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Growth of HgCr2Se4 single crystals by chemical transport
Journal
of’ Crystal
Growth 6 (1910) 319-322
GROWTH
‘0 North-Holland
OF HgCr,Se,
SINGLE
Publishing
CRYSTALS
TATSUO RCA
Research
Co., Amsterdam
TRANSPORT
TAKAHASHL
Laboratories,
Received
BY CHEMICAL
Inc., Tokyo,
13 October
Japan
1969
Single crystals of undoped, Ag-doped and In-doped HgCr2Se4 have been grown by chemical transport from the HgCr2Se4CrCla system. A mixture of HgCr2Se4, either undoped or doped, and CrCla was sealed in an evacuated quartz ampoule and heated in a two-zone furnace (Thigh N 700 “C, and T,,, N 670 ‘C).
Single hedral of the these those
1. Introduction
form of In,Se, up to 2.0 mol% of the Hg atoms. All the materials used had a purity of 99.9% or better. The ampoule was evacuated to below 10m3 Torr and sealed. It was then placed in a two-zone furnace with a small temperature gradient [most typically Thigh = 700 “C and T,,,= 670 “C (AT = 30 “C)]. The two end temperatures were controlled independently by two SCR (silicon control rectifier) temperature regulators. After heating for seven to ten days, the ampoule was furnace-cooled. Single crystals up to 3 x 3 x 1 mm, most of them having a truncated octahedral shape, were found on the wall in the low temperature zone of the ampoule. Single crystals were successfully grown in both horizontal and inclined furnaces. The angle of inclination seemed to have little effect on crystal growth. Crystal growth could be reproduced quite well, but the maximum size of crystals seemed to fluctuate a little from one batch to another even under apparently the same heating conditions. A large temperature gradient (AT > 50 “C) was apparently disadvantageous for crystal growth and yielded either very small crystals or no crystals at all. A temperature gradient of 20 to 30 “C gave the most satisfactory results. Higher reaction temperatures had very harmful effects on crystal growth. For Thigh = 720 “C and T,,,= 670 “C, the deposition of HgSe at the low temperature end of the ampoule became excessive even though the crystal growth of HgCr,Se, was not completely hindered. For Thigh = 800 “C, no HgCr,Se, were found
Mercury chromium selenide, HgCr,Se,, has been found’,2) to be a ferromagnetic semiconductor with a normal spine1 structure. Single crystal growth of undoped-HgCr,Se, has been recently reported by Lehmann and Emmenegger3). They have obtained octahedral crystals up to I .5 mm in edge length using HgCl, or CrO,Cl, as a transport agent. They report that transport with Br, or CrCl, yielded smaller crystals and that Cl, is an even less suitable transport agent. In this report, single crystal growth of undoped and doped HgCr,Se, using CrCl, as a transport agent will be described. Large single crystals of a truncated octahedral shape up to 3 mm in edge length have been grown by this technique. The room temperature Seebeck coefficient for these single crystals will also be reported. 2. Single crystal growth of HgCr,Se, One mmol (-0.6 g) of polycrystalline powder of doped- or undoped-HgCr,Se,, and a small amount of CrCl, [from 0.05 mmol (-0.008 g) to 0.7 mmol (-0.105 g)] were placed in a quartz ampoule of lOO120 mm length and 12 mm inner diameter. The HgCr,Se, powders were synthesized from HgSe, Cr and Se plus dopant, following the procedure described by Baltzer et a1.l). For the doped-HgCr,Se, powders, Ag was added in the form of metallic powder up to 1.55 mol% of the Hg atoms and In was added in the 319
crystals of up to 3 mm on an edge, with a truncated octashape, were found on the wall of the low temperature end ampoule. The room temperature Seebeck coefficients of single crystals have been measured and compared with obtained for pressure-sintered samples.
TATSUO
320
TAKAHASHI
to grow regardless of T,,,. It, therefore. seems that HgCr,Se, becomes thermally unstable at a temperature slightly above 700 “C under the present growth condition. On the other hand, the lower limit of 7;ligh for crystal growth seems to be around 650 + 10 “C since, for Thigh = 620 “C and T,,, = 590 “C no crystals were found to grow.
