt to the solution
and reduce the solution temperature a degree or so with the
thermoregulator. By letting the solution stand in this way,
it can be assured that the solution will reach saturation
exactly at the starting temperature set on the thermoregu 
lator. Scrap crystals that grow in the tank during this
period will take salt from solution only if the solution is
supersaturated; as they grow, the solution is reduced to sa 
turation. Once the solution becomes saturated, scrap crys 
tals will stop growing and the solution can be stored under
temperature control in its starting state.
As a precaution one should not let the scrap crys 
tals build up at the bottom of the growing tank to the ex 
tent that they entirely insulate the bottom surface of the
tank from solution. If this should happen, the solution
will not acquire the heat applied to the tank with the in 
candescent heater, and effective temperature control of the
solution will no longer be possible. Moreover, a tempera 
ture stress will occur at the walls of the tank, which
might cause it to crack. If scrap-crystal growth does be 
come excessive, temporarily transfer the solution to another
container, clean the tank of waste crystals, and then re 
sume stabilizing the solution at saturation.
15
Growing Crystal Seeds
Crystal seeds are merely small single crystals. The
valuable ones are those that can be harvested from a solu 
tion before they become imperfect. These seeds are used to
start the formation of larger crystals in the Rotary Crys 
tallizer. Any fragment of the solid, no matter how tiny, is
a potential seed. But in order to be conveniently suspended
by a thread to the crystal support rod of the Rotary Crys 
tallizer, a seed must be 1/8 to 1/4 inch long. Furthermore,
it must be a single crystal so that the crystal growing from
it will also be single.
To grow seeds one must first start with a solution
of the salt to be crystallized. The excess solution pre 
pared in the foregoing procedure should be adequate for seed
formation. However, if there is not enough left, one can
make a little more in the same way.
When the solution becomes supersaturated, crystals
will usually form spontaneously as excess salt comes out of
solution. If these crystals are allowed to grow for a while,
they will reach a size large enough for convenient handling.
As the seeds are growing, they should be inspected from time
to time to be sure that potentially good seeds are not being
interfered with by other crystals. When useful crystal
seeds have grown, about 1/8 to 1/4 inch long between opposi 
te faces, they are removed from the solution with tweezers
and dried carefully with facial tissue or a soft cloth.
16
These crystals seeds are then inspected for clarity, symme 
try, singularity, and size. The best crystals are stored
in a labelled and well-sealed vial.
Seeds may also be prepared by allowing a solution
that is saturated at room temperature to stand exposed to
the atmosphere in a small glass container. As solvent eva 
poration proceeds, the solution will supersaturate and seeds
will form.
Since it is the tetragonal nickel sulfate crystals
which we are interested in, the solution used for growing
seeds must be kept in an oven, which has a thermoregulator,
so that the temperature of the solution and its surrounding
air can be kept between the temperature limits of 35 deg C
and 50 deg C during the entire seed-growing process.
The ease with which one can grow seeds depends
largely upon the material being crystallized. For example,
some solutions can hold excess solute while they become more
supersaturated. In this case seeds will not form spontane 
ously with ease. If a highly supersaturated solution of
this sort is sUddenly disturbed, the salt will be urged out
of the solution. Solute then crystallizes so rapidly that
useful seeds can not form. This may be overcome by placing
a few grains of solid solute into the seed-producing liquid
at the very beginning of the process. Thus the solution
will not be allowed to store solute and go further and
further into supersaturation. A bit of residue that remains
from the evaporation of a drop of the solution may also be
17
used, in lieu of solute grains, to begin the growth of
seeds.
The rate at which seeds grow is most important. If
a solution that is saturated at a rather high temperature
is allowed to cool rapidly to room temperature, crystalli 
zation may occur at a rate too rapid for good seed forma 
tion. In this case the remedy is to dilute the solution
with added solvent, so that the saturation point is brought
closer to room temperature.
When working with material whose crystals break
apart or cleave well, such as nickel sulfate hexahydrate
whose crystals cleave beautifully along the plane which is
perpendicular to its axis of four-fold symmetry, seeds may
be made readily from imperfect crystals by cleaving away the
imperfections. A single good seed of such a material may
also be sliced up and each slice used as a seed.
