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Growth of high purity LaB6 single crystals by multi-float zone passage
Journal of Crystal Growth 30 (1975) 193--197 © North-Holland Publishing Company
GROWTH OF HIGH PURITY LaB6 SINGLE CRYSTALS BY MULTI-FLOAT ZONE PASSAGE Takaho TANAKA, Eisuke BANNAI and Shichio KAWAI National Institute for Researches in Inorganic Materials, Kurakake, Sakura-mura, Niiharigun, Iharaki 300-31, Japan
and Tsuneko YAMANE The Tokyo Metropolitan Industrial Technology Centre, Nishigaoka, Kita-ku, Tokyo, Japan
Received 28 April 1975; revised manuscript received 28 May 1975
High purity LaB6 single crystals have been grown by multi-float zone passage. Crystal growth was done under a pressurized atmosphere to prevent vaporization and dissociation of LaB6. No seed crystals for the selection of a preferred growth axis were used. The existence of an easy axis of growth was not found. The impurity concentrations were analysed by enhission spectrography and residual resistance ratio measurement. The results show that the purity of the crystals after three passes were improved by about one order of magnitude compared with the crystals after only one zone pass. The grown crystals consist of sub-grains whose crystal axes were slightly misorientated with respect to each other. Electrical resistivity measurements for the purest crystal reveal the characteristic feature of conduction property of LaB6, i.e., polar phonon scattering of electrons plays an important role in the scattering mechanism of LaB6 in addition to impurity scattering and acoustic phonon scattering.
1. Introduction
2. Experimental
The application of LaB6 as an electron emission source material requires the preparation of carefully characterized pure crystals. The relation between the life time of the emission tip of LaB6 and its purity has not yet been proved, but it seems that a higher purity crystal should have longer life times. Therefore, the preparation of high purity material is of prime importance. Since the crystal growth of LaB6 is difficult due to its high melting point (about 2400 °C), the crystals so far reported by various authors [L—5] were relatively low in purity. In this article, the growth single crystals by multipassagesofofhigh floatpurity zones LaB6 has been reported.
2.1. Preparation of polycrystalline rods
The preparation of polycrystalline rods for the floating zone technique and the multi-float zone pass process will be described next, followed by the results concering the purity of the crystals. Finally, the deetronic properties of LaB 6 crystals will be briefly reported.
Powder material of LaB6 was purchased from Cerac Inc. Its nominal purity was 99.9% up and the powder size was under 325 mesh. Since this powder size is too coarse to be compacted into pellets, the powder was ground by a high speed ball-mill of stainless steel with stainless steel balls to a mean powder size of 4 pm. Contamination from the stainless steel was leached out by boiling in 1/1 HC1 solution. After decantation the powder was dried and pressed pelletssection of 3. The density into and cross of lox 10 X 200 mm the polycrystalline rod needs to be as uniform as possible. The powder compacting procedure used in the present work is relatively complicated: at first, a rubber sheet was inserted between a die and a bottom plate and the powder was pressed at 200 kg/cm2. In this way, the upper the compact at part 2, butpart the of pressure appliedwas to pressed the lower 200 kg/cm
194
T Tanaka eta!. /Growth of high purity LaB
6 single crs’stals
300 kg/cm2
2
2.2. Growth flf single crystals
200 kg/cm
~
There is no crucible available to contain molten LaB
Die mpact Rubber sheet
plate Bottom
No, 1
No. 2
Fig. 1. Powder compacting process: (1) usual die press, where a rubber sheet is inserted between a die and a bottom plate, (2) press of floating die system. By tile removal of the rubber sheet, the die floats from the bottom plate.
