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Vapor transport and crystal growth in the mixed system MnS-MnSe
Journal of Crystal Growth 6 (1969) 67—7 1 © North-Holland Publishing Co., Amsterdam
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM MuS~MnSe*
HERIBERT WIEDEMEIER and A. GARY SIGAI Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12181, U.S.A.
Received 4 March 1969; revised manuscript received 20 May 1969
Mass transport rate studies in the multi-component multi-reaction system MnS—MnSe—iodine were performed on atomic mixtures of high purity elements in the temperature gradient 945 850°C and with iodine pressures ranging from 0.7 to 7.8 atm (based on diatomic iodine). Experimental data evaluated in terms of material flux in moles/cm2 sec as a function of iodine pressure show that the mass transfer process is predominantly by convection within the applied pressure range. Growth studies of single crystals of MnS, MnSe, and solid solution by vapor
transport of the elements with iodine yielded crystals of 2—5 mm edge length (octahedra and platelets) using iodine concentrations 3 tube volume and small temperature gradients of 1—3 mg/cm (zJT < 50°C). The as-grown surfaces of the platelets are (111) planes. Thermoelectric measurements indicate p-type conductivity of the single crystals. The effects of different iodine concentrations, temperature gradients, and etching of the transport ampules on crystal growth are discussed.
1. Introduction
and MnSe form a continuous series of solid solutions. This work deals with the investigation of transport properties and crystal growth of the alloy system MnS— MnSe. The transport with iodine represents a multicomponent multi-reaction system, and it is desirable to determine whether the mass transport rate and mode are comparable with those of less complex systems. The
The term “vapor transport” or “chemical transport reaction” as defined by Schafer’) applies to reactions in which a solid source material reacts with a gaseous transport agent at a temperature T, to produce exclusively gaseous products, while at another location in the system at temperature T 2 the reverse reaction occurs, causing the deposition of the solid. Depending upon the experimental conditions the transported material can be obtained in the form of well-developed single crystals. The concentration gradient required to achieve a transport from the source region to the crystallization region of the system is established by means of temperature differences. In a closed system the transport direction is dependentupon the thermodynamic properties of the transport reaction. It is a decisive advantage of this method that the source material can be introduced in form of high purity elements, and that the required temperatures are well below the melting of theproducts. compounds. Both aspects are in favor ofpoints high purity In previous phase studies2) it was shown that MnS Based on part of a thesis submitted by A. Gary Sigai to the Graduate School of Rensselaer Polytechnic Institute as partial fulfillment of the requirements for the degree of Master of Science. *
67
effect of the concentration of iodine on the gross cornposition of the transport product and on the growth habit of the crystals is studied. The influence of other experimental parameters such as temperature gradient and treatment of transport ampules on crystal morphology are considered. 2. Experimental procedures 2. 1. PREPARATION OF TRANSPORT TUBES The rate studies were carried out in closed quartz tubes of 15 mm inner diameter and about 11 cm in length. The volume of the tubes was actually 3 measured after the and did not change more than ±0.5 cm tubes were sealed off. The cleaning of the quartz ampules followed a standard procedure in which a HF! HNO, solution containing 1—2% HF was used for about 5 mm and subsequent rinsing with distilled and finally with demineralized water. The tubes were then outgassed under a pressure of 10
6
Torr or less at a
68
HERIBERT WIEDEMEIER AND A. GARY SIGAI
temperature of about 1000 °Cfor 10—12 hr. These conditions were the same for all ampules used in the rate studies. Different HF concentrations and etching times were applied in connection with observations of the crystal morphology. 2.2.