When the harvested seeds are examined, it may be
noticed that one face of each of these little crystals is
slightly concave. It is the face that rested on the bottom
of the glass. Not much solution is able to get under such
a face to make it grow. But slight vibrations of the glass
let a tiny amount of liquid under the edges of the crystals,
and those edges slowly grow until finally the hollow left
at the center of the face becomes deep enough to be notice 
able. But this hollow will have little effect upon the
quality of the seed as a starting element for the experi 
ments.
18
Seeding the Solution in the Tank
The first step in seeding the Rotary Crystallizer
is to fasten the seeds to the crystal support rods that are
attached to the stirring shaft. Double over a short length
of white cotton thread and pass it through a piece of pla 
stic tubing that has been cut to the length of the support
rod. The thread should protrude from the tubing as a loop
at one end and two tails at the other.
Slide the tubing and the thread inside it completely
over one of the crystal support rods. Select a crystal seed
from the supply vial and place it in the thread loop. Hold
the seed in place with fingers and pull the tails of the
thread until the seed is firmly secured at the end of the
rod. Then cut off the loose ends of the thread with a razor
blade. It will be helpful to clamp the stirring rod in some
fixture while these manipulations are being performed be 
cause one must use both hands to fasten the seed crystals in
place. Repeat this procedure for the other three support
rods and then set the assembly aside until one is ready to
plant the seeds in the solution.
Then pour the saturated solution from the tank into
a 4-liter beaker so that the tank can be cleaned of all
waste crystals that have formed. Immediately heat the solu 
tion to about 5 deg C above what its temperature had been in
the ta~~. In being transferred, the solution will have
cooled by giving up heat to the new container. This cooling
19
can result in undesired crystallization from solution. The
beaker should be covered with a pyrex watch glass, or clean
plastic wrap, during this operation to minimize solvent eva 
poration, which would upset the proportion of solvent to so 
lute.
Remove any waste crystals from the tank and allow
them to dry. They can then be stored in a clean, sealed
container and later reused to prepare a new solution. Tho 
roughly wash the tank, lid and other parts of the apparatus
that have been in contact with the solution. Affix the
stirring shaft and seeded crystal-support rod assemble to
the motor shaft of the Rotary Crystallizer and lock it in
place.
Then, pour the solution from the beaker back into
the tank. Allow it to cool, monitoring its temperature with
the thermometer as it does so. When the solution reaches a
temperature of from 1 deg C to 1.5 deg C above its satura 
tion temperature, place the tank back on the heater base and
set the lid on top of the tank, immersing the seeds in the
solution. Turn on the temperature control system, but do
not turn on the motor.
As the solution cools to its saturation temperature,
all minute solute particles that might act as seeds for
waste crystals will dissolve. The presence of these seeds
is practically inevitable and their elimination is essen 
tial. Just as these particles dissolve in the cooling so 
lution, so will these specially selected and planted seeds.
20
However, these seeds are large, and they will only be par 
tially lost to the solution before they begin to grow.
Occasionally one or two of these seeds will fallout of the
thread loops holding them and drop to the bottom of the
tank. This happens if the solution is too far from the
state of saturation. If a seed does drop off, the solution
should be reseeded.
When the solution reaches saturation, we set the
thermoregulator into operation. To begin growth of the
seeds into large single crystals, reduce the temperature
setting on the thermoregulator 0.25 deg C by turning the
control magnet. This will allow the solution to cool down
from saturation and become slightly supersaturated. Turn
on the motor of the Rotary Crystallizer, and the crystals
will begin to circulate through the solution. The seed
crystals will begin to grow, as will be most evident in one
day's time. Leave the apparatus undisturbed until then.
Growing Crystals
The day after seeding the solution, the crystals
should have grown partially, or completely, around the
thread loops that hold them in place. If the crystals have
grown much larger, either the solution saturation point was
not determined correctly, or the temperature-regulation sys 
tem had failed. Crystals must not grow too rapidly if they
are to retain a high degree of perfection. If, on the other
hand, the crystals have disappeared altogether, the solution
21
must have been too far into unsaturated state, and the tank
will have to be seeded again.
If all has gone well, continuing the growth of the
seeds to maturity requires only successive reduction of the
solution temperature. Reduce the thermoregulator setting
another 0.25 deg C and wait a day to see how crystal growth
proceeds. Continue to reduce the temperature in such small
increments each day for an over-all period of several weeks
until the crystals have grown to the desired size.