The amount of CrCI, added to the system (from 0.05 mmol to 0.7 mmol for each I mmol of HgCr2Se,) seemed to have some effect on growth rate of crystals. That is, crystals grown in the ampoules containing 0.5 mmol of CrCI, were usually larger in size than those grown in the ampoules containing less CrCI,. However, the yield of single crystal (i.e. total weight of single crystal/weight of HgCr,Se, powder) was not high in either case and was at best about 2O”i,. No further analysis of growth rate was attemped because of the wide variation in the size of the crystals grown in a single ampoule. Dopants, i.e. Ag or In, did not seem to have any significant effect on the crystal growth. No attempts have been made to formulate the exact chemical reactions at this time. However, the major reactions seem to be: CrCI, HgCr,Se,
Fig. 1. transport
Undoped in the
HgCrzSe4 single HgCrzSe4-CrC13 T ,ow ” 670
Fig.
2.
crystals grown by chemical system (Thjph N 700 “C; C).
Group
of growth
hillocks
on
* CrCl, + tClz.
+ 2CI, F? 2CrCI, + HgSe + 3Se.
(1) (2)
The presence of chlorine gas was confirmed by its characteristic odor when the ampoule was broken after heating. At the same time, residual reactants at the high temperature end acquired a dark green color. This coloration indicates the presence of either and that minor reactions CrCl, . xH,O or Cr,O,, other than just (I ) and (2) also take place.
(111) face of undoped
HgCr2Se4
crystal.
0~
GROWTH
Fig.
3.
HgCr,Se,
Growth
Several undoped HgCr,Se, crystals present technique are shown in fig. 1.
SINGLE
spirals
grown
BY CHEMICAL
CRYSTALS
on a (111) face of undoped
by the
3. Some properties of single crystal HgCr,Se, In figs. 2 and 3, some microscopic features on “asgrown” crystal faces of undoped HgCr$e, single crystals are shown. Typical growth hillocks, which can be identified by their conspicuous ridges, are clearly seen in fig. 2. Growth spirals of fairly large size could be observed on some crystal faces, as shown in fig. 3. In either case, these defects reflect on the symmetry of the (1 I 1) face, i.e. the three-fold rotational symmetry. Two etching solutions, namely, a solution of HNO, and HCI (HNO, : HCl: H,O = 2: 6: 3 by volume) and that of KOH, glycerol, H,O and H,O, (KOH 10 g, H,O 10 ml, glycerol I ml and 30”/, H,02 0.5 ml), were found to produce a clean pitless surface without any staining. However, attempts to develop etch pits have not been successful. X-ray Laue photographs of the single crystal faces and X-ray powder patterns of the pulverized crystals were taken using a Siemens X-ray unit. The Laue photograph of a slightly inclined (111) face is shown in fig. 4. Lack of any streaks or asterism of individual Laue spots indicates that the crystal grew in a stress free state. The room temperature lattice parameters
HgCr,Se,
321
TRANSPORT
crystal.
determined from X-ray powder patterns are in agreement with those of polycrystalline powder within the limit of the experimental error (a,, = 10.743 + 0.002 A). The room temperature Seebeck coefficients (M) were also measured by using the hot-point technique with
Fig.
4.
Laue
photograph undoped
of a slightly inclined HgCrzSo4 crystal.
(111)
face
of
322
TATSUO
0. I mm chromel-alumel thermocouples. The thermal voltage across the sample was measured between the two chrome1 leads. The Seebeck coefficient data for both undoped and doped single crystals are presented in table I and compared with those for the pressure sintered HgCr,Se, samples4). These data are the average of measurements on at least five different samples or crystals. TABLE
Seebeck
I
coefficient
single
G(at 298 -’ 2 K
(/(VI C) _
Sample Undoped Ag l.O:,, Ag 1.55”/,, In I.Oy,, In 2.0”’/ 0
Single crystal 33 229 181 24
23
sentative cxvalue (+33 pV/“C) given in the table is for 0.5 mmol CrCI,. The small a for the undoped single crystals could be due to the presence of both holes and electrons (mixed conduction), which have opposing contributions to X. More detailed information on the stoichiometry 01 the single crystals is still required to provide a detinite explanation of this behavior. Further discussion of the thermoelectric data as well as electrical
coefficient data for undoped and doped HgCrzSel crystals and pressure-sintered samples Seebeck
TAKAHASHI
.._ Pressure-sintered 312 204 164 24 22
For the doped crystals, z’s are in fair agreement with those for the pressure sintered samples. This seems to indicate that fractionation of dopants during crystal growth was not excessive, if indeed there had been any. The Ag-doped crystals had large positive values for r which is reasonable because the replacement of Ag+ ions for Hg ‘+ ions should lead to the creation of extrinsic holes. The small positive r for the In-doped crystals had not been expected and is different from the case of CdCr,Se,, in which In-doping leads to a large negative E (- 600 pV/“C) as would be expected by the replacement of In3+ ions for Cd2+ ions5). A recent study’) on the solid solubility of In in HgCr,Se, has indicated that In3+ ions have the tendency to go into the octahedral sites, i.e. to replace the Cr3+ ions rather than the Hg ‘+ ions. It appears therefore that the In-doping of HgCr,Se, can not be simply explained by the direct replacement of In3’ ions for Hg”’ ions. The undoped crystals had a much smaller a than undoped pressure-sintered samples. Although t( for the doped crystals did not change with the amount of CrCl, present in the system, r* for the undoped single crystals changed anomalously (from -7.5 to + 50 /.LV/“C) with the amount of CrCI, present in the system. The repre-
reported
and magnetic
studies of these materials
are
separately4).