FURNACE TUBE 85O’C<~
~
NUCLEATION REGION
~95O
945’C
SOURCE REGION
~9o4
VAPOR TRANSPORT EXPERIMENTS
~ 85O~-
Stoichiometric amounts of high purity manganese 0 0 0 (99.99 /0), sulfur (99.999 /~, and selenium (99.999 /0) in atomic ratios of 1:0.5:0.5 were introduced into the pretreated tubes. After evacuating, a known amount of iodine was sublimed and condensed into the tube which was then sealed off at a pressure of 10-6 Torr or less. The loaded tube was placed in the temperature gradient of a two-zone tubular resistance furnace so that the source material was at the higher temperature. After an appropriate transport time the tubes were removed from the furnace in such a way that the gas phase content condensed at the location of the residual source material, For the crystal growth experiments, the position of the transport tube in the furnace was selected so that the source material was initially in the low temperature range in order to cause reverse transport or cleaning of the final deposition region of the ampule from condensation sites. After about 12 hr under this condition the ampule was put in the desired temperature gradient with the source material in the hot zone of the furnace. For both the transport rate and crystal growth experiments the furnace and tube were in a horizontal position. A typical temperature profile and gradient of the furnace assembly is shown in fig. 1. 2.3. ANALYSIS OF THE TRANSPORT PRODUCT The transport product is a solid solution whose composition was deduced from the lattice parameter
~80O
~o
25
35
40
45
DISTANCE INTO THE FURNACE (CMI
Fig. 1. Aapplied typicalin temperature profile gradient ofthetemperafurnace assembly the transport rate and studies. Different ture gradients were used in the crysal growth experiments. Nudeation occurred at the cooler end of the tube usually over a 1.5 cm region. The total length of the horizontal furnace was 60 cm.
of the solid solution based on the linear relationship between lattice constant and composition as established previously2). For this purpose Debye—Scherrer powder photographs were taken in a 114.59 mm diameter camera using Ni-filtered CuKc~-radiationand evaluated as described earlier2). Based on this method the uncertainty in the composition of the transport product is less than ±2.5 %. 3. Results and discussion 3. 1. MASS TRANSPORT RATE STUDIES During the rate studies no emphasis was placed on crystal growth. The deposited crystals showed various habits, octahedral and platelet, and twinning was observed. The experimental results of the rate of transport studies are listed in table 1. The transport rate was determined from the weight of the crystals recovered and the total elapsed time. This assumes that the transport rate is constant during the experiment. The transport times in this series of experiments were long enough to average out initial transient variations in the transport
TABLE 1
Experimental parameters in the determination of the transport rate as a function of iodine concentration in the system Mn—S—Se; atomic ratio of elements in the source material: 1 Mn: 0.5 S : 0.5 Se Run
Weight of
Weight of
No.
source material (g) 2.2101 2.2091 2.2105 2.2091 2.2090 2.2087
product (g) 0.4379 0.6863 0.6769 1.0664 1.2986 0.7113
C 1,
1 2 3 4 5 6
3) (mg/cm 1.8 3.0 4.0 6.5 12.2 20.7
Tube volume (cm’) 18.6 19.4 20.7 18.0 20.1 20.1
Temperature gradient (°C) 913 945 945 942 945 945
850 852 -~ 850 —~ 850 -÷ 852 -÷ 850 —~—~-
Transport time (hr)
Transport (mg/hr) rate
27.75 25.75 24.00 24.75 15.75 5.00
15.78 26.65 28.20 43.09 82.45 142.26
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM
MnS—MnSe
69
TABLE 2 Results of transport rate studies in terms of flux and iodine pressure in the system Mn—S—Se; lattice constant and composition of the transported mixed single crystals (experimental conditions as in table 1) Run
C
1,
No. 3) -
2 3 4 5 6
(mg/cm 1.8 3.0 4.0 6.5 12.2 20.7
Transport rate (mg/hr) 15.78 26.65 28.20 43.09 82.45 142.26
Iodine P~ (atm)
Pressure F1, (atm) 0.7 1.1 1.5 2.5 4.6 7.8
1.4 2.2 3.0 4.9 9.3 15.6