This amount of daily temperature reduction is only
an average figure; the optimum will vary with the material
been used. The best growth rate and the best rate of tem-
perature reduction are to encourage the crystals to grow at
a more rapid rate when they are small than when they are
large.
As shown in Figure 2, crystal perfection is inverse 
ly proportional to the rate of cooling. So it is very im-
CRYSTALPERFECTION
COOLING RATE
Fig. 2.--Crystal perfection
as a function of cooling rate.
22
portant to determine the correct rate of cooling during
growth. This is a difficult task and is usually approached
by the trial and error method. For example, we started with
a supercooling by 0.25 deg G. It was found that the growth
was satisfactory. After 24 hours, the temperature was re 
duced by 0.25 deg G. Everything was still in order. After
the next 24 hours, the temperature was reduced by a further
0.25 deg C. gverything was still all right. So another
0.25 deg G was reduced and so on until toward the end of one
week, it was observed some cracks within some of the crys 
tals. Since one of the reasons of the appearance of cracks
in crystals is too great a drop in temperature, the tempera 
ture setting on the thermoregulator was reduced 1/8 deg C at
every 24 hours interval in the next run. But crystals still
cracked. So the next run, the temperature setting was re 
duced at the rate of 1/16 deg C per 24 hours interval. It
was found that at this rate of reduction in temperature, we
were able to grow better crystals. It becomes clear that
the trial and error method of searching for optimum growth
conditions may take months. For a crystal with a simple
geometrical shape, one can use a ruler to measure externally
the approximate dimensions of the crystal and weigh its
weight. One can then calculate roughly the actual satura 
tion temperature under given growth conditions. Using such
a calculation, one can deduce how far one can reduce the
temperature in the crystallizer.
23
Harvesting Crystals
The crystallization process may be regarded as com 
plete after 24 hours from the last reduction of temperature.
Then, the crystals are extracted from the crystallizer.
Certain precautions must be observed in removing crystals
from the solution in which they have grown. The crystals
are quite fragile and must not be suddenly exposed to a
radically different temperature. If they are removed from
the solution and allow to cool rapidly to room temperature,
they are likely to crack. One can avoid this by transfer 
ring the crystals to an empty container that provides an en 
vironment of the same temperature as their solution bath.
In this new container they can cool slowly and thus be kept
from breaking.
To do this, obtain a container of the same size as
the growing tank. Fill a sink with water of the same tem 
perature as that of the growing solution. Place the new
container in the water-filled sink. (One may need to put
ballast at the bottom of the harvesting vessel to keep it
from floating.) Turn off the Rotary Crystallizer and dis 
connect the motor power line. Then remove the thermoregu 
lator from the tank lid.
Quickly transfer the tank lid and its crystal-hold 
ing assembly from the growing tank to the cooling tank in
the sink. In a few hours, when the temperature of the water
bath in the sink has reached room temperature, the crystals
24
may be taken from the cooling tank. They should then be re 
moved from the support rods, and carefully dried immediately
with a paper tissue or a soft cloth. These crystals must be
handled very carefully, especially if one intends to use
them in the optical experiments such as the one described
later, for they are soluble in water, and their clear, plane
surfaces will be easily etched and damaged by the perspera 
tion on one's hands. The best way to store them is to wrap
them in a scrap of cloth and put them in a screw-topped jar
to keep them from damage in either too dry or too humid air.
The remaining solution can be used to grow more
crystals, but an amount of the original substance equal to
the weight of these newly grown crystals must be added to
it. During long runs, the solvent losses may be considera 
ble. Therefore, after filling the crystallizer, it is ne 
cessary to measure the level of the solution and, at the end
of a run and after the addition of a new portion of the sub 
stance, solvent must be added up to the previous level of
the solution. The saturation temperature is determined
again and the process is repeated as described above.
Even when very pure substances are used, the repeat 
ed growth of crystals from the same solution results in an
accumulation of impurities in the solution which, sooner or
later, begin to affect the growth and quality of the crys 
tals. It follows that it is necessary to change the solu 
tion from time to time.
25
In this method of growing crystals from solutions,
the greatest difficulty is the prevention of the formation
of stray crystals. Single or multiple stray crystals may
appear quite quickly during the first few hours after the
beginning of growth of the main crystal or they may appear
in the later stages. In the former case, stray crystals are
due to errors in the solution of the correct experiment con 
ditions while in the latter case, they are due to a distur 
bance in the normal growth process. The main causes of the
formation of stray crystals due to incorrect initial condi 
tions are as follows:
(1) The capture of crystalline dust particles from air
during the introduction of seeds into the crystallizer.