4. Conclusions Undoped and doped single crystals of HgCrzSe4 sufficiently large for electrical, magnetic and optical studies, have been grown from the HgCrzSe,&rCI, system. Single crystals of up to 3 mm on an edge, with a truncated octahedra1 shape, were found on the wall of the low temperature end of a sealed ampoule after heating for 7 to IO days. The room temperature Seebeck coefficients for the doped crystals were comparable with those for the respective doped pressure-sintered samples, while a for the undoped crystals was anomolously small as compared with that for undoped pressure-sintered samples. Acknowledgements The author wishes to thank Drs. F. Okamoto and Y. Wada for their useful suggestions and stimulating discussions during the course of this work. He also wishes to thank Mr. T. Oka and Mr. S. Harada for their assistance in the experiments. He is also grateful to Drs. P. K. Baltzer and H. Pinch for much helpful advice during the preparation of this manuscript. References P. K. Baltzer, H. W. Lehmann and M. Robbins, Phys. Rev. Letters 15 (1965) 493. P. K. Baltzer, P. J. Wojtowicr, M. Robbins and E. Lopatin. Phys. Rev. 151 (1966) 367. H. W. Lchmann and F. P. Emmenegger, Solid State Commun. 7 ( 1969) 965. K. Minematsu, K. Miyatani andT. Takahashi, to be published. H. W. Lehmann, J. Appl. Phys. 163 (1967) 488. T.Takahashi, K. Minematsu and K. Miyatani, to bepublished.
of’ Crystal
Growth 6 (1910) 319-322
GROWTH
‘0 North-Holland
OF HgCr,Se,
SINGLE
Publishing
CRYSTALS
TATSUO RCA
Research
Co., Amsterdam
TRANSPORT
TAKAHASHL
Laboratories,
Received
BY CHEMICAL
Inc., Tokyo,
13 October
Japan
1969
Single crystals of undoped, Ag-doped and In-doped HgCr2Se4 have been grown by chemical transport from the HgCr2Se4CrCla system. A mixture of HgCr2Se4, either undoped or doped, and CrCla was sealed in an evacuated quartz ampoule and heated in a two-zone furnace (Thigh N 700 “C, and T,,, N 670 ‘C).
Single hedral of the these those
1. Introduction
form of In,Se, up to 2.0 mol% of the Hg atoms. All the materials used had a purity of 99.9% or better. The ampoule was evacuated to below 10m3 Torr and sealed. It was then placed in a two-zone furnace with a small temperature gradient [most typically Thigh = 700 “C and T,,,= 670 “C (AT = 30 “C)]. The two end temperatures were controlled independently by two SCR (silicon control rectifier) temperature regulators. After heating for seven to ten days, the ampoule was furnace-cooled. Single crystals up to 3 x 3 x 1 mm, most of them having a truncated octahedral shape, were found on the wall in the low temperature zone of the ampoule. Single crystals were successfully grown in both horizontal and inclined furnaces. The angle of inclination seemed to have little effect on crystal growth. Crystal growth could be reproduced quite well, but the maximum size of crystals seemed to fluctuate a little from one batch to another even under apparently the same heating conditions. A large temperature gradient (AT > 50 “C) was apparently disadvantageous for crystal growth and yielded either very small crystals or no crystals at all. A temperature gradient of 20 to 30 “C gave the most satisfactory results. Higher reaction temperatures had very harmful effects on crystal growth. For Thigh = 720 “C and T,,,= 670 “C, the deposition of HgSe at the low temperature end of the ampoule became excessive even though the crystal growth of HgCr,Se, was not completely hindered. For Thigh = 800 “C, no HgCr,Se, were found
Mercury chromium selenide, HgCr,Se,, has been found’,2) to be a ferromagnetic semiconductor with a normal spine1 structure. Single crystal growth of undoped-HgCr,Se, has been recently reported by Lehmann and Emmenegger3). They have obtained octahedral crystals up to I .5 mm in edge length using HgCl, or CrO,Cl, as a transport agent. They report that transport with Br, or CrCl, yielded smaller crystals and that Cl, is an even less suitable transport agent. In this report, single crystal growth of undoped and doped HgCr,Se, using CrCl, as a transport agent will be described. Large single crystals of a truncated octahedral shape up to 3 mm in edge length have been grown by this technique. The room temperature Seebeck coefficient for these single crystals will also be reported. 2. Single crystal growth of HgCr,Se, One mmol (-0.6 g) of polycrystalline powder of doped- or undoped-HgCr,Se,, and a small amount of CrCl, [from 0.05 mmol (-0.008 g) to 0.7 mmol (-0.105 g)] were placed in a quartz ampoule of lOO120 mm length and 12 mm inner diameter. The HgCr,Se, powders were synthesized from HgSe, Cr and Se plus dopant, following the procedure described by Baltzer et a1.l). For the doped-HgCr,Se, powders, Ag was added in the form of metallic powder up to 1.55 mol% of the Hg atoms and In was added in the 319
