Full text of DOI:10.1016/0022-0248(72)90226-6

Journal of Crystal Growth 17 (1972) 3-18 © North-Holland Publishing Co.
KINETIC AND THERMODYNAMIC ASPECTS OF HIGH TEMPERATURE
CHEMICAL TRANSPORT AND SUBLIMATION OF EuS*
E. KALDIS
Laboratorium fur Festkorperphysik ETH, Honggerberg, 8049 ZOrich, Switzerland
Kinetic and thermodynamic aspects of vapour growth are discussed and the importance of the surface reac-
tions as rate determining steps is particularly stressed. Both sublimation and chemical transport of EuS have
been studied from this point of view.
Thermogravimetric measurements of the equilibrium vapour pressures of EuS in the temperature range
1700—2200°Cusing Knudsen cells have been made for the first time. The enthalpy of sublimation is 100 kcal/
mole. Free evaporation measurements (Langmuir) show that the activation energy of vaporisation is higher,
110 kcal/mole and the Langmuir vapour pressure much lower. The evaporation coefficient is therefore
3.3—6.Ox l0_2 and the vaporisation kinetics are rate determining. Morphological evidence from the EuS
crystals grown by sublimation indicate that here also kinetics are rate determining. Electronmicroscopic
investigations showed that at high temperatures a transition from singular to non-singular faces takes place
for EuS.
The chemical transport of EuS has been possible for the first time, using a high pressure, high temperature
chemical transport and starting material Eu3S4. There is some indirect evidence that a complex polysulfide
of the type E u~ M o~ Sm, ight be the transporting agent. Both growth kinetics and interface morphology show
that kinetics and not diffusion is the rate determining process. During these investigations, no evidence of
constitutional supercooling from the vapour phase was found.
1. Introduction
Following the increasing interest of science and
with high purity, homogeneity and good stoichiometry
is great.
Recently the high temperature chemical transport in
technology for high temperature materials, the demand sealed refractory metal crucibles has been used in order
for single crystals of compounds with high melting to grow single crystals of highmelting compounds from
point has been continuously increasing in the last 10 the vapour phase’ 2). Careful characterisation by
years. Up to now the great majority of the attempts to physical, chemical and crystallographic methods2’3)
grow such crystals involved growth from the melt. It is has shown that these crystals are pure, stoichiometric
well known, however, that although the operating and perfect to a high degree and do not contain any
temperatures of melt growth methods have been con- impurities of the crucible wall (less than 1 ppm at.).
tinuously increased and these techniques have become The wide-spread opinion that only crystals with a few
very sophisticated, there are still many compounds mm dimensions can be grown from the vapour phase,
which cannot be grown from the melt. This is partic- has also been proved incorrect by the above method.
ularly true for compounds which decompose at high Thus single crystals of EuTe with dimensions of
temperatures, melt peritectically, or undergo undesir- 10 x 6 x 6 mm, and of EuSe with 15 x 7 x 8 mm can be
able phase transitions at high temperatures. In the last grown3). The latter have been grown in a molybdenum
years a much larger group of compounds grown from crucible of 15 mm diameter. By using crucibles with
the melt have been found with improved methods of 30 mm diameter, even larger crystals can be obtained.
detection to be non-stoichiometric, inhomogeneous or
For further development of the vapour growth
impure. With increasing recognition of the great methods at temperatures higher than 2000 °C ,know-
influence which these qualities have on the physical ledge of the growth mechanism would be very helpful.
and mechanical properties of most compounds, the This is particularly true in the case of EuS. At tempera-
demand for single crystals of high melting compounds tures where single crystals of EuTe with appreciable
*
Dedicated to Professor H. Schafer on his 60th birthday for his
dimensions have been grown by sublimation, only very
thin deposits of EuS can be obtained. Also the high
primary role in the field of chemical transport.
3

4
E. K A LD IS
temperature chemical transport, using halogens or might be rate determining in several cases. This is due
halides as transporting agents, was not successful in both to the lack of experimental methods giving direct
growing single crystals of EuS’). Due to these reasons, insight into the in situ growth process and also to a
a research program was started aiming at the investi- general underestimation of the importance of this
gation of (a) the mechanism of sublimation (evapora- phenomena for experimental vapour growth. In the case
tion) and its rate determining process, and (b) mis- of chemical transport this underestimation is due to the
cellaneous transporting agents suitable for appreciable widespread assumption that for most chemical re-
transport of EuS. Both these investigations were actions at medium temperatures (higher than 500 °C )
expected to furnish data for optimal conditions of diffusion in the vapour phase and not chemical kinetics
growth of EuS single crystals. The results of these are rate determining9~’).True as this statement may
investigations are reported in this work. The conditions be, it must be pointed out that kinetics of the surface
of a new process
chemical transport of EuS
—
the high pressure, high temperature reaction on the growing interface and not the usual
are given, and the mech- chemical kinetics are important during growth of single
—
anism of this reaction is discussed as it relates to the crystals from the vapour. The different influence on
growth rate and the surface morphology of the crystals vapour growth of these two processes becomes ap-
grown.
parent when growth at high and low supersaturation is
considered. At high supersaturation, the vapour species
2. The importance of surface reactions in experimental can reach the activation energy necessary for homo-
vapour growth
geneous nucleation and the formation of clusters in the
In the last few years there have been considerable vapour phase is possible. Therefore, clusters of the
discussions on the rate determining step in vapour compound condense on the growing surface. Due to
growth under conditions suitable for the growth of their reduced migration velocity and varying orien-
single crystals. These discussions have been initiated by tations the growing surface will tend to become poly-
the work of Reed and LaFleur4) on the constitutional crystalline. We expect, therefore, that at higher super-
supercooling in the vapour phase and were further saturation fast nucleation phenomena will be important
promoted by the work of Toyama’), Faktor6) and and not the single crystal growth kinetics. As the
others7) on the constitutional supercooling during equilibrium can be easily reached at these temperatures
growth from the vapour of non-stoichiometric com- it is very probable that in this case mass transport
pounds. The problem of the rate determining step in through the gas phase will be the slower process which
experimental vapour growth is not only of theoretical controls the deposition rate9’ 1 )~
interest but also an economic one. If diffusion alone is
At low supersaturations, which alone are suitable for
rate determining, then high carrier gas velocity and the growth of large single crystals2”2) from the vapour,
steep temperature gradients at the interfacecan be used. only slow growth phenomena are important. Here the
“Once a suitable temperature profile is established by surface reaction enters the picture. Consider a growing
increasing the carrier gas velocity, the growth velocity face of a single crystal, e.g. CdS. The condensing
may be increased indefinitely without causing inter- species are not molecules of CdS, but according to the
face instability”8). Were this the real situation, the reaction’3) of the dissociative sublimation
growth of crystals from the vapour phase would no
longer be a slow process and the cost of devices and CdS
single crystals from the vapour could be reduced
Cd++ S2
appreciably.
Cd atoms and S2 molecules. The “reaction” of these
Although diffusion problems have received the right two species on the surface, necessary for the incorpora-
attention and have been sufficiently discussed, there is tion of Cd and S atoms into the lattice, needs some
very little information about the growth kinetics of extra activation energy which decreases the growth
compounds, especially those grown by chemical rate. In growth by chemical transport, the situation
transport.
becomes even more complicated. The following reac-
The same is also true about surface reactions which tion’4) takes place for CdS during iodine transport:

HIGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLIMATION OF EuS
5
sation of solids is given from the value of the evapo-
ration/condensation coefficient. From these values
/
/
some conclusions can be drawn about the mechanism of
sublimation. For a given compound, it is not generally
/
/ /
possible to change the surface reaction during evapo-
ration. A few exceptions have been found when liquid
catalysts are used2). Thus, e.g., thallium melt covering
~ /
~//
/ ~ t ~ _ , /
the surface of evaporating arsenic increases nearly
100 times the vapour pressure’ 7 ) of this element. On
/1
/ ~ ~ / /
the other hand, a great advantage of chemical transport
is the large choice of surface reactions available for the
Growth kinetics Diffusion in the gasphase
rate determ. rate determining
P0
transport of a given compound. By using various
Fig. 1. Growth rate versus supersaturation. Schematic compari- transporting agents the optimal surface reaction can be
son of the theoretical growth rate of a (IOU) face of a metal
crystal growing from its monatomic vapour with the growth rate
found. Following this aproach Parker’8) found that
although ZnSe can be transported with HC1 and HBr,
expected when diffusion is rate determining. At low super-
saturation the rate of kinetically controlled growth will be lower
than that of the diffusion controlled growth. The former should
be rate determining,
only polycrystals, dendrites and twins grow when these
agents are used and confirmed earlier experiments’ 9 )
which have shown that large single crystals of ZnSe can
be grown by iodine transport.
CdS + 2 I ~± Cd12+ ~ S2.
In the past only the retarding influence of the surface
reaction during chemical transport has been pointed
2
‘~ ‘
Here, at low supersaturation, the condensing species out’ 2 ) However, the selection of a suitable transporting
[eq. (1), 2] are even more different from the lattice units agent for a given compound may lead to a surface
than in the case of sublimation. Further, some pro- reaction which needs less activation energy than the
ducts of the surface reaction (in the present case I) must activation energy of sublimation. At high temperatures
be desorbed from the growing surface, a fact which the choice in transporting agents becomes larger. This
generally will increase the overall activation energy of is due to entropy changes which allow the occurrence
the surface reaction. Therefore, it is reasonable to of chemical reactions and vapour species which are
expect that at low supersaturation when single crystal unstable at lower temperatures. An example of this
growth is considered, the growth kinetics might be rate effect is the high pressure, high temperature, chemical
determining,
transport of EuS with Eu3S4 in sealed molybdenum
Even in the case of ideal growth kinetics (a metal crucibles, which isreported in this work.
crystal growing out of its monatomic vapour) a com-
parison of the growth rate expected by the BCF 3. High temperature sublimation of EuS
theory’ 5) with the rate of a simple diffusive growth,
indicates that at low supersaturations the slowest
process should be growth kinetics and not diffusion
~ ~ VAPORISATION MEASUREMENTS
As mentioned above, low sublimation rates for EuS
through the vapour (fig. 1). In the case of chemical have been observed in sealed molybdenum crucibles at
transport the parabolic law of the BCF theory has temperatures up to 2100 °C .In principle, this could be
been shown to be valid for Cd4GeS6 i2 ) Due to ad- the result of either low equilibrium vapour pressures,
sorption effects’6 ) from the transporting agent, it can i.e. a thermodynamic effect, or low evaporation!
be expected that the parabolic part will be shifted condensation coefficient due to a kinetic barrier of the
towards higher supersaturations and therefore the evaporation/condensation process. Kinetic barriers are
region where growth kinetics is rate determining will be the result of the activation energies necessary for the
extended.
various steps of the surface reaction. As there are no
Information about the existence of a rate determining thermodynamic or evaporation data of EuS in the
surface reaction during the evaporation or conden- literature, an investigation of the above problem must

6
E. K A LD IS
include both thermodynamic measurements of the
equilibrium vapour pressures using Knudsen cells, and
free evaporation (Langmuir) measurements in vacuum.
The latter can give information about kinetic barriers
of the surface reaction which should decrease the rate
of evaporation under vacuum2’20). Both investigations
have been performed thermogravimetrically. The
vapour pressures have been calculated from the Hertz—
Knudsenequation
_ _ _ _ _ _ _ _ _ _ _ _
‘1
-—
-.
P
J
=
—
-
— . ,
(2IzRTM)T
/ ~ ~ / ~ /
where J is the number of atoms impinging on the unit
area of the wall at unit time, P the vapour pressure, R
N
.
I
0.532 mm
d (Microscope)
°
0.1t5mm
the ideal gas constant, T the absolute temperature and
—
d (Surtace integrator)
M the average molecular weight of the vapour species.
0.109mm
In terms of the weight loss of the cell, the pressure is Fig. 2. Design of Knudsen cells, which were used in the high
given in atm by
temperature furnace of the Mettler Thermoanalyser for thermo-
gravimetric vapour pressure measurements. The ground joint at
m
/ T \t
the bottom allows refilling of the cell and use during several
P
=
0.02256 ( - )
,
experiments. Before each use the areas of the joint have to be
tA
ground again. The dimensions shown belong to tungsten cells
electromachined by the Elox technique.
where rn/tA is the weight loss in g/sec cm2
For the
.
free evaporation experiments the Langmuir pressure electromachined by the “Elox” technique. Holes with
was calculated for a value of the evaporating surface 0.1 mm diam. were drilled in the lids of the Mo and Ta
A equal to the cross sectional area of the cell. Normally cells and holes with 0.1 and 0.2 mm diam. electro-
the evaporating surface will be larger and this is the machined in the W cells. The internal dimensions of the
lowest possible value for A; therefore, the highest cells were 6 mm diam., 6 mm high. The temperature
value for ~L and ~. For the equilibrium pressure meas- range investigated was 1700—2200 ° C for the Knudsen,
urements with a Knudsen cell the cross section A of and 1450—1950 ° C for the Langmuir measurements.
the orifice is multiplied with the Clausing factor to The large range of 500 °C ,which was easily possible
account for the flow resistance of the vapour through in the full automatic balance, increased the accuracy of
the small orifice,
the measurements. On the other hand, the high tem-
peratures necessarily increased the danger of conta-
mination of the divalent Eu, and the possible falsifi-
3 .2. EXPERIMENTAL CONDITIONS
The vaporisation measurements were performed in cation of the values of both the vapour pressures and
cylindrical cells especially designed to fit the crucible of the enthalpies of evaporation. Nevertheless, the new
support of the SHT-furnace of the Mettler Thermo- SHT high temperature vacuum furnace of the Mettler
analyser. In order to be able to use the same cell for Thermoanalyser, which was used for these measure-
several experiments and still close the lid hermetically, ments, was rapidly covered by an evaporatedfilm of the
a conical ground joint was used at the bottom of the investigated compound which was gettering the fur-
cell (fig. 2)*. After each experiment the lid and crucible nace atmosphere keeping the vacuum near 10_b torr
can be taken apart and the joint ground again for the even at the highest temperatures. The danger of con-
next experiment. Molybdenum and tantalum cells tamination was thus decreased. Another great ad-
were machined on the lathe and tungsten cells were vantage of the balance is its extreme thermal stability
which maintains the temperature ± 1.5 ~Cin the range
*
The author is indebted to Mr. R. Haug who suggested this
of 600—2400 °C . Scattering of measurements due to
temperature fluctuations is therefore negligible.
design and to Mr. W. Loertscher for the skilled electromachining
of the tungsten cells,

HIGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLIMATION OF EuS
7
One of the most important tasks of such thermogravi-
metric, high temperature vapour pressure studies is the
avoidance of side reactions which would result in
erroneous evaporation data. These reactions may
induce chemical evaporation of either the crucible or
2200
2000
1800
1600
* —
C~
m gh
the charge. Due to this reason the weight changes of the
crucible were carefully controlled after each experiment.
A slight increase of the tungsten Knudsen cells was
100
Langmuir (Mo Cell)
~H 110.5 kcal Mol
observed (4 mg in 7.0 g total weight) probably due to
transport of tungsten from the colder baffles to the
crucible.
In order to calculate the Clausing factor of the cells,
the surface area of the orifice was measured both with
the microscope (measurement of the diameter) and with
the planimeter after photographic magnification (surfa-
ce area measurement). These measurements were made
before and after each experiment. In tungsten cells
to
1,0
0,1
Knudsen(Ta Cell)
- 110.3 kcal Mol
onlyvery small changes ofthe orifice area were observed
due to the growth of tiny crystallites in the orifice. The
height of the orifice was determined by measuring the
deplacement of the microscope table with a very
sensitive micrometer. The opened cell was put on a
polished glass rod to make the lower surface of the
orifice visible. At high magnification the scratches from
lo~
400
450
500
550
Fig. 3. Examples of thermogravimetric weight loss measurements
of EuS versus temperature (10 6 JT, K ). The decrease ofweight
loss rate by the Langmuir measurements is explained by the d e-
the abrasive were visible, showing the position of the crease of the evaporating surface due to recrystallisation of the
charge.
bottoni of the orifice.
curves measured at decreasing temperature were paral-
3.3. Tui~EVAPORATION COEFFICIENT OF EuS
lel to each other. Parallel shifts of the evaporation
Fig. 3 shows the measured weight loss rates. The use curves were found alsoin some Knudsen measurements.
of a molybdenum cell for the Knudsen measurements probably due to minor recrystallisation of the orifice
was not feasible, because the weight losses from the cell under the influence of the high temperature vapour.
became comparable to the effusion losses. In the plot of
The enthalpy of evaporation calculated by the second
fig. 3, linear dependence of the weight losses could be law method from the Knudsen cell data was found to be
achieved by the Langmuir measurements only by 110.3 ± 2.0 kcal/mole (in the temperature range of
decreasingthe temperature. This was understood as the 1800—2250 °C ) for the tantalum cell and 100.0 ± 2.0
result of a change of the evaporating surface area. kcal/mole (in the temperature range of 1720—2200 °C )
When the surface area is formed at a high temperature, for a tungsten cell. As the tantalum cell showed a large
it is not substantially changed by the evaporation at decrease of its weight and the tungsten cell almost
lower temperature due to the exponential dependence none, we trust only the latter value. The activation
of P on T. On the contrary, when the measurements are enthalpy of free evaporation (Langmuir) was found to
performed in the order of increasing temperature, each be the same for experiments with both molybdenum and
new measurement would increase the evaporation rate tunsten cells AH*
=
110.0 ± 2.0 kcal/mole (in the
and change appreciably the evaporation surface. This temperature range of 1450—1950 °C).The higher value
effect was observed also when single crystals with di- of AH* in comparison to LiH (both measured in the
mensions of the order of 0.5—1 mm were used as charge. W cells) indicates that an activation energy is necessary
It was found also that during evaporation extensive for the surface reaction of evaporation.
recrystallisation of the charge took place.The Langmuir
Fig. 4 shows the equilibrium (Knudsen) and free