(2) Insufficiently careful washing of the cover, crystal
holder, and seed crystals.
(3) Defects in the seed crystals (cracks and occlusions).
(4) Insufficient duration of the storage or too Iowa
temperature of the solution after the determination of the
saturation temperature by means of a test seed.
(5) Too high a value of the initial supersaturation,
which may be due to the evaporation of the solvent as a
result of the poor sealing of the crystallizer during stor 
age before the introduction of the seeds or due to errors in
the determination of the saturation temperature.
All these faults can be remedied quite easily, but
if a crystal film or floating crystals appear on the surface
of the solution in spite of the measures that have been
26
taken to ensure the correct initial conditions, it follows
that the temperature at the beginning of the process should
be altered or the apparatus should be modified. If the her 
metic seal is satisfactory, the appearance of floating crys 
tals on the surface of the solution is the result of strong
supercooling of the surface. This can be prevented either
by additional thermal insulation of the crystallizer cover
or by special heating of the cover.
Stray crystals may appear several days after the be-
ginning of the growth of a crystal for the following reasons:
(1) The appearance of single crystals, particularly at
the same points in the crystallizer in several consecutive
runs, indicates cracks or scratches in which residues of the
crystal phase may remain even after careful washing. This
happens in a crystallizer in which a solution is permitted
to dry out: such a crystallizer should be filled with a sol 
vent and left in the filled state for several days before
using in the next run.
(2) The formation of many stray crystals usually indi 
cates that the crystallizer is not hermetically sealed.
(3) A disturbance in the normal operation of the drive
from the motor, which may shake the crystal holder and frac 
ture the growing crystals attached to it.
(4) Cracks may appear in a crystal and these may emerge
on the surface.
(5) The rate of cooling may be too high, resulting in the
transition of the solution to the labile state (this is a1-
27
most always preceded by imperfections in a growing crystal,
the appearance of inclusions, the skeletal growth of a
crystal, etc.).
Naturally, several of these factors may be acting
simultaneously and measures to prevent the effect of one of
them may not give any perceptible improvement. When stray
crystals are already present we can heat the solution to
such an extent as to ensure a higher temperature in the low 
er part of the crystallizer.
28
CHAPTER IV
OPTICAL ABSORPTION MEASUREMENTS
Preparing the Sample
In a tetragonal crystal, the dielectric permittivity
tensor only has two independent components. This means that
the crystal has, at most, two distinct light absorption
characteristics if they exist. One of them is in the direc 
tion of the optic axis of the crystal while the other one is
along any of the directions which are perpendicular to the
optic axis. The harvested crystal is cut along the two di 
rections described above in the search for possible dichro 
ism or a shift in the absorption band edges.
In nickel sulfate hexahydrate crystals, the family
of cleavage planes is perpendicular to the four-fold axis
of symmetry. Thus, thin square windows of quite large sur 
face can be obtained by simple cleavage of a crystal block
along the basal plane. A thin sharp knife is used for this
cleavage; it is placed on in the desired cleavage direction
and gently hit with a light hammer. As a rule the piece to
be cleaved off should always have about the same thickness
as the piece remaining; however, after a little practice
this rule need not to be strictly followed. Cuts against
the natural cleavage planes must be made with saws; for this
fret saws or thin metalled saw blades or diamond cutting
29
saws are used, sometimes with alcohol or water as lUbricant.
The cleaved surfaces usually are very fine surfaces for op 
tical measurements. But for those surfaces which do not
cleave well or those surfaces which are cut against the nat 
ural cleavage planes, they must be polished before they can
be used for optical measurements.
Nickel sulfate hexahydrate crystals are consider 
ably soft. This property enables easier finishing and pol 
ishing compared with glass, but also causes greater sensi 
tivity towards mechanical action by pressure and scratching,
and a lower quality of the final polish. The equipment
needed for finishing and polishing is quite simple: one or
more glass grinding plates, which can be purchased at any
optical firm; abrasive papers of various fineness, grits 2,
1, 0, 00, 000, 0000; abrasive powder of various fineness,
aluminum oxide powder grit 600 and gamma micropolish 0.05
micron; a polishing powder of special fineness (jeweller's
rouge); polishing clothes, metcloth and microcloth which can
be found in metallurgical laboratories; and a suitable non 
aqueous liquid as lubricant (kerosene). All these abrasive
papers and powders and polishing clothes can be purchased
from Buehler Ltd., 2120 Greenwood Avenue, Evanston, Ill.