rate. The result of the last run shows that any initial variations were negligibly small. The analytical results in table 2 show that there is substantially no change in the gross composition of the deposited crystals with respect to different iodine concentrations in the gas phase and that thecomposition of the mixed crystals is in reasonable agreement with the stoichiometric ratio of the elements in the starting mixture. This shows that the gas phase composition remains constant during the transport experiment, This confirms previous results that alloys of any composition can be obtained by this method2). In table 2 are also listed the iodine pressure in atm and the material flux in moles/sec. cm2. The iodine pressure was computed from the iodine concentration for the mean temperature of the tube, assuming ideal behavior and the exclusive presence of either monatomic or diatomic species. The linear dependence with positive slope of the transport rate (mg/hr) on the iodine pressure (atm) indicates that convection exceeds diffusion as the major mass transfer process1). This is supported by the observation that the crystals frequently grew in a flow pattern on the upper wall of the transport tube. For a more quantitative interpretation our experimental results are compared with the work of Jona and Mandel3). These authors3) investigated the rate of solid transport in the system ZnS—iodine and compared experimental rates with rates computed on the basis of diffusion theory. The similarity of the MnS—MnSe system to the ZnS system suggests a direct comparison. For this purpose the mass flux (moles/ sec’ cm2) was plotted as a function of the iodine pressure (atm) from table 2. Fig. 2 shows that the observed transport rates in the MnS—MnSe system are con-
(cm__‘sec ~ moles /
Lattice constant a0 (A)
Mole % MnSe final product by X-ray analysis
2.25 3.80 4.02 6.14 11.74 20.26
5.352 5.343 5.342 5.338 5.342 5.343
52.2 49.0 48.7 47.5 48.7 49.0
Flux x 108 2
siderably larger than if the process were only by diffusion. This supports the previous conclusions that mass transport is predominantly by convection within the applied pressure range. Considering that the thermodynamic properties and the diffusion coefficients in the system MnS—MnSe—iodine are different from those of the ZnS—iodine system, our results are in good agreement with Jona and Mandel’s observations. Our data in combination with Jona and Mandel’s work indicate that the range of predominant convection is _________________________
/ / (2)/
.~
~
I / /,/,.“
.
/./
,‘
/ ld’8~
,/
,/
~‘
/
~
/ ,,j’
-. ~-~‘
~
I
IODINE PRESSURE
I
(ATM)
. 2. The rate of transport in the system MnS—MnSe—iodine Fig. (curves 4 and 5, this work) compared to calculated and experimental transport rates for the ZnS—iodine system (curves 1—3, Jona and Mandel3)). Curve 1: theoretical transport rate calculated from diffusion theory for the system ZnS—iodine. Curve 2: experimental rates, 22 mm i.d. tubes. Curve 3: experimental rates, 8 mm id. tubes. Curves 1—3 are based on diatomic iodine in gas phase. 4: experimental rates, 15 mm i.d.the tubes, basedCurve on diatomic iodine.MnS—MnSe Curve 5: experimental MnS—MnSe rates, 15 mm i.d. tubes, based on monatomic iodine.
70
HERIBERT WIEDEMEIER AND A. GARY SIGAI
extended to lower total pressures with increasing diameter of the reaction tube. The difference in slope of the curves for the ZnS and Mn—chalcogenide systems in fig. 2 is most likely caused by differences in the thermodynamic and diffusion properties of the systems. The concept developed by Jona and Mandel provides an excellent means to elucidate the predominant mass transport mode for related systems. 3.2,
TRANSPORT MECHANISM
Instead of using polycrystalline MnS and MnSe or the preannealed solid solution, mixtures of high purity elements Mn, S, and Se were employed as starting materials for the rate studies. These conditions were chosen deliberately to investigate the applicability of simpler models to a more complex system. The cornplexity of this system is even more increased through the fact that the transport product is a solid solution and hence distinctly different from the starting mixture, Thermodynamic calculations concerning the transport mechanism are not reasonable under these conditions without further knowledge of the thermodynamic properties of the product solid solution. Based on previous studies2) the formation of the compounds MnS and MnSe from the elements under annealing conditions is fast compared to the formation of a solid solution. This leads to the conclusion that shortly after initiating the transport the starting mixture consists mainly of MnS and MnSe. Under these conditions the transport of the mixed system from higher to lower temperature can be described by the following reactions: High temperature region: Source material MnS (s) + MnSe (s) + 2 12 (g) 2 Mn12 (g)-i-~S2 (g)--.~Se2 (g). Low temperature region: Final product Mn1 2 (g)+1 S2 (g)+~Se2 (g) = MnS05Se05 (s)+12 (g).
(1)
3.3.