8
E. KALDIS
Plorr
/ /
/ /
/
evaporation coefficients of the order of 10-
2
There-
/
/
fore, the low sublimation rate of EuS as compared
to that of EuTe must be the result of lower vapour
77
~
Y
-o
// a’6,O’1c5° ,‘
:‘
pressures. Measurements of the vapour pressures of
EuTe confirmed this assumption. Extrapolation of the
lo
//
//
~ //
~“
. ‘
‘
vapour pressure of EuTe in the temperature range of
the above measurements showed that the EuS vapour
pressure is approximately one order of magnitude
V
/
//
.i
1 ~ i
/
,‘
lower than that of EuTe2’).
~,Mo-cell
4. High pressure, high temperature chemical transport
-
a .3 .3 1 6 ° / ///
B
~
1425-1950’C
4. 1. TRANSPORT OF EuS WITH Eu3S4
Growth of EuS single crystals by halogen or halide
~Ht 110’2 t55o-17go~C
/ /
/
C P W-cell
transport (12, EuBr2) has not been successful up to
now’). Due to this reason transport experiments have
/; / /
n p Ta-cell
oHt liO ’2 1706-225O ~C
been started using miscellaneous transporting agents.
The ideal transporting agent from the point of view of
purity is clearly one of the component elements of the
compound. In principle, such transport reactions are
1 0 0 ’2
1750-213O~C
i~ ©
,/‘
/
/
500
,
450
2000
400
1 ,?~ ~ c
°C
1650
1810
2227
Fig. 4. Equilibrium vapour pressures PK and Langmuir vapour possible. Thus, e.g., transport of EuO takes place in Eu
pressures PL of EuS calculated from weight loss data measured
with cells of various materials. The tungsten cell data (curves B
and C) are the correct ones. Only the heavily drawn parts of the
curves correspond to measured data. The lightly drawn parts are
extrapolated to enable calculation of the evaporation coefficient.
The temperature range of each group of measurements is given in
the figure.
vapour2). Another example is the transport of sulfides
under sulfur pressure. This reaction has been observed
for the first time by the growth of small crystals of
Cd4SiS6 at 1100°C22)and wassystematically studied for
.
.
many transition metal sulfides (including MoS2 and
WS2) by Schafer et al.23). These investigators assumed
evaporation (Langmuir) vapour pressures calculated that polysulfide species in the vapour might be responsi-
from the thermogravimetric measurements. For rea- ble for this transport.
sons of comparison, not only the data from tungsten
In order to grow single crystals of EuS and to prove
cells, but also those of tantalum and molybdenum cells this assumption, it was decided to use sulfur for the
are given. From the ratio of the Knudsen to Langmuir transport. However, free sulfur cannot be used as a
measurements the gross evaporation coefficient of EuS charge component in a molybdenum crucible. The
can be calculated. Between 1650 and 2100 °C , the volatility of this element hinders the sealing of the
evaporation coefficient takes the values
crucible by electron bombardment’3). To avoid this
difficulty the polysulfides of europium Eu3S4 and
~ev
=
=
3.3—6.0 x l0_2
-
EuS2 ~, were considered as possible sources of sulfur.
K
Preliminary thermogravimetric measurements showed
It isclear, therefore, that a kinetic barrier is determining that Eu 3S4 begins to decompose under a vacuum of
the rate of evaporation of EuS. Assuming that the 1 0 ~ torr near 350 °C .After the end of decomposition
gross condensation coefficient is equal to the gross and annealing at 2000 ° C the residue has the X-ray
evaporation coefficient, it can be concluded that one of structure and vapour pressure of EuS. However, the
the reasons of the absolutely low sublimation rate of thermalstability ofthis compound increasesappreciably
EuS is the low evaporation/condensation coefficient. with pressure.Thus,during synthesis from the elements,
However, similar investigations of other RE-chalcoge- it is possible to heat it in a sealed silica ampoule up to
riides have shown that both EuTe and EuSe have2’21) 600 ° C without decomposition. At 628 °C free sulfur

HIGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLiMATION OF EuS
9
crucibles were charged with Eu3S4 sealed with electron
bombardment’3) and heated at 2050 °C .It was found
that transport of EuS takes place very rapidly from
higher to lower temperatures and that single crystals of
dimensions up to 7 x 5 x 3 mm are grown (fig. 5).
—
_ _ _ _ _
Control experiments with EuS—Eu3S4 mixtures showed
smaller transport velocities and a tendency to dendritic
crystallisation, both indications of the expected lower
supersaturation. Due to the inferior quality of the
cr~stals, these experiments were not continued. The
following unusual features were observed during the
_ _ _ _ _ _ _ _ _ _
_ _ _ _ _
_ _ _ _ _ _ _
_ _ _ _ _ _ _
transport of EuS:
_ _ _ _ _ _
(a) Small parts of the crucible walls (mainly near the
_ _ _ _ _
lid or the bottom) were corroded during a transport
experiment and allowed short erruptions of transport-
~
ing gases into the vacuum furnace. Dendritic molyb-
denum was instantaneously deposited around the
channels and pores of the crucible walls from which
the erruptions took place (fig. 5a).
(b) In the interior of the crucible a plug of dendritic
molybdenum wool was formed on the charge.
These results gave the impression that the transport
of EuS was not as simple as it could have been assumed
from reaction (2) but that molybdenum might be
actively involved in this transport. The importance of
pressure and its relief by the erruptions was also not
clear. These questions and the high growth rate
(several times higher than the usual rate of EuS
sublimation) of this chemical transport reaction
made a further investigation of the transport and
growth mechanism attractive.
Fig. 5. Crystals of EuS grown by high pressure, high temperature
chemical transport in sealed molybdenum crucibles; starting
material Eu3S4, total pressure, if S2 were the only vapour species
4.2. ERRUPTIONS FROM THE CRUCIBLE
35 atm. Temperature 2030 C, undercooling AT
=
30 ~C. The
Information about the erruptions from the crucible
was easily achieved by monitoring with a recorder the
crystals have a green metallic highly reflecting colour. The various
amounts of reflected light from the individual crystals are due to
different angles towards the inclining light,
vacuum of the furnace, in which the crucible was
heated. Fig. 6 shows the dependence of the pressure in
vapour first appears in the ampoule due to the de- the furnaceas a function of time during a typical experi-
composition of Eu3S4. ment of EuS transport. The time scale is arbitrary and
This appreciable dependence of the decomposition schematic showing only the sequence of the various
on pressure implied that at higher pressures and changes in pressure. At the beginning of each experi-
temperatures a gaseous polysulfide molecule might be ments as the furnace is rapidly heated from room
stable in the vapour phase and a transport equilibrium temperature to 2060 ° C (in approximately 55 mm) the
of the general type
3 EuS (s)+* S2 (v)
pressure increases due to degassing (period 1). When the
(2) highest temperature is reached, degassing decreases
rapidly and the pumping speed limits the decrease of
Eu3S4 (v)
might exist. To prove this hypothesis, molybdenum pressure (period 2). This period is interrupted by an

10
L. K \L D iS
increase ol’ the pressure of’ the furnace accom pan~ing
~
lhe first erruptioii of gases o u t of the crucible (period 3).
The time elapsing het~~eetnhe start of an experiment
5 0 ’
and the first erruption, the duration of tli is erruplion
and the rate of pressure inci’ease v~ith the time, are
factors depending appreciahI~on the thickness of’ the
I
crucible and the microstructure of’ the ~~alls.T ~pical
~alues for ‘ ~ arious hatches of’ crucibles are: start of’ the
erruption ca. I fi Ii after reaching the highest tempera—
510
1 iire : d LIi’atlOn of the erruption ca. I h. O b ~iousi~ not
all gases are diffusing out of’ the crucible. other~itse no
transport of EuS could take place. -l herefore. it seems
that due to the deposition of m ol~hden tim the pores iii
a
___________________________________________________________________
2
3
4
5
6
°
°
‘ ~
the ~sall close again after a ‘ ~hile and tile crucible
1-ig. 0. Changes 01 pressure as a l’unLtion ol’tinie l~chem aiicalll~in
ac u urn lu m ace containing sealed Mo—crue,Ne u here the
[uS Iranspori ssuh I u3S4 Likes place. Sudden incre,isc ol pres—
becomes ‘ ~ acuum tig h t.
1—rom th is point on. I lie speed
a
a
.
.
of. the pumps is again limiting the pressure decrease
,
sure is due io erruptions of gases from ihe crucible see ~
.
(period 4). When the cooling of the I Lirnace starls
0
—
.
‘0 ’
‘
‘ .
.
.
-
~t\
~
0
.
.
~.
-
¼
Fig. 7. Initial sttige ol’gross Ill on the lid of ihe M o crucible before the tirsi errupiion. \lagn,lft.,tion 100 (Sl.M operator It. W essikenl.

HtGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLIMATION OF EuS
11
(point 5) the vacuum improves further (period 6).
However, when the crucible reaches a temperature of
Ca. 700 ° C a second erruption takes place. The slow
improvement of the vacuum (period 7) indicates that
self sealing of the crucible is not feasible at this tem-
perature. The pressure decrease which leads to the
second erruption can be associated with the decom-
position of endothermal molecules formed at high tem-
peratures, i.e. the molecules of the transporting agent.
4.3. ROLE OF MOLYBI)ENUM
The important role which molybdenum plays in the
EuS transport is manifested by experiments performed
in tungsten crucibles. In these crucibles the reactions
2 Eu3S4 (s)
—
~
6 EuS (s)+S2 (v),
(3)
(4)
4,
W (s)+S2 (v) - ÷ WS2 (s),
-
-
take place, but no transport of EuS over the vapour
phase is possible. The pressure increase due to the
decomposition of Eu354 [eq. (3)] is relieved by the
reaction of sulfur with tungsten [eq. (4)]. The impor-
5.
Fig. S. The line crystalline deposit of molshdenum shossn in 11g. 7
at magnification IlOOx (SEM operator R. W essiken).
tance of molybdenum in the transport of EuS is also have lost pieces (fig. 7). Thus, it becomes visible that
demonstrated by the following experiment, if MoS2 they are bubbles with a crystal-like shell. These bubbles
and Eu metal are used as starting material (with a have been probably formed due to decomposition of a
ratio 3 :4 of Eu to 5) the transport of EuS takes place compound which is stable at high temperatures and
exactly as if Eu3S4 was used.
decomposes during cooling. This is also in agreement
Therefore, it is reasonable to assume that the vapour with the build up of pressure in the crucible during
species which are responsible for the transport of EuS, cooling, which finally leads to the second erruption.
are containing molybdenum. The transporting agent
A qualitative analysis of both the large and small
might be a gaseous complex of the type Eu~Mo~ST0.his crystallites of the lid with the microprobe (fig. 9)
assumption is also supported by an investigation of the shows two peaks of europium L5 at 5.82 keY and L4
first nucleation layer on the lid of the crucible, before at 6.65 keY. The ratio of the heights of the Mo and Eu
erruption, with a microprobe attached to the scanning peaks is approx. 10%. Detection of sulfur was not
electron microscope, and by a sparc source mass possible due to overlapping of the K ~peak of S and the
spectrographic analysis of the crucible walls and of the L5 peak of molybdenum at 2.3 keV. Analysis of pure
dendritic molybdenum overgrowth on the outside of the molybdenum (fig. 9a) could not help in dissolving the
crucible. Before the first erruption, the morphology of two peaks. However, S and Eu were detected in the
the lid surface (where condensation takes place) is molybdenum by spark source mass spectragraphic
quite different than after the erruption when conden- analysis to be also of the order of 10%. These results
sation of EuS begins. Fig.
7
shows a part of the lid, support the above made assumption of a transporting
Crystallites with a typical elongated orthogonal habit agent of the formula EUXMOYS~
and degenerated step structure, and also a molyb-
denum deposit (fig. 8) with much smaller nuclei, have
been found on the initially flat surface of the lid. Both
~
GROWTH RATE
Experiments with various total heating periods, all
kinds of crystallites have the colour of molybdenum other conditions kept constant, gave widely different
with a faint violet overtone. The larger crystallites values of the average transport rate. Thus, experiments,
with the orthogonal habit bear deep cracks and some with crucibles heated several hours but cooled before

12
E. KALDIS
5 Ø G S E C
6 S ~ 1 c ,S
rogh
TK TTTV~~
-
t.~ B u 5 t5 s 0 .,.K 5 7 *
Tin,e hi
0
0
10 ~
I U
Fig. 10. Mass transport rate of EuS using Eu3S4 starting material,
D ~
(a)
x
as a function of time, High pressure, high temperature chemical
transport in sealed molybdenum crucibles. AT — 2060—2030
C
=
30 C. Calculated pressure of S2 at total decomposition of
~
Eu3S4 is 35 atm. Each point is a separate experiment. All experi-
ments are normalised at zero growth time, Ihe moment of (he
first erruption from the crucible (see text).
growth times are normalised with zero time, the time of
first erruption. The maximum growth time was 12 h
(after the erruption) simply because in this period
almost all charge was transported. The charge was
for all experiments the same, 10 g. The temperatures
and undercooling for all these experiments were
AT = 2060—2030 = 30 °C .Deep drown molybdenum
crucibles (Metallwerk Plansee,Tyrol, Austria) of 16mm
diam., 100 mm length and 1 mm wall thickness were
used throughout these experiments The results of
fig. 10 show two distinct regions of the dependence of
the average growth rate on time. Two hours after the
first erruption the growth rate is still very small to be
(b)
Fig. 9. Microprobe analysis performed in the SEM by an energy accurately measured. From this time on, an almost
dispersive analyser; (a) standardisation with molybdenum (b)
parabolic increase of the average growth rate takes
analysis of the crystallite and deposit shown in fig. 7. The two
sm aller peaks belong to europium.
place and at 6 h growth time the rate reaches the unusu-
ally high value of more than 700 mg/h (‘— ‘ 10 ‘cm/sec).
or shortly after the first erruption showed no transport This should be compared to ca, 20 mg/hr which is the
at all. On the contrary in crucibles which were heated order of the growth rate of EuS by sublimation at simi-
also a few hours after the erruption, high transport lar temperatures. This maximum value decreases for
rates were observed. Therefore, it became clear that the growth times6—8 hand remains then constant up to 12 h.
average growth rate was not constant with time and
that the first erruption was very important for the
growth to take place.
In general, changes of the growth rate with time are
due to changes of the supersaturation. In the present
case this might be the result of the formation of a
A systematic investigation of the growth rate as a complex transporting agent. The kinetic law of the
function of time gave the results shown in fig. 10. Each change of supersaturation cannot be deduced from these
point of the curve is a separate experiment and shows experiments. Assuming the simplest case, i.e. a linear
the average growth rate calculated from the weight of dependence of the supersaturation on time, a similar
the transported crystals and the growth time. The curve like that of fig. 10 should be obtained for the

HIGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLIMATION OF EuS
13
change of growth rate with supersaturation and the
b091~
following model can be used to explain these results.
If kinetics of growth are rate determining then, accord-
ing to the BCF theory, a parabolic dependence of the
growth rate on supersaturation is expected. On the
contrary, if diffusion in the gas phase of the transport-
ing vapours is rate determining, then a linear depend-
25
6546
2 0
1.5
ence of the growth rate on supersaturation should be
observed. For chemical transport, both these cases have
been observed earlier’2 ) by the growth of Cd4GeS6,
ZnSe and CdS with iodine. In the present case the
~
°
io
6237
os,
0 3 0 9
experimental results favour the former possibility. For
growth periods longer than 6 h, obviously the growth
mechanism changes and the rate becomes constant.
This is probably the result of the above mentioned
deposition of molybdenum on the charge, which at a
__________________________ ___________
1000
1500
2000
2500
lInro)
Fig. II. Optical density and absorption coefficient K of tra n s-
ported EuS crystals as a function of wavelength. Thickness of
the crystal: d 0.097 mm,
=
later stage of each experiment becomes appreciable. As crystals. However, no molybdenum could be detected,
the transporting gases have to diffuse through this Molecular species of EuS were found only in concen-
molybdenum plug, diffusion becomes rate determining tration of 3 ppm and the mass 91.5 corresponding to
and the rate increases linearly with supersaturation and EuS2~(ref. 28).
therefore time. if the increase of growth rate due to
(c) Optical properties.’ The optical transmission as a
increasing supersaturation is compensated by the function of wavelength was measured for the EuS
increasing resistance to diffusion due to the growing transported crystals. Fig. 11 shows the optical density
thickness of the molybdenum deposit, a constant value as a function of wavelength. From these measurements
of the growth rate may result. This would explain the and the crystal thickness d = 0,097 mm, the absorption
horizontal part of the curve in fig. 10. The above model coefficient K (cm_1) was calculated for selected values.
is only a working hypothesis. A mass spectronietric The adsorption edge has a perfect shape corresponding
investigation might give further information about the to the band gap of EuS, AE = 1,65 eV. The low re-
chemistry of this high pressure, high temperature sidual absorption with K values between 47.5 and
chemical transport reaction.
30.9 cm ‘ for wavelengths 650 and 2400 nm respect-
ively indicates considerable perfection of the lattice. As
there are no optical data about Eu3S4 a comparison
4.5. CHARACTERISATION
To find out the degree of purity and stoichiometry of with those of EuS was not possible. To enable this corn-
the EuS crystals grown by the above discussed method parison the absorption power of a polycrystalline
following analytical methods have been used:
Eu3S4 sample was measured for the first time. Fig. 12
(a) X-ray fluorescence analysis: The crystals were shows this curve which has a rather complicated shape.
dissolved in mineral acid and the solution analysed. To The absorption edge is in the same wavelength region
increase the sensitivity for sulfur, BaSO4 was preci- as that of EuS butit shows an additional step at 400 nm.
pitated and then the solution of a barium salt was The residual absorption shows two steps. As Eu3S4 is in
measured. The europium content of the EuS crystals a closed silica ampoule stable only up to 600 °C and
was found to be 83.01 ± 0.6% (theor. 82.58%) and the cannot be recrystallised at higher temperatures, it is
sulfur content 17.94 ± 0.54% (theor. 17.42%). Within possible that the sample was not very homogeneous and
the limit of accuracy of the method this result shows contained some EuS. However, this could not be
that the crystals are stoichiometric.
confirmed from X-rays powder diagrams.
(b) Mass spectrographic analysis.’ An AEI MS-701
(d) Lattice constants: To investigate the lattice
sparc source mass spectrometer was used for this work, perfection of EuS grown both by sublimation and
to find out if molybdenum has been incorporated in the chemical transport, a lattice constant statistic of various

14
E. K A LD IS
: ~ k
500
1000
1500
2000
2500 A no,)
W~oe)nngth
Fig. 12. Optical reemission measurements of polycrystalline
Eu3S4 as a function of wavelength.
EuS
a
A
Subilmat. Transpori Deviation
•
w4hEo.
withBr
*
E u S E ~
5 .9 7 1 0
11
Ia)
018°
___
Fig. 13. Reproducibility of the lattice constant of EuS crystals
grovnn by sublimation and chemical transport,
_ _ _ _ _ _ _
samples was made in order to determine the re -
producibility. Fig. 13 shows the obtained results for
sublimated crystals from five experiments. A deviation
of 0. 18°/oowas found for sublimated and for transported
crystals of 0.410 / ~ ~ The reproducibility for readings of
the same film was 0.1 5 ° / ~o f~the lattice constant. These
results show that the reproducibility of the lattice
constants of transported crystals is lower than that of
sublimated ones. This might be the result of the varying
conditions of growth of the investigated crystals, which
were rapidly cooled from high temperatures.
.
__________________
It can be concluded from the above measurement
that the transported crystals are pure and stoichio-
metric and have good optical properties. No incor-
poration of molybdenum takes place. Slow cooling
may be necessary to improve lattice perfection.
Fig. 14. Habit of chemically transported EuS crystallites at an
early stage of growth (2.5 h after the erruption). (a) (100) faces
with and without vicinal facets; magnification 480 (b) (110)
- .
face with disturbed growth steps. Between EuS crystals the
face of the ltd can be seen: magnification 960
R. Wessiken.)
.
(SEM operator
5. Surface morphology
for the investigation of growth mechanism, a scanning
Due to the importance of the surface morphology
electronmicroscope study was performed with EuS

H IG H 1 E \IP I R \iL ’R E
(‘HI rili( Al 1 R A N S P O R I
\ND sviiLiM A rlo\ Ui EuS
IS
—
I ,
~
~.0’
I ig. IS. Small single ersstals ot’ FuS transporied l’rom Fu3S4: magnification 40
.
Regions ssith high orthogonal steps are
separated froni ~hantielo (along grain hottndaries). ‘I he ‘ t aIls of (hese groos es are eos ered ss illi loss grossili s(eps (SF NI o p er-
ator It. \\ essi ken).
cr\ stals g ro ~ ~ nboth h ~ the Eu3S4 transport and h ~ treatments concerning the importance of’ non—singuilar
sublimation tinder the conditions discussed above. An faces in \ apour phase growth and some indications
additional reason f’or this in~’estigationwas that. ~ ith about the rate limiting step. The surface morphology
the possible exception of SiC, almost no electron— of the condensation area (lid of the crucible) before the
microscopic data exist on the surface morphology of
first erruiption is shos~n in fig. 7 . As it ssas mentioned
en stals gross ii f’roni the vapour at ~en high tempera— ah o ~e. ~arious an al~ses. habil and colour of the en’s—
tures. At these temperatures. the transition from sin— tallites indicate that the latter are the products of’ de—
gular to non—singular faces might occuri 7 ) (surf’ace composition ofan other conipouind. probably the trans—
melting). This transition of’ faceted to non—faceted
grossIh is interesting because it has been often con—
sidered as an indication for diffusion controlled grossth.
Hosse~er.as it has been shoss ii b~’BLirton. Cabrera
porting agent.
At the first stage ol’ EuS deposition (2.5 Ii aller the
erruption) mainly (100) singular faces and (110) step
rich faces dominate the habit (fig. 14), The steps of the
(110) laces (fig. I 4h) are shown at a magnification of
antI 1-rank
‘
) some time ago. growth by non—singular
laces can appear. at high temperatures also ss hen 960 ‘/ to he “hunched’, bounded h~’cursed faces, and
g~’ossth k in etics are rale determining due to surface disturbed, as it couild he expected near the transition
melting. Morphological data might gise lirst experi— to a non—singular lace. The (100) laces (fig. l4a) are
mental int’ormation about the alidil~ of Iheoretical well deseloped and at a magnification 01’ 480 x show