The various types of abrasive powder must be carefully elu 
triated and if necessary winnowed in order to obtain an
even particle size. Great care must be taken that a finer
type is not contaminated by a rougher type because even a
single large particle will spoil all the previous work.
30
If the surface in question is not a good quality
cleavage surface, it is first rubbed to the approximate
thickness wanted with grits 2, 1, and 0 abrasive papers.
The grinding process itself is then carried out by placing
the component to be polished on the grit 00 abrasive paper
and then rubbing the component with a gentle even pressure
and small circular or in the form of figure "8" movements
gradually over the whole surface of the abrasive paper.
This process is repeated with grits 000 and 0000 abrasive
papers of increasing fineness until a good flat surface is
obtained with the appearance of a ground glass plate and so
that after working on both sides the components has the de 
sired thickness. The flatness of the surface is tested by
means of mechanical precision instruments or even (after
polishing) by optical methods. Further treatment, i.e.,
polishing, is carried out with the constant use of rubber
gloves, or at least finger-stalls, in order to avoid the
influence of perspiration on the surface; breathing on the
component should also be avoided. For polishing, the pro 
cess is carried out by placing a little aluminum oxide
powder grit 600 on the polishing glass plate, which is
covered by a polishing metcloth, with a few drops of kero 
sene as the lubricant and then rubbing the component to be
polished with a gentle even pressure and small circular or
in the form of figure lI8" movements gradually over the
whole surface of the grinding plate (which in time may be 
come hollowed out). This process is repeated with the
31
gamma micropolish 0.05 micron and the jeweller's rouge
until the dull surface gradually vanishes and a smooth near
mirror-like surface appears. The final polishing is carried
out on the microcloth by dropping a little 95% ethyl alco 
hol, which is being saturated with nickel sulfate hexahy-
drate, on it and polishing the component until it is com 
pletely dry. The component now is ready for optical mea-
surements.
Optical Measurements
The polished crystal is then mounted in a sample
support and put in the sample fixture of Perkin-Elmer Nodel
202 Ultraviolet-Visible and Model 457 Infrared Spectrophoto-
meters. Its light absorption in the wavelength ranges of
ultraviolet, visible, and infrared is studied. It is found
that for a sample of 2.53 ? 0.01 mm. thickness, after being
treated as described above, and an exposure area of 6 mm.,
both the A crystal component whose exposy~ed plane is paral 
lel to the optic axis and the C crystal component whose ex 
pos~red plane is perpendicular to optic axis of the nickel
.,,-'"
sulfate hexahydrate crystal show nearly complete absorption
in the infrared region while in the ultraviolet and visible
regions the crystal has two transmitting bands. Since it is
the differences in absorption in the Ita" and lie" directions
we are looking for, the investigation is then emphasized in
the light transmitting characteristics of the A and the C
crystal components in the ultraviolet and visible wavelength
32
ranges by utilizing the Ultraviolet-Visible Spectrophoto 
meter.
The Perkin-Elmer Model 202 Ultraviolet-Visible Spec 
trophotometer is a double-beam instrument designed for lab 
oratory use in quantitative analysis. This instrument,
developed as an absorbance-recording spectrophotometer, per 
forms throughout the ultraviolet and visible spectral re 
gions with high resolution.
The Model 202 UV-VIS Spectrophotometer covers two
ranges: 190 to 360 millimicrons in the ultraviolet, and 350
to 700 millimicrons in the visible. Two scanning rates,
either wavelength range selected can be scanned in two or
eight minutes, offer the analyst a choice of either sur 
vey or precise spectra recorded on a 8! x 11-in. paper.
Host ultraviolet recording spectrophotometers are
electronic null instruments. That is, they are dependent
upon a precision potentiometer to balance the signal pro 
duced by different light levels in the two optical channels.
In such instruments, the signal is intrinsically propor 
tional to transmittance and therefore, some type of elec 
trical or mechanical component is required to produce the
logarithmic (or absorbance) presentation that is required
for the solution of most analytical problems.