CRYSTAL MORPHOLOGY
In order to obtain large sized single crystals it is important to keep the number of growth sites at a minimum. Besides the surface quality of the quartz tube the number of critical nuclei is generally dependent upon the concentration of the transport agent (in this case iodine) and on the temperature gradient. Large and well-developed crystals of MnS, MnSe, and solid solution were grown with iodine concentrations of 1—3 mg/cm3 tube volume and with the higher and lower temperatures in the range from 900 to 800 °C.These crystals were usually octahedrons and platelets of several mm edge length. At concentrations of iodine greater than about 5 mg/cm3, the crystals are still well defined, but they are smaller and extensive nucleation occurs. This is caused by the iodine delivering too much feed-material for the already nucleated crystals, thus exceeding the critical saturation limit. Under such conditions twinning and growth of crystals in form of extended aggregates was also observed. This results from a combination of too many growth sites and supersaturation of the gas phase. It was frequently observed here that large temperature gradients (AT> 100 °C)yielded microcrystalline material. This can be explained by the fact that the transport rate is too large. Best results in terms of crystal size and surface quality were obtained with small temperature gradients (AT < 50 °C). It is worthwhile noting the effect of etching of the transport tube during the cleaning procedure on the crystal growth of MnS and MnSe, For this purpose the tubes were cleaned and outgassed as described above, but a HF/HNO 3 solution containing 1 % HF was applied for periods ranging from one3)toand thirty minutes. the temperaThe iodine concentration (3 mg/cm ture gradient (900 850 °C) were kept constant -+
during this series of experiments. After etching the tube
(2)
The X-ray diffraction patterns of the solid solution show a high quality in terms of sharpness and resolution of the back reflection lines. With respect to the rate of formation and good crystallinity of the product the vapor phase transport serves as a strong promoter of the “solid state reaction” between MnS and MnSe.
for one minute with the 1% HF solution the transport yielded platelets. A five minute treatment resulted in platelets and some octahedrons. The number of octahedrons increased with increasing etching times, and after thirty minutes etching polynucleation occuied with only octahedrons visible. Similar results were obtamed with a 2% HF solution. Higher HF concentrations in the cleaning solution caused polynucleation in the subsequent transport to occur even after short
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM
71
•
~w
a Fig. 3.
MnS—MnSe
b
c
Single crystals of MnS (a), MnS
05 Se0.5 (b), and MnSe (c). Diagonal length of the platelets 4—8 mm, edge length of the octahedra 2—5 mm.
applications of the etchinga solution. Polynucleation can be explained through large number of growth sites caused by extensive etching of the quartz tube. The preferential growth of octahedrons with increasing etching is not yet understood. Representative samples of single crystalline MnS, MnSe, and MnS 05Se05 are shown in fig. 3. Laue photographs of MnS and MnSe platelets indicate that the as-grown surfaces are (111) planes. Under these conditions the fast growing direction is in the (ill) plane, and the slow growing direction is in the <111> direction. Qualitative thermoelectric measurements on MnS, MnSe, and MnS—MnSe single crystals grown from the vapor by this technique indicate that all are of p-type conductivity. This is consistent with p-type conductivity found for MnO which is explained in 4). terms of cation vacancies Semi-quantitative emission spectrographic analysis of several of the above MnS and MnSe crystals showed the following impurities in ppm by weight: Si (6—60), Cs (30—300), Mg (3—30), Fe (3—30), Al (15—150), Ca (10—100), and Cu (6—60). The total mean impurity content of spectroscopically detectable elements is about 4 x 10—2 wt %. The incorporation of iodine into chalcogenide and other systems has been studied by
5) by radioactive tracer method, Schafer and Odenbach Under present transport conditions for the crystal growth experiments (temperature limits within the range 900—800 °C)and with iodine concentrations between 1—3 mg/cm3 tube volume the iodine content of the MnS crystals is estimated to be in the order of l0-~ wt% and that of the MnSe crystals in the order of 10_2 wt°/.
Acknowledgements The authors. are pleased to acknowledge the partial support of this work by the National Science Founda. tion and by the National Aeronautics and Space . Administration. References 1) H. Schafer, Chemical Transport Reactions (Academic Press, 2) H. Wiedemeier and A.’G. Sigai, High Temp. Sci. 1 (1969) 18. 3) F. Jona and G. Mandel, J. Phys. Chem. Solids 25 (1964) 187. 4) A. Duquesnoy and F. Marion, Compt. Rend. (Paris) 256 5) H. Schafer and H. Odenbach, Z. Anorg. AlIg. Chem. 346 (1966) 127.