crystals of up to 3 mm on an edge, with a truncated octashape, were found on the wall of the low temperature end ampoule. The room temperature Seebeck coefficients of single crystals have been measured and compared with obtained for pressure-sintered samples.
TATSUO
320
TAKAHASHI
to grow regardless of T,,,. It, therefore. seems that HgCr,Se, becomes thermally unstable at a temperature slightly above 700 “C under the present growth condition. On the other hand, the lower limit of 7;ligh for crystal growth seems to be around 650 + 10 “C since, for Thigh = 620 “C and T,,, = 590 “C no crystals were found to grow.
The amount of CrCI, added to the system (from 0.05 mmol to 0.7 mmol for each I mmol of HgCr2Se,) seemed to have some effect on growth rate of crystals. That is, crystals grown in the ampoules containing 0.5 mmol of CrCI, were usually larger in size than those grown in the ampoules containing less CrCI,. However, the yield of single crystal (i.e. total weight of single crystal/weight of HgCr,Se, powder) was not high in either case and was at best about 2O”i,. No further analysis of growth rate was attemped because of the wide variation in the size of the crystals grown in a single ampoule. Dopants, i.e. Ag or In, did not seem to have any significant effect on the crystal growth. No attempts have been made to formulate the exact chemical reactions at this time. However, the major reactions seem to be: CrCI, HgCr,Se,
Fig. 1. transport
Undoped in the
HgCrzSe4 single HgCrzSe4-CrC13 T ,ow ” 670
Fig.
2.
crystals grown by chemical system (Thjph N 700 “C; C).
Group
of growth
hillocks
on
* CrCl, + tClz.
+ 2CI, F? 2CrCI, + HgSe + 3Se.
(1) (2)
The presence of chlorine gas was confirmed by its characteristic odor when the ampoule was broken after heating. At the same time, residual reactants at the high temperature end acquired a dark green color. This coloration indicates the presence of either and that minor reactions CrCl, . xH,O or Cr,O,, other than just (I ) and (2) also take place.
(111) face of undoped
HgCr2Se4
crystal.
0~
GROWTH
Fig.
3.
HgCr,Se,
Growth
Several undoped HgCr,Se, crystals present technique are shown in fig. 1.
SINGLE
spirals
grown
BY CHEMICAL
CRYSTALS
on a (111) face of undoped
by the
3. Some properties of single crystal HgCr,Se, In figs. 2 and 3, some microscopic features on “asgrown” crystal faces of undoped HgCr$e, single crystals are shown. Typical growth hillocks, which can be identified by their conspicuous ridges, are clearly seen in fig. 2. Growth spirals of fairly large size could be observed on some crystal faces, as shown in fig. 3. In either case, these defects reflect on the symmetry of the (1 I 1) face, i.e. the three-fold rotational symmetry. Two etching solutions, namely, a solution of HNO, and HCI (HNO, : HCl: H,O = 2: 6: 3 by volume) and that of KOH, glycerol, H,O and H,O, (KOH 10 g, H,O 10 ml, glycerol I ml and 30”/, H,02 0.5 ml), were found to produce a clean pitless surface without any staining. However, attempts to develop etch pits have not been successful. X-ray Laue photographs of the single crystal faces and X-ray powder patterns of the pulverized crystals were taken using a Siemens X-ray unit. The Laue photograph of a slightly inclined (111) face is shown in fig. 4. Lack of any streaks or asterism of individual Laue spots indicates that the crystal grew in a stress free state. The room temperature lattice parameters
HgCr,Se,
321
TRANSPORT
crystal.
determined from X-ray powder patterns are in agreement with those of polycrystalline powder within the limit of the experimental error (a,, = 10.743 + 0.002 A). The room temperature Seebeck coefficients (M) were also measured by using the hot-point technique with
Fig.