nleter of 12 nim. The polycrystalline rod was inserted in the rf coil and the rodAs was driven down with speed ofofabout 20 mm/hr. a consequence of thea passage the zone
Viewing port Gas inlet I
~i~_~~nge Quartz
Graphite
tube
susceptor ._.—
Viewing port
REcoil
Graphite wool
.~ ,...‘
6 because of the high melting point and high reactivity. So the crystals of LaB6 were prepared by the floating zone technique under a pressurized argon gas 2) which prevented vaporization and dissociation of LaB atmosphere (15 kg/cm 6. The crystals were grown by using a high pressure type furnace of ADL Inc. The polycrystalline rods were heated by direct induction heating with a frequency of 200 kHz. The rf coil used was four turns two steps coil which had an inner dia-
Green of LaB5 BN spacer
BN bottom plate
i r
a single crystal boule was obtair1ed; if, however, one tried to repeat the zone passage, difficulties did occur. The reason is as follows: since the rod density was about 73% of the theoretical density, the rod largely shrank on melting, and both the molten zone and the cross section of the crystal boule obtained became small as compared with the cross section of the polycrystalline rod. The molten zone in such a state could float stably in the rf coil and the process of growth from the polycrystalline rod to the single crystal boule was easily done. After the first pass, apparently there was no shrinkage on the melt as was the case during the first pass and tile molten zone remained too large. This caused an instability of the molten zone and in the worst case, the zone dropped off. This difficulty
Gas outlet Drive shaft
Fig. 2. Schematic diagram of sintering furnace. BN holder
was decreased by the friction between the powder and the die. Then the rubber sheet was removed and athe 2 under friccompact condition was pressed again 300 kg/cm tion-free (fig. 1). at Finally, the compact was pressed again by a hydrostatic press (1 ton/cm2) in order to obtain uniform density. This procedure prevents the pellets to bend during sintering. The pellets of LaB 6 were sintered using a graphite susceptor heated by induction heating under an argon gas atmosphere for 0.5 hr at 2000 °C.The obtained
View direction Polycrystalline Upper crystal rod
.::
Drive Molten zone RE coil ~
I 0
00 00
direction
/Mirror
Lower crystal
‘ “
Polycrystalline rod BN holder
~polycrystalline rods had about 73% of the theoretical density. A schematic diagram of the sintering furnace is shown in fig. 2.
Drive shaft Fig. 3. Process of multi float zone passage.
T Tanaka et al.
/ Growth of high purity LaB6 single crystals
which occurred after the first pass could be avoided by setting the drive speed of the upper boule to be less than the growing speed of the lower crystal, so that both the molten zone and the cross section of the lower crystal became small as compared with the cross section of the upper boule. In this way repeated zone passes can be made possible. However, if the diameter of the upper crystal is changed, this also can cause the failure of the multiple zone passages. Therefore, the polycrystalline rod for the first pass needs to be uniform and straight so that the diameter of the crystal obtained at the first pass is almost constant. When these conditions are completely satisfied, the multi-float zone passages can be achieved continuously. The crystal growing system is shown schematically in fig. 3.
chemical analysis. Furthermore, in order to check if the ratios fluctuate from the stoichiometry along the growth axis, the concentration of lanthanum ion has been measured by electron probe microanalysis. The X-ray intensity from the lanthanum ion was about 8.5 X 102 c/sec and the total counts of 20 sec fluctuate only less than 3% from one point to another along the growth axis. Thus any appreciable deviation from the stoichiometry has not been observed. Impurities have been qualitatively analysed by emission spectrography which revealed the presence of Co, Si, Mn, Cr, Fe, Mo, Ti, Zr and Ni from the raw material, but only Si, Mg and Fe were detected in the single float zone passed crystals. For the triply passed crystals, no impurities were detected by emission spectrography. Carbon impurities were detected to be 0.6 wt% from the raw material but could not be detected in any crystals where the detection limit of carbon was 0.04 wt%. The residual resistance ratio R/R0, which is defined approximately by the ratio of room temperature resistance R to liquid helium temperature resistance R0, was also used to characterize the crystals. The R/R0 ratio of singly passed crystals was about 20 and is comparable with those reported previously [2, 41. On the other hand, the R/R0 ratio for the triply passed crystals had a maximum value of 450 and a minimum of 200. Thus the triple passage of the molten zone increases the purity more than one order of magnitude
3. Results Two kinds of LaB6 crystals have been synthesized. One is a crystal passed once by the molten zone and the other is a triply passed one. One of these crystals is shown in fig. 4 where the size of the crystal is about 7 mm diam. X 60 mm. LaB6 is well known to have a non-stoichiometry region [61.Those crystals were chemically analysed and the results show that the ratio of La/B is the stoichiometric ratio of 1/6 within the accuracy of the
~
195
iT
—
—
___
-----t’J
II IItt~~~ll ~
15
16
17
118
19
210
21
Fig. 4. Single crystal of LaB6 whose dimension is 7 mm diaiii. X 60 mm.
t96
T. Tanaka et at
/ Growth of/ugh purity LaB6 single
crystals
0.5
Hg. 5. Ilerg -Barrell pholograpliy of I all~hs 12111 dilfr.iction.
~i 0.2 U
as compared with the single passage crystals. From the results of these analyses, it is shown that the purity of the crystals obtained now was markedly improved by the multi-float zone passage. After the triple passage of the molten zone, the boules of LaB6 became single crystalline which occasionally contained several crystallites of different orientations. Seed crystals for the selection of a preferred growth axis were not used. The existence of an easy axis of crystal growth was not found. The further observation by Berg—Barrett photographs (fig. 5) has shown that this LaB6 crystal con- 3. sists of sub-grains whose dimensions are about 1 mm The sub-grains observed in fig. 5 are misorientated by about several seconds to each other. The diffractions from the sob-grains of the large misorientation are not recorded on fig. 5. The maximum misorientation of the sob-grains measured by back-Laoe photographs was about a few degrees, and was observed mainly at the outer side of the crystal bonles. The sub-grains were not eliminated by slowing down the speed of crystal growth from 20 mm/hr to 10 mm/hr.