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM MuS~MnSe*
HERIBERT WIEDEMEIER and A. GARY SIGAI Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12181, U.S.A.
Received 4 March 1969; revised manuscript received 20 May 1969
Mass transport rate studies in the multi-component multi-reaction system MnS—MnSe—iodine were performed on atomic mixtures of high purity elements in the temperature gradient 945 850°C and with iodine pressures ranging from 0.7 to 7.8 atm (based on diatomic iodine). Experimental data evaluated in terms of material flux in moles/cm2 sec as a function of iodine pressure show that the mass transfer process is predominantly by convection within the applied pressure range. Growth studies of single crystals of MnS, MnSe, and solid solution by vapor
transport of the elements with iodine yielded crystals of 2—5 mm edge length (octahedra and platelets) using iodine concentrations 3 tube volume and small temperature gradients of 1—3 mg/cm (zJT < 50°C). The as-grown surfaces of the platelets are (111) planes. Thermoelectric measurements indicate p-type conductivity of the single crystals. The effects of different iodine concentrations, temperature gradients, and etching of the transport ampules on crystal growth are discussed.
1. Introduction
and MnSe form a continuous series of solid solutions. This work deals with the investigation of transport properties and crystal growth of the alloy system MnS— MnSe. The transport with iodine represents a multicomponent multi-reaction system, and it is desirable to determine whether the mass transport rate and mode are comparable with those of less complex systems. The
The term “vapor transport” or “chemical transport reaction” as defined by Schafer’) applies to reactions in which a solid source material reacts with a gaseous transport agent at a temperature T, to produce exclusively gaseous products, while at another location in the system at temperature T 2 the reverse reaction occurs, causing the deposition of the solid. Depending upon the experimental conditions the transported material can be obtained in the form of well-developed single crystals. The concentration gradient required to achieve a transport from the source region to the crystallization region of the system is established by means of temperature differences. In a closed system the transport direction is dependentupon the thermodynamic properties of the transport reaction. It is a decisive advantage of this method that the source material can be introduced in form of high purity elements, and that the required temperatures are well below the melting of theproducts. compounds. Both aspects are in favor ofpoints high purity In previous phase studies2) it was shown that MnS Based on part of a thesis submitted by A. Gary Sigai to the Graduate School of Rensselaer Polytechnic Institute as partial fulfillment of the requirements for the degree of Master of Science. *
67
effect of the concentration of iodine on the gross cornposition of the transport product and on the growth habit of the crystals is studied. The influence of other experimental parameters such as temperature gradient and treatment of transport ampules on crystal morphology are considered. 2. Experimental procedures 2. 1. PREPARATION OF TRANSPORT TUBES The rate studies were carried out in closed quartz tubes of 15 mm inner diameter and about 11 cm in length. The volume of the tubes was actually 3 measured after the and did not change more than ±0.5 cm tubes were sealed off. The cleaning of the quartz ampules followed a standard procedure in which a HF! HNO, solution containing 1—2% HF was used for about 5 mm and subsequent rinsing with distilled and finally with demineralized water. The tubes were then outgassed under a pressure of 10
6
Torr or less at a
68
HERIBERT WIEDEMEIER AND A. GARY SIGAI
temperature of about 1000 °Cfor 10—12 hr. These conditions were the same for all ampules used in the rate studies. Different HF concentrations and etching times were applied in connection with observations of the crystal morphology. 2.2.