In an optical null system, the light that is removed
from the beam by the sample is balanced by a mechanical
light attenuator wedge in the reference beam. The attenu 
ator wedge in the Nadel 202 is shaped so that the light is
33
removed logarithmatically as the wedge is moved in the beam.
Motion of the attenuator is therefore directly proportional
to the absorbance of the sample.
Since the taper of the attenuator wedge is logarith 
mic, the rate of change of transmission is not constant;
in fact, it is approximately 30 times greater at one end of
the range than it is at the other. Because of this 30 to 1
change in the slope of the attenuator, a similar error of
the attenuator, i.e., 0.01 absorbance unit, will produce an
electrical error signal 30 times larger at zero absorbance
than at 1.5 absorbances. To compensate for this variation
in gain, an automatic gain control keeps the output of the
multiplier anode constant thereby keeping a constant pen
response over a wide range of energy variation.
Two light sources are provided with Model 202: an
air-cooled deuterium lamp is for the ultraviolet spectral
range, and a ribbon-filament incandescent lamp for the vis 
ible range. In either range only the associated lamp is
lighted. Range selection is made by rotating the drum until
the metal paper holder passes the wavelength indicator. At
this point, a switch is actuated which turns on the proper
lamp source and control the position of a small mirror. In
the VIS range, this mirror reflects radiation from the
tungsten source onto the toroidal mirror; in the UV range,
the small mirror is removed and the light from the deuteri 
um source falls directly on the toroidal mirror. All the
aluminum mirrors are coated with magnesium fluoride to pro-
34
tect them from atmospheric deterioration. The large amount
of available energy results in low stray light even at 190
millimicrons.
In Model 202, the light from either source is dis 
persed by the monochromator and separated by a rotating
chopper into two beams, each beam being interrupted at 26
cps. The sample is placed in one beam (the sample beam)
and the air is used as a reference in the second beam. The
beams are then recombined on the detector and the resulting
signal is amplified. The main 26 cps control signal is
then amplified and used to drive a servomotor which moves
the attenuator wedge in the reference beam to a position
where the intensity in the two beams is equalized. The
pen which records the position of the attenuator is also
driven by this same servomotor. A 52 cps control signal is
also generated by the chopper and is used to control the
loop of the servo system. This signal is amplified in the
automatic gain control the high voltage across the photo 
multiplier in such a way as to keep the anode signal con 
stant.
In addition to providing improved performance at
short wavelengths, the end-on photomultiplier used in the
Model 202 has high sensitivity in the red end of the spec 
trum and provides good performance out to 700 millimicrons.
Furthermore, the application of the end-on photomultiplier
design makes the instrument very insensitive to sample cell
orientation and other effects that tend to change the beam
35
position on the detector.
Figure 3 and Figure 4 show that f~r the crystal com 
ponent whose surface exposed to the light source is perpen 
dicular to the optic axis of the crystal has total absorp 
tion bands in the wavelength ranges of 190 to 195 millimi 
crons, 350 to 428 millimicrons, and 585 to 700 millimicrons
and transmission bands in the wavelength ranges of 195 to
350 millimicrons, 428 to 585 millimicrons with maximum
transmission occurs at 310 millimicrons and 490 millimi 
crons, while for the crystal component whose surface exposed
to the light source is parallel to the optic axis of the
crystal has total absorption bands in the wavelength ranges
of 190 to 195 millimicrons, 352 to 425 millimicrons, and
590 to 700 millimicrons and transmission bands in the wave 
length of 195 to 352 millimicrons and l~25 to 590 millimi 
crons with maximum transmission occurs at 305 millimicrons
and 490 millimicrons. Furthermore, from these two plots, it
is found that the light transmission of both A crystal com 
ponent and C crystal component are the same except the mag 
nitude of the curves. Apparently, there is no shift in the
edges of the absorption bands in the alpha-hexahydrate of
nickel sulfate crystals.