4.
Laue
photograph undoped
of a slightly inclined HgCrzSo4 crystal.
(111)
face
of
322
TATSUO
0. I mm chromel-alumel thermocouples. The thermal voltage across the sample was measured between the two chrome1 leads. The Seebeck coefficient data for both undoped and doped single crystals are presented in table I and compared with those for the pressure sintered HgCr,Se, samples4). These data are the average of measurements on at least five different samples or crystals. TABLE
Seebeck
I
coefficient
single
G(at 298 -’ 2 K
(/(VI C) _
Sample Undoped Ag l.O:,, Ag 1.55”/,, In I.Oy,, In 2.0”’/ 0
Single crystal 33 229 181 24
23
sentative cxvalue (+33 pV/“C) given in the table is for 0.5 mmol CrCI,. The small a for the undoped single crystals could be due to the presence of both holes and electrons (mixed conduction), which have opposing contributions to X. More detailed information on the stoichiometry 01 the single crystals is still required to provide a detinite explanation of this behavior. Further discussion of the thermoelectric data as well as electrical
coefficient data for undoped and doped HgCrzSel crystals and pressure-sintered samples Seebeck
TAKAHASHI
.._ Pressure-sintered 312 204 164 24 22
For the doped crystals, z’s are in fair agreement with those for the pressure sintered samples. This seems to indicate that fractionation of dopants during crystal growth was not excessive, if indeed there had been any. The Ag-doped crystals had large positive values for r which is reasonable because the replacement of Ag+ ions for Hg ‘+ ions should lead to the creation of extrinsic holes. The small positive r for the In-doped crystals had not been expected and is different from the case of CdCr,Se,, in which In-doping leads to a large negative E (- 600 pV/“C) as would be expected by the replacement of In3+ ions for Cd2+ ions5). A recent study’) on the solid solubility of In in HgCr,Se, has indicated that In3+ ions have the tendency to go into the octahedral sites, i.e. to replace the Cr3+ ions rather than the Hg ‘+ ions. It appears therefore that the In-doping of HgCr,Se, can not be simply explained by the direct replacement of In3’ ions for Hg”’ ions. The undoped crystals had a much smaller a than undoped pressure-sintered samples. Although t( for the doped crystals did not change with the amount of CrCl, present in the system, r* for the undoped single crystals changed anomalously (from -7.5 to + 50 /.LV/“C) with the amount of CrCI, present in the system. The repre-
reported
and magnetic
studies of these materials
are
separately4).
4. Conclusions Undoped and doped single crystals of HgCrzSe4 sufficiently large for electrical, magnetic and optical studies, have been grown from the HgCrzSe,&rCI, system. Single crystals of up to 3 mm on an edge, with a truncated octahedra1 shape, were found on the wall of the low temperature end of a sealed ampoule after heating for 7 to IO days. The room temperature Seebeck coefficients for the doped crystals were comparable with those for the respective doped pressure-sintered samples, while a for the undoped crystals was anomolously small as compared with that for undoped pressure-sintered samples. Acknowledgements The author wishes to thank Drs. F. Okamoto and Y. Wada for their useful suggestions and stimulating discussions during the course of this work. He also wishes to thank Mr. T. Oka and Mr. S. Harada for their assistance in the experiments. He is also grateful to Drs. P. K. Baltzer and H. Pinch for much helpful advice during the preparation of this manuscript. References P. K. Baltzer, H. W. Lehmann and M. Robbins, Phys. Rev. Letters 15 (1965) 493. P. K. Baltzer, P. J. Wojtowicr, M. Robbins and E. Lopatin. Phys. Rev. 151 (1966) 367. H. W. Lchmann and F. P. Emmenegger, Solid State Commun. 7 ( 1969) 965. K. Minematsu, K. Miyatani andT. Takahashi, to be published. H. W. Lehmann, J. Appl. Phys. 163 (1967) 488. T.Takahashi, K. Minematsu and K. Miyatani, to bepublished.