FURNACE TUBE 85O’C<~
~
NUCLEATION REGION
~95O
945’C
SOURCE REGION
~9o4
VAPOR TRANSPORT EXPERIMENTS
~ 85O~-
Stoichiometric amounts of high purity manganese 0 0 0 (99.99 /0), sulfur (99.999 /~, and selenium (99.999 /0) in atomic ratios of 1:0.5:0.5 were introduced into the pretreated tubes. After evacuating, a known amount of iodine was sublimed and condensed into the tube which was then sealed off at a pressure of 10-6 Torr or less. The loaded tube was placed in the temperature gradient of a two-zone tubular resistance furnace so that the source material was at the higher temperature. After an appropriate transport time the tubes were removed from the furnace in such a way that the gas phase content condensed at the location of the residual source material, For the crystal growth experiments, the position of the transport tube in the furnace was selected so that the source material was initially in the low temperature range in order to cause reverse transport or cleaning of the final deposition region of the ampule from condensation sites. After about 12 hr under this condition the ampule was put in the desired temperature gradient with the source material in the hot zone of the furnace. For both the transport rate and crystal growth experiments the furnace and tube were in a horizontal position. A typical temperature profile and gradient of the furnace assembly is shown in fig. 1. 2.3. ANALYSIS OF THE TRANSPORT PRODUCT The transport product is a solid solution whose composition was deduced from the lattice parameter
~80O
~o
25
35
40
45
DISTANCE INTO THE FURNACE (CMI
Fig. 1. Aapplied typicalin temperature profile gradient ofthetemperafurnace assembly the transport rate and studies. Different ture gradients were used in the crysal growth experiments. Nudeation occurred at the cooler end of the tube usually over a 1.5 cm region. The total length of the horizontal furnace was 60 cm.
of the solid solution based on the linear relationship between lattice constant and composition as established previously2). For this purpose Debye—Scherrer powder photographs were taken in a 114.59 mm diameter camera using Ni-filtered CuKc~-radiationand evaluated as described earlier2). Based on this method the uncertainty in the composition of the transport product is less than ±2.5 %. 3. Results and discussion 3. 1. MASS TRANSPORT RATE STUDIES During the rate studies no emphasis was placed on crystal growth. The deposited crystals showed various habits, octahedral and platelet, and twinning was observed. The experimental results of the rate of transport studies are listed in table 1. The transport rate was determined from the weight of the crystals recovered and the total elapsed time. This assumes that the transport rate is constant during the experiment. The transport times in this series of experiments were long enough to average out initial transient variations in the transport
TABLE 1
Experimental parameters in the determination of the transport rate as a function of iodine concentration in the system Mn—S—Se; atomic ratio of elements in the source material: 1 Mn: 0.5 S : 0.5 Se Run
Weight of
Weight of
No.
source material (g) 2.2101 2.2091 2.2105 2.2091 2.2090 2.2087
product (g) 0.4379 0.6863 0.6769 1.0664 1.2986 0.7113
C 1,
1 2 3 4 5 6
3) (mg/cm 1.8 3.0 4.0 6.5 12.2 20.7
Tube volume (cm’) 18.6 19.4 20.7 18.0 20.1 20.1
Temperature gradient (°C) 913 945 945 942 945 945
850 852 -~ 850 —~ 850 -÷ 852 -÷ 850 —~—~-
Transport time (hr)
Transport (mg/hr) rate
27.75 25.75 24.00 24.75 15.75 5.00
15.78 26.65 28.20 43.09 82.45 142.26
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM
MnS—MnSe
69
TABLE 2 Results of transport rate studies in terms of flux and iodine pressure in the system Mn—S—Se; lattice constant and composition of the transported mixed single crystals (experimental conditions as in table 1) Run
C
1,
No. 3) -
2 3 4 5 6
(mg/cm 1.8 3.0 4.0 6.5 12.2 20.7
Transport rate (mg/hr) 15.78 26.65 28.20 43.09 82.45 142.26
Iodine P~ (atm)
Pressure F1, (atm) 0.7 1.1 1.5 2.5 4.6 7.8
1.4 2.2 3.0 4.9 9.3 15.6