360280260
~c
240
a LIGHT SOURCE PERPENDICULAH TO OPTIC AXIS
c LIGHT SOURCE PARALLEL TO OPTIC AXIS
a~ ~
1.0
0.9
0.8
~0
0.7
~8
8 0.6
Hs
CI2z 0.5
.~r.r::
8 0.4
0?3
0.2
0.1
a
190 200 220
WAVELENGTH (millimicrons)
Fig. 3.--I,ight transmittance characteristics of alpha-NiS04 ?6H20 crystals in
the ultraviolet region. Sample thickness = 2.53 ~ 0.01 mm. W
Q'\

38
CHAPTER V
Sm1MARY AND DISCUSSION
By comparing this experimental data with the crystal
growth data given by Holden and Singer, it is fOWld that
both data agree with each other except the saturation tem 
perature. With the same amount of distilled water as a sol 
vent and nickel sulfate hexahydrate as a solute, the satu 
ration temperature of the solution, according to Holden and
Singer, will be approximately 50 deg C while in this experi 
ment it was only 45 deg C. The cause of this deviation
probably is the difference in the amount of impurities con 
tained in the nickel sulfate hexahydrate chemical used. The
nickel salt used by Holden probably contained more impuri 
ties than that of used in this experiment. These impurities,
in general, tend to decrease the solubility and increase the
saturation temperature of the chemical.
From Figure J and Figure 4, since the curves for the
two crystal components look practically the same, it may be
concluded that there is no major shift in the absorption
edge in the alpha-hexahydrate of nickel sulfate crystals.
Furthermore, it looks safe to conclude that the light trans 
mittance characteristics of the A and C crystal components
are exactly the same because the small magnitude difference
may be caused by the differences in final polishing, actual
39
thickness and the spectrophotometer's machine errors.
In Model 202 Ultraviolet-Visible Spectrophotometer,
the sources of machine errors are as follows: (1) A wave 
length reading on the wavelength scale is different by + 0.5
millimicron in the Tv, and ? 1 millimicron in the VIS, from
the wavelength actually being scanned by the instrument.
(2) The wavelength readings registered on the wavelength
scale are different up to + 0.3 millimicron in the UV and
? 0.5 millimicron in the VIS from one run to the next. (3)
The readings on the absorbance scale, for any fraction of
full scale energy, have errors of ? 0.01 A between 0.0 and
0.1 A and ? 1% between 1.0 and 1.5 A. (4) Absorbance read 
ings differ by 0.005 A from one run to the next. However,
during the measurements, it is found that the instrument
has greater errors both in wavelength and absorbance read 
ings than those just described above from one run to the
next. Sometimes the fluctuations go as high as 10%, espe 
cially in absorbance readings.
The human errors in absorption measurements are as
follows: (1) The thickness of A and C crystal components
after the final polishing may differ by 0.02 mm. The
thicker the sample is/the greater the absorption. (2) The
difference in the fineness of the polished surfaces of the
A and C crystal components. It is impossible to have
exactly the same fineness of final polish on the surfaces
of these two crystal components. (3) Because of the lack
of enough absorption measuring instrument components,
40
~ t
samples have to be mounded and dismounded from the sample
support every time another sample's absorption character-
tistic is to be measured. During these mounding and dis-
tmounding, chances of damaging the fine polished surfaces
increases directly proportional to the number of measure-
(j/ments taken. All these human errors can
~ accounted for
the differences in the magnitudes of the transmission
curves.
The fact that A and C crystal components were not
cut from the same crystal may also ~f account~ for the./
deviations.
In this experiment, a major shift in the absorp 
tion edge between A and C crystal components was not ob 
served. Apparently, the UV or VIS absorption is not due to
Ni++ transitions even though the Ni++ is located in the low
symmetry site. Instead, it is believed due to the Ni(H20)6
octahedra structure and its vibration states.9 Furthermore,
the arrangement of Ni(H20)6 octahedra and 304 tetrahedra
within the tetragonal NiS04?6H20 structure is in such a way
that the absorption characteristics are the same in the two
orthogonal directions of which one is along the optic axis
10of the crystal.
9C? Furlani, "Ni(II) Kom lexe." Zeitschrift fur
Ph sikalisch-Chemie Frankfurter. X (1957), 291-305.
10 ++R. Trehin, "Bande d'Absorption de IlIon Ni
II
Annalen der Physik, ~( (1945), 372-90.
41
In this experiment, nor was the linear dichroism of
nickel sulfate hexahydrate found. The reason might be the
sample were too thick to show the detail absorption charac 
teristics of the A and C crystal components. Thus thin film
samples which are less than 0.5 mm. thick are recommended
for future optical absorption measurements.
It is observed, from Figure 3 and Figure 4, that
these transmission curves have very sharp cut off points
which make nickel sulfate hexahydrate crystals very good
filters in the UV-VIS regions.
42
BIBLIOGHAPHY
Books
Holden, Alan and Singer, Phylis.
ing. Garden City, N. Y.:Company, Inc., 1960.