4. Electronic properties of LaB6 Hall coefficients of some crystals have been measored. The values of the Hall coefficients are to be independent of the crystal purity and are —4.5 X 10—12 V cm/A G. This value corresponds to the assumption that one carrier exists per unit cell and this means that one lanthanum ion offers one electron as a carrier. The present result agrees with the prediction by Longuet-Higgins et al. [71. The crystal purity of LaB6 was markedly improved by the multi-float zone passes and the effect of im-
D1 0.D5 0.02 0,01
5
10
20
50
100
300
Temperature I It I Fig. 6. Electrical resistivity of LaB6, open circles show experimental data and solid line is calculated curve.
purities on the electrical resistivities of those crystals whichthe gives the residual resistance is much suppressed. Thus intrinsic conduction property of LaB 6 can be revealed at a wider temperature range. En fig. 6 the electrical resistivity versus temperature curve is shown for the crystal whose residual resistance ratio is 450. Electrical resistivities of metals are usually expressed by the summation of two terms [81.One is a constant term ascribed to impurity scattering and the other is the Bloch—Gruneisen term which is the expression of the scattering of electrons by acoustic phonons, respectively. The electrical resistivity of LaB6 shown in fig. 6 can not be fitted by the summatioii of these two terms, and therefore a further scattering mechanism should be considered. A proposed mechanism is the polar phonon mode scattering of electrons, because LaB6 has some properties of ionic crystals [71and it suggests the existence of polar phonon scattering which does not exist in the usual metals. Thus tIle solid line in fig. 6 is calculated as the resistivity to the summation of the impurity scattering term, the acoustic phonon scattering term and the polar phonon scattering term. It seems that the calculated curve agrees quite well with the experinlental data. The characteristic temperatures of the acoustic phonon mode and the polar phonon mode which give the best fit in this cal-
T Tanaka etat
/ Growth of IliAd? purity LaB6 single crystals
eulation are 245 K and 920K, respectively [9]. The value of 245 K for the acoustic phonon mode is in good agreement with the Debye temperatures reported [2,10]. On the other hand, there are no data for the characteristic temperature for the polar mode of LaB6. It should be noted that the measurement of this value is left as an unsolved problem.
197
Acknowledgements The authors wish to express their gratitude to Drs. 0. Fukunaga, Y. Ishizawa and K. Uchida for their valuable advice. They also thank Mrs. M. Kobayashi for the chemical analysis, Mr. A. Ono for the EPMA measurement and Mr. S. Momma for the Berg—Barrett photography. References
5. Conclusions [11 T. Niemyski and F. Kierzek-Pecohd, J. Crystal Growth High purity LaB6 single crystals have been obtained by the multi-float zone passes. The process was achieved by controlling the drive speed of the upper crystal to be slower than that of the growing crystal. The triply passed crystals had a higher purity than the singly passed ones of at least one order of magnitude, and the ratio of La/B was held in the stoichiometrie ratio in every crystal. The crystal boules became single crystalline and consisted of sub-grains. These sub-grains were inisorientated by small angles to each other. The electrical resistivity measurement for the highest purity crystal revealed the characteristic feature of the conduction property of LaB6, i.e., in the scattering mechanisms of LaB6 the polar phonon mode scattering plays an important role in addition to the impurity scattering and the acoustic phonon scattering.
3/4 (1968) 162.
121 J. Etourneau, J. Mereurio, R. Naslain and P. HagenmUller, J. Solid State Chem. 2 (1970) 332.
131 Yu.B. Paderno, ES. Garf, T. Niemyski and I. Pracka, Poroshkovaya Met. No. 10 (82) (1969) 55. 141 Yu.B. Paderno, VI. Novikov and ES. Garf, Poroshkovaya Met. No. 11(83) (1969) 70.
151 B.J. Curtis and H. Graffenberger, Mater. Rcs. Bull. 1 (1966) 27.
161 G.V. Sanhsonov, L.Ya. Markovskii, A.F. Zhigach and 171 [81 191 1101
MG. Valyashko, Boron, Its Compounds and Alloys (Acad. Sci. Ukrainian S.S.R., Kiev, 1960). H.C. Longuet-Iliggins and M. de V. Roberts, Proc. Roy. Sue. (London) 224 (1954) 336. A.F1. Wilson, Theory of Metals, 2nd ed. (Cambridge Univ. Press, 1965). T. Tanaka, Y. Ishizawa, T. Akahane, S. Kawai and E. Bannai, detail process of the calculation to be published. J. Mercurio, J. Etourneau, R. Naslain and J. Bornnerot, Compt. Rend. (Paris) B 268 (1969) 1766.