rate. The result of the last run shows that any initial variations were negligibly small. The analytical results in table 2 show that there is substantially no change in the gross composition of the deposited crystals with respect to different iodine concentrations in the gas phase and that thecomposition of the mixed crystals is in reasonable agreement with the stoichiometric ratio of the elements in the starting mixture. This shows that the gas phase composition remains constant during the transport experiment, This confirms previous results that alloys of any composition can be obtained by this method2). In table 2 are also listed the iodine pressure in atm and the material flux in moles/sec. cm2. The iodine pressure was computed from the iodine concentration for the mean temperature of the tube, assuming ideal behavior and the exclusive presence of either monatomic or diatomic species. The linear dependence with positive slope of the transport rate (mg/hr) on the iodine pressure (atm) indicates that convection exceeds diffusion as the major mass transfer process1). This is supported by the observation that the crystals frequently grew in a flow pattern on the upper wall of the transport tube. For a more quantitative interpretation our experimental results are compared with the work of Jona and Mandel3). These authors3) investigated the rate of solid transport in the system ZnS—iodine and compared experimental rates with rates computed on the basis of diffusion theory. The similarity of the MnS—MnSe system to the ZnS system suggests a direct comparison. For this purpose the mass flux (moles/ sec’ cm2) was plotted as a function of the iodine pressure (atm) from table 2. Fig. 2 shows that the observed transport rates in the MnS—MnSe system are con-
(cm__‘sec ~ moles /
Lattice constant a0 (A)
Mole % MnSe final product by X-ray analysis
2.25 3.80 4.02 6.14 11.74 20.26
5.352 5.343 5.342 5.338 5.342 5.343
52.2 49.0 48.7 47.5 48.7 49.0
Flux x 108 2
siderably larger than if the process were only by diffusion. This supports the previous conclusions that mass transport is predominantly by convection within the applied pressure range. Considering that the thermodynamic properties and the diffusion coefficients in the system MnS—MnSe—iodine are different from those of the ZnS—iodine system, our results are in good agreement with Jona and Mandel’s observations. Our data in combination with Jona and Mandel’s work indicate that the range of predominant convection is _________________________
/ / (2)/
.~
~
I / /,/,.“
.
/./
,‘
/ ld’8~
,/
,/
~‘
/
~
/ ,,j’
-. ~-~‘
~
I
IODINE PRESSURE
I
(ATM)
. 2. The rate of transport in the system MnS—MnSe—iodine Fig. (curves 4 and 5, this work) compared to calculated and experimental transport rates for the ZnS—iodine system (curves 1—3, Jona and Mandel3)). Curve 1: theoretical transport rate calculated from diffusion theory for the system ZnS—iodine. Curve 2: experimental rates, 22 mm i.d. tubes. Curve 3: experimental rates, 8 mm id. tubes. Curves 1—3 are based on diatomic iodine in gas phase. 4: experimental rates, 15 mm i.d.the tubes, basedCurve on diatomic iodine.MnS—MnSe Curve 5: experimental MnS—MnSe rates, 15 mm i.d. tubes, based on monatomic iodine.
70
HERIBERT WIEDEMEIER AND A. GARY SIGAI
extended to lower total pressures with increasing diameter of the reaction tube. The difference in slope of the curves for the ZnS and Mn—chalcogenide systems in fig. 2 is most likely caused by differences in the thermodynamic and diffusion properties of the systems. The concept developed by Jona and Mandel provides an excellent means to elucidate the predominant mass transport mode for related systems. 3.2,
TRANSPORT MECHANISM
Instead of using polycrystalline MnS and MnSe or the preannealed solid solution, mixtures of high purity elements Mn, S, and Se were employed as starting materials for the rate studies. These conditions were chosen deliberately to investigate the applicability of simpler models to a more complex system. The cornplexity of this system is even more increased through the fact that the transport product is a solid solution and hence distinctly different from the starting mixture, Thermodynamic calculations concerning the transport mechanism are not reasonable under these conditions without further knowledge of the thermodynamic properties of the product solid solution. Based on previous studies2) the formation of the compounds MnS and MnSe from the elements under annealing conditions is fast compared to the formation of a solid solution. This leads to the conclusion that shortly after initiating the transport the starting mixture consists mainly of MnS and MnSe. Under these conditions the transport of the mixed system from higher to lower temperature can be described by the following reactions: High temperature region: Source material MnS (s) + MnSe (s) + 2 12 (g) 2 Mn12 (g)-i-~S2 (g)--.~Se2 (g). Low temperature region: Final product Mn1 2 (g)+1 S2 (g)+~Se2 (g) = MnS05Se05 (s)+12 (g).
(1)
3.3.