Crystal and Crystal Grow Anchor Books Doubleday &
Laudise, R. A. The Growth of Single Crystals. Englewood
Cliffs, N. J.: Prentice-Hall, Inc., 1970.
Seidell, Artherton. Solubilities of Inorganic and Metal Organic Copounds. Fourth Edition. Princeton,
N. J.: Van Nostrand, 1958.
Articles
Furlani, C. IlAbsorptionsspektren Nagnetisch Normaler
Ni(II) Komplexe. 1l Zeitschrift fur Physikalisch 
Chemie (Frankfurter). X (1957), 291-305.
Ignersoll, L. R. et ale "Optical Activity, CircularDichroism, and Absorption of Crystalline Nickel
Sulfate.II Physical Review. LVII (1940), 1145-53.
Trehin, R. "Etude Esperimentale de la bande d'Absorptionde IlIon Ni++ Dans Ie Proche Ultra-Violet pour les
Cristaux So4Ni, 60H2 (Quadratique) et So4Ni , 70H2
(Orthorhombique)," Annalen der Physik, XX (1945) ,
372-90.
R.EFERENCES
Books
Brugel, Werner. An Introduction to Infrared SpectroscoPI.
New York: John Wiley & Sons Inc., 1962.
Buerger, Martin Julian. Elementary CrystallographY. New
York: Wiley, 1956.
C. R. C. Handbook of Chemistry and Physics. 51st Edition.
Cleveland: Chemical Rubber Publishing Company,
1970, p. B-114.
Gmelin, 1eo}Old. Gmelins Handbuch der Anor~anischen ChemieNi. Weinheim: Verlag Chemie, 19 3, Vol. LVII.
International Union of Crystallography. International
Tables for X-Ray Crystallography. Vol. I,
Birmingham, England: Kynoch Press, 1962, p. 426.
Landolt, Hans Heinrich. Landolt-Bornstein Physikalisch Chemische Tabellen. Berlin: Springer, 1965.
Lindsay, p. A. Introduction to Quantum Mechanics for Elec trical Engineers. New York: McGraw-Hill Publish 
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National Bureau of Standards. Selected Values of Chemical
Il~hermodynamic Properties. IIJ"'BS 500. Washington:
Government Printing Office, 1970, Series 1, p. 248;Series 2, p. 690.
Nye, J. F. Physical Properties of Crystals. Oxford:
Oxford University Press, 1957.
Perkin-Elmer Corp. Perkin-Elmer Model 202 Ultraviolet 
Visible Spectrophotometer Operation Hanual.
Norwalk, Connecticut: Perkin-Elmer Corporation,
1970.
Wyckoff, Ralph W. G. Crystal Structures. Second Edition,
Vol. II. New York: Interscience Publishers, 1963.
44
Articles
Brieger, K. "Zum Optischen Verhalten des Kristallwassers,"Annalen der Physik, LVII (1918), 287-320.
Dietz, Ralph W. and Bennett, Jean M. Il Bowl It'eed Technique
for Producing Supersmooth Optical Surfaces," Applied
Optics, V (1966), 881-2.
Hartert, E. llUber den Einfluss der Bindung des
Kristallwassers auf dessen Deformationsschwingung in
Ultrarot,1l Naturwissenschaften, XLIII (1956), 275-6.
Jacolson, Jacob L. and Nixon, Eugene R. "Polishing of Cesium
Halide Windows," Applied Optics, VI (1967), 1583.
Krishnamurti, D. "Raman Spectra of Nickel Sulfate Crystals,"
Proceedings, Indian Academy of Sciences, XLIIA(1955 ), 77-80?
Levine, N. A. llPolishing of CsI Windows for Use in the
Infrared,ll Applied Optics, V (1966), 1957.
Hitzner, bernard H. "Technique for Polishing Silver Chloride
Plates," Journal of Optical Society of America,
XLVII (1957), 328.
Slack, F. G., ~ aI, "Magneto-Optical Activity of Crystal line Nickel Sulfate, Alpha-Hexahydrate," Physical
Review, LIV (1938), 355-7.
Smakula, A., et aI, "Optical Haterials & their Preparation,"
Applied Optics, III (1964), 323-8.
Vierling, J., ~ aI, "Improving the Diffracting Properties
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?QPZ, XXIII (1969), 342-5.