GROWTH OF HIGH PURITY LaB6 SINGLE CRYSTALS BY MULTI-FLOAT ZONE PASSAGE Takaho TANAKA, Eisuke BANNAI and Shichio KAWAI National Institute for Researches in Inorganic Materials, Kurakake, Sakura-mura, Niiharigun, Iharaki 300-31, Japan
and Tsuneko YAMANE The Tokyo Metropolitan Industrial Technology Centre, Nishigaoka, Kita-ku, Tokyo, Japan
Received 28 April 1975; revised manuscript received 28 May 1975
High purity LaB6 single crystals have been grown by multi-float zone passage. Crystal growth was done under a pressurized atmosphere to prevent vaporization and dissociation of LaB6. No seed crystals for the selection of a preferred growth axis were used. The existence of an easy axis of growth was not found. The impurity concentrations were analysed by enhission spectrography and residual resistance ratio measurement. The results show that the purity of the crystals after three passes were improved by about one order of magnitude compared with the crystals after only one zone pass. The grown crystals consist of sub-grains whose crystal axes were slightly misorientated with respect to each other. Electrical resistivity measurements for the purest crystal reveal the characteristic feature of conduction property of LaB6, i.e., polar phonon scattering of electrons plays an important role in the scattering mechanism of LaB6 in addition to impurity scattering and acoustic phonon scattering.
1. Introduction
2. Experimental
The application of LaB6 as an electron emission source material requires the preparation of carefully characterized pure crystals. The relation between the life time of the emission tip of LaB6 and its purity has not yet been proved, but it seems that a higher purity crystal should have longer life times. Therefore, the preparation of high purity material is of prime importance. Since the crystal growth of LaB6 is difficult due to its high melting point (about 2400 °C), the crystals so far reported by various authors [L—5] were relatively low in purity. In this article, the growth single crystals by multipassagesofofhigh floatpurity zones LaB6 has been reported.
2.1. Preparation of polycrystalline rods
The preparation of polycrystalline rods for the floating zone technique and the multi-float zone pass process will be described next, followed by the results concering the purity of the crystals. Finally, the deetronic properties of LaB 6 crystals will be briefly reported.
Powder material of LaB6 was purchased from Cerac Inc. Its nominal purity was 99.9% up and the powder size was under 325 mesh. Since this powder size is too coarse to be compacted into pellets, the powder was ground by a high speed ball-mill of stainless steel with stainless steel balls to a mean powder size of 4 pm. Contamination from the stainless steel was leached out by boiling in 1/1 HC1 solution. After decantation the powder was dried and pressed pelletssection of 3. The density into and cross of lox 10 X 200 mm the polycrystalline rod needs to be as uniform as possible. The powder compacting procedure used in the present work is relatively complicated: at first, a rubber sheet was inserted between a die and a bottom plate and the powder was pressed at 200 kg/cm2. In this way, the upper the compact at part 2, butpart the of pressure appliedwas to pressed the lower 200 kg/cm
194
T Tanaka eta!. /Growth of high purity LaB
6 single crs’stals
300 kg/cm2
2
2.2. Growth flf single crystals
200 kg/cm
~
There is no crucible available to contain molten LaB
Die mpact Rubber sheet
plate Bottom
No, 1
No. 2
Fig. 1. Powder compacting process: (1) usual die press, where a rubber sheet is inserted between a die and a bottom plate, (2) press of floating die system. By tile removal of the rubber sheet, the die floats from the bottom plate.
nleter of 12 nim. The polycrystalline rod was inserted in the rf coil and the rodAs was driven down with speed ofofabout 20 mm/hr. a consequence of thea passage the zone
Viewing port Gas inlet I
~i~_~~nge Quartz
Graphite
tube
susceptor ._.—
Viewing port
REcoil
Graphite wool
.~ ,...‘
6 because of the high melting point and high reactivity. So the crystals of LaB6 were prepared by the floating zone technique under a pressurized argon gas 2) which prevented vaporization and dissociation of LaB atmosphere (15 kg/cm 6. The crystals were grown by using a high pressure type furnace of ADL Inc. The polycrystalline rods were heated by direct induction heating with a frequency of 200 kHz. The rf coil used was four turns two steps coil which had an inner dia-
Green of LaB5 BN spacer
BN bottom plate
i r
a single crystal boule was obtair1ed; if, however, one tried to repeat the zone passage, difficulties did occur. The reason is as follows: since the rod density was about 73% of the theoretical density, the rod largely shrank on melting, and both the molten zone and the cross section of the crystal boule obtained became small as compared with the cross section of the