CRYSTAL MORPHOLOGY
In order to obtain large sized single crystals it is important to keep the number of growth sites at a minimum. Besides the surface quality of the quartz tube the number of critical nuclei is generally dependent upon the concentration of the transport agent (in this case iodine) and on the temperature gradient. Large and well-developed crystals of MnS, MnSe, and solid solution were grown with iodine concentrations of 1—3 mg/cm3 tube volume and with the higher and lower temperatures in the range from 900 to 800 °C.These crystals were usually octahedrons and platelets of several mm edge length. At concentrations of iodine greater than about 5 mg/cm3, the crystals are still well defined, but they are smaller and extensive nucleation occurs. This is caused by the iodine delivering too much feed-material for the already nucleated crystals, thus exceeding the critical saturation limit. Under such conditions twinning and growth of crystals in form of extended aggregates was also observed. This results from a combination of too many growth sites and supersaturation of the gas phase. It was frequently observed here that large temperature gradients (AT> 100 °C)yielded microcrystalline material. This can be explained by the fact that the transport rate is too large. Best results in terms of crystal size and surface quality were obtained with small temperature gradients (AT < 50 °C). It is worthwhile noting the effect of etching of the transport tube during the cleaning procedure on the crystal growth of MnS and MnSe, For this purpose the tubes were cleaned and outgassed as described above, but a HF/HNO 3 solution containing 1 % HF was applied for periods ranging from one3)toand thirty minutes. the temperaThe iodine concentration (3 mg/cm ture gradient (900 850 °C) were kept constant -+
during this series of experiments. After etching the tube
(2)
The X-ray diffraction patterns of the solid solution show a high quality in terms of sharpness and resolution of the back reflection lines. With respect to the rate of formation and good crystallinity of the product the vapor phase transport serves as a strong promoter of the “solid state reaction” between MnS and MnSe.
for one minute with the 1% HF solution the transport yielded platelets. A five minute treatment resulted in platelets and some octahedrons. The number of octahedrons increased with increasing etching times, and after thirty minutes etching polynucleation occuied with only octahedrons visible. Similar results were obtamed with a 2% HF solution. Higher HF concentrations in the cleaning solution caused polynucleation in the subsequent transport to occur even after short
VAPOR TRANSPORT AND CRYSTAL GROWTH IN THE MIXED SYSTEM
71
•
~w
a Fig. 3.
MnS—MnSe
b
c
Single crystals of MnS (a), MnS
05 Se0.5 (b), and MnSe (c). Diagonal length of the platelets 4—8 mm, edge length of the octahedra 2—5 mm.
applications of the etchinga solution. Polynucleation can be explained through large number of growth sites caused by extensive etching of the quartz tube. The preferential growth of octahedrons with increasing etching is not yet understood. Representative samples of single crystalline MnS, MnSe, and MnS 05Se05 are shown in fig. 3. Laue photographs of MnS and MnSe platelets indicate that the as-grown surfaces are (111) planes. Under these conditions the fast growing direction is in the (ill) plane, and the slow growing direction is in the <111> direction. Qualitative thermoelectric measurements on MnS, MnSe, and MnS—MnSe single crystals grown from the vapor by this technique indicate that all are of p-type conductivity. This is consistent with p-type conductivity found for MnO which is explained in 4). terms of cation vacancies Semi-quantitative emission spectrographic analysis of several of the above MnS and MnSe crystals showed the following impurities in ppm by weight: Si (6—60), Cs (30—300), Mg (3—30), Fe (3—30), Al (15—150), Ca (10—100), and Cu (6—60). The total mean impurity content of spectroscopically detectable elements is about 4 x 10—2 wt %. The incorporation of iodine into chalcogenide and other systems has been studied by
5) by radioactive tracer method, Schafer and Odenbach Under present transport conditions for the crystal growth experiments (temperature limits within the range 900—800 °C)and with iodine concentrations between 1—3 mg/cm3 tube volume the iodine content of the MnS crystals is estimated to be in the order of l0-~ wt% and that of the MnSe crystals in the order of 10_2 wt°/.
Acknowledgements The authors. are pleased to acknowledge the partial support of this work by the National Science Founda. tion and by the National Aeronautics and Space . Administration. References 1) H. Schafer, Chemical Transport Reactions (Academic Press, 2) H. Wiedemeier and A.’G. Sigai, High Temp. Sci. 1 (1969) 18. 3) F. Jona and G. Mandel, J. Phys. Chem. Solids 25 (1964) 187. 4) A. Duquesnoy and F. Marion, Compt. Rend. (Paris) 256 5) H. Schafer and H. Odenbach, Z. Anorg. AlIg. Chem. 346 (1966) 127.