16
F. K A LD IS
5j1’
regions is quite different. The large steps are bounded
with flat, low index signular faces and show an unusual
regular pattern (fig. 16 magnification 1200x). These
faces do not reveal any steps even at a magnification ol
22000 x or even 60000 x. Replica techniques of trans-
mission electronmicroscopy are necessary to reveal
possihls existing growth ledges with atomic dimensions.
Nevertheless, there are in these regions also disturbed
Pr
- ~
_____________________________________________ steps sshich are bounded both by singular and non-
~
singular faces with steps height of ca. 6 .tm. Direct
incorporation of the impinging atoms in these steps
~
_ _ _ _ _
should be possible. Contrary to the above, the steps
on the walls of the channels are bounded partly from
curved non-singular faces and have heights down to
7
100 A . Therefore, they seem more suitable to incorpo-
rate fastly adatoms and sustain rapid growth. For the
time being, it is not clear how the regions with the large
_ _ _ _ _
steps (fig. 16) can grow as rapidly as the experimentally
measured rate suggests. It seems more reasonable to
assume that the large steps are the result of a secondary
Fig. 16. Detail of the interlace region ‘,s ith the large orthogonal
steps shown in fig. 15. Step height 1—10 gm , m agnification 1200x
ISEM operator R. W essiken).
recrystallisation or growth process. Such processes
some vicinal faces. At longer growth times coalescence could be due to:
of the crystallites takes place but the habit remains the
same,
—
Recrystallisation during the end phase ol the
experiment, leading to equilibrium faces. Such effects
The (100) face was observed also in EuS crystals have been observed, e.g., by Bienfait and Kern24’25) in
grown by sublimation at growth rate l0 8 cm/sec and small NaC1 crystals. initially grown in dendritic form
similar temperatures (2060 °C). Even at the highest due to high supersaturation, these crystallites recrystal-
useful magnification of the microscope, approx. lised after some time to the low index equilibrium form,
60000 x
,
no steps or other features of this surface In the present case, such recrystallisation of the initial-
could be detected, On the other hand, transported iy formed fine steps could be enhanced by a decrease of
crystals grown at high growth rates show often a quite the supersaturation towards the end phase ol the
different morphology. Fig. IS shows an overall view experiment and/or the “mineraliser” action of the
of several small crystals grown together at magnifi- transporting agent.
cation 40><
.
A step-rich structure covers all the growing
—
Secondary nucleation and growth (overgrow th)
surface, consisting of regions with large orthogonal during the decomposition of the transporting agent. As
steps (step height of the order of 1—10 l.tm) and deep both Eu and S are contained in the transporting agent,
channels with a V-shaped profile. These channels its decomposition might lead to a secondary deposition
follow the grain boundaries. Their walls are covered of EuS. EuS single crystals grown by sublimation at
with long ledges having step heights between I ~tm and higher temperatures ~~-~225~0C, supersaturation P-1 /P2-.
less than 100 A . An exact identification of the orien-
= 1.5 and growth rates of the order of 106cm/sec
tation of the grains and their steps is on the way now, show under the scanning electronmicroscope, up to
For the time being, it appears that the various grains magnifications of 60000 x, a curved liquid-like growth
grow on the molybdenum substrate with symmetrical interface with no structural features of any kind.
orientations which are nearer to (Ill) than (100). Therefore, at temperatures between 2060 ~C and
Growth in these directions takes probably place 2250 °C , a transition between growth with singular
through step rich faces. At higher magnification, it is faces to growth with non-singular faces seems to take
seen that the morphology of the steps of the two
place.

HIGH TEMPERATURE CHEMICAL TRANSPORT AND SUBLIMATION OF EuS
17
6. Rate determining step
scale) when surface melting takes place and rough
(interface instability) when constitutional supercooling
In order to decide if diffusion in the vapour phase or from the vapour phase is existing. Our investigation
growth kinetics are the rate determining step during the has shown that the curved non-singular interface of
high temperature chemical transport and sublimation EuS resulting by sublimation at around 2250 ° C does
of EuS, the most important results from this point of not show any roughness up to magnifications of
view are summarised here.
Growth by chemical transport at 2060 ° C has high determining process in high temperature sublimation
rate. The average linear growth rate (ALGR) varieswith of EuS ~ = 40 torr) is not diffusion in the vapour
time between nearly zero and 10 ‘ cm/sec. The phase. This is directly supported by the results of the
60000 x. It seems reasonable to assume that the rate
chemical transport reaction is not known (transport by vaporisation study. The low evaporation coefficient
a ternary compound seems very probable) and also the (a ,~-~5.0_~l0_o2f)EuS shows that the surface reaction
supersaturation. The pressure during an experiment is rate determining. The activation enthalpy of evap-
reaches high values. The interface shows a step rich oration is higher than the enthalpy of evaporation.
structure.
The mechanism of the surface reaction is not clear.
Growth by sublimation at temperatures around However, it is very probable that as in the case of
2060 ° Ctakes place by (100) singular faces with ALGR CdS’3) and other dissociative sublimations2) a disso-
l0 8 cm/sec. The supersaturation is approx. 1.5. ciation of EuS takes place in Eu atoms and S molecules.
At temperatures near 2250 °C ,ALGR ‘ ~ - ~10 6 cm/see, This is qualitatively supported by the fact that in spark
the total pressure P = 40 torr and the supersaturation source mass spectrographic investigations28) of EuS
PT1/P72
=
1.5. Contrary to chemical transport the crystals almost no EuS molecules appear. The for-
interface is concave and smooth without steps. mation of these species on the evaporating surface or
Non-faceted growth with curved non-singular inter- their dissociation and rearrangement during conden-
facehas beenassociated several timesin the lastyears7’ 8) sation, need an activation energy which decreases
with diffusion control. In fact when diffusion in the evaporation and growth rate and makes both evap-
vapour phase is rate determining all directions in the oration and condensation coefficientslowerthan unity*.
crystal should grow at the same rate and in the extreme
case a no otherwise disturbed interface should have a EuS is its high growth rate. Direct comparison with
spherical shape. sublimation is difficult due to lack of knowlegde of the
An interesting result of the chemical transport of
However, it must be pointed out that a curved non- supersaturation. However, as it has been discussed in
singular interface might also be the result of “surface the past, in the case of CdS2”2) a useful criterion for
melting”. Burton, Cabrera and Frankt 5 ) have shown such comparisons is the value of growth rate at which
that in principle also when growth kinetics are rate single crystal growth can still take place. High values
determining, it is expected from the theory that at high indicate a rapid, low activated surface reaction. The
temperatures a transition of singular to non-singular high growth rate of the EuS transport can be inter-
faces will take place. This is due to a rapid increase of preted this way. Diffusion in the vapour phase as rate
the surface roughness in atomic scale at a certain determining in this transport, is not probable due to
transition temperatures. However, it is not clear
because it depends on bond strength and lattice dy- can give us a clue about the high growth rate. As it was
namics if for a given compound this transition tern- discussed in section 4, the large steps by bounded low
—
the step rich interface morphology. This morphology
—
perature can be lower than the melting point26). Pay- index faces are probably the results of secondary
lovska and Nenov2 7 ) havefound this transition in nega- recrystallisation or growth. In the actual first stage of
live crystals (inclusions) of diphenyl and naphthalene.
To differentiate between “surface melting” and dif-
growth the interface was probably covered with the
* In this connection it must be reminded however, that although
fusion controlled growth a scanning electronmicro- in most cases evaporation and condensation coefficients have the
same value, it is in principle possible that they have different v al-
scopic study is very useful. The microstructure of the
curved interface should be smooth (rough in atomic different.
ues when the mechanism of evaporation and condensation are

18
E. K A LD iS
low steps which can be still seen in the saddle regions. microscope work has been performed in the Labora-
The high density of these steps could give an explana- torium für Elektronenmikroskopie 11 of the ETH in
tion for the high growth rate.
collaboration with Mr. R. Wersiken. The author wishes
to thank both him and Dr. U. Nissen.
7. Conclusions
M any thanks are due to Dr. W. Beckenbaugh for
—
No evidence of constitutional supercooling in the critically reading the manuscript.
vapour phase has been found both in high temperature
This work was partly supported by the Swiss National
chemical transport and sublimation of EuS in a sealed Fonds.
Mo crucible, although the former shows the highest
References
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single crystal growth by chemical transport. Kinetic
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2) E. Kaldis, Principles of the Vapour Growth of Single
and morphological data indicate that growth kinetics
are probably the rate determining step in the chemical
crystals, in: Crystal Growth, Theory and Techniques’, Ed.
C. H. Goodman (Plenum, New York, 1972).
transport of EuS.
3) E. Kaldis, in: Proc. li/A ll-U nion Conf. of USSR, T zakhka-
zor, September 1972.
—
The vaporisation study of EuS shows that the surface
4) T. B. Reed and W. J. LaFleur, AppI. Phys. Letters, 5 (1964)
reaction is rate determining, the evaporation coefficient
being ~ = 3,3—6.Ox 1 0 2 in the region of 1700—1950
° CThese and morphological observations indicate that
growth kinetics are probably the rate determining step
191.
5) M . Toyama, Japan. J. AppI. Phys. 5(1966)1204.
6) M. M. Faktor, R. Heckingbottom and I. Garrett, J. Chem.
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J. Crystal Growth 7 (1970) 79.
in the sublimation of EuS.
8) T. B. Reed, W. J. LaFleur and A. J. Strauss, J. Crystal
—
In the sublimation of EuS a transition from singular
(100) to non-singular faces has been found at tempera-
tures near the melting point.
Growth 3,4 (1968) 115.
9) H. Schafer, J. Crystal Growth 9(1971)18: Che,nische Tra,is-
porrreokt,onen (Verlag Chemie, 1962).
10) J. H. F. Jeffes, J. Crystal Growth 3,4 (1968) 13.
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II) C. F. Goodeve, Discussions Faraday Soc. 4(1948)14.
of optimal conditions for crystal growth by sublimation.
12) E. Kaldis, J. Crystal Growth 5 (1969) 376.
—
The high pressure, high temperature chemical
13) P. Goldfinger and M. Jeunehomme, Trans. Faraday Soc.
59 (1963) 2851.
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possibilities for the crystal growth of high melting
compounds from the vapour phase. Some evidence is
5) W. K. Burton, N. Cabrera and F. C. Frank. Phil Trans. Roy.
Soc. London A243 (1950) (99.
16) J. P. l—lirth and G. NI. Pound, Condensation and Ei’aporation
(Pergamon, London, 1963).
existing that the transporting agent is a ternary corn-
pound of the general formula Eu~M~S0T.he purity of
17) L. Brewer and J. S. Kane, J. Phys. Chem. 59(1955)105.
the EuS crystals grown by this method is remarkable.
18) S. G. Parker and J. E. Pinnell, Trans. Al ME 245 ((969) 451;
J. Crystal Growth 9(1971)177.
Acknowledgements
19) E. Kaldis, J. Phys. Chem. Solids 26(1965)1701.
20) W. L. Winterbottom, in: Phiysicocheinical Mea,surenzent.s in
The author is deeply indebted to the director of the
Metals Research, Vol. IV, I, Ed. R. A . Rapp (lnterscience,
New York, 197O)p. 95.
Laboratorium für Festkorperphysik, Prof. Dr. G. 21) E. Kaldis, to be published.
Busch, for his continuous support and encouragement 22) E. Kaldis, L. Krausbauer and R. Widmer, Cd4SiS6 atid
during this work, as well for many fruitful discussions.
The success of this work has been positively influ- 23) H. SchEfer F. Wehmeier and M. Trenkel, J. Less Commoti
Cd4SiSe6 New Ternary Photoconductors and Phosphors,
RCA Laboratories Zurich, Internal Report, 1965.
enced by the skilled experimental collaboration of
Messrs. G. Sampietro, V. Hildebrandt and P. Wilhelm.
Metals 16 (1968) 290.
24) M. Bienfait and R. Kern, Bull. Soc. Franc. Mineral. Crist.
87 (1964) 604.
The author wishes to express his thanks to them. The 25) M . Bienfait,Thèse, Faculté des Sciences, Université de Nancy
author wishes to thank also Prof. P. Wachter for the
966.
optical measurements, Dr. J. Muheim for the mass 26) N. Cabrera, Discussions Faraday Soc. 28(1960) 16.
27) A. Pavlovska and D . N enow , Surface Sci. 27(1971) 211; J.
Crystal G row th 8(1971)209; J. Crystal Growth 12(1972)9.
spectragraphic analysis and Dr. B. Magyar for the
X-ray fluorenscence analysis. The scanning electron- 28) J. M uheim, private com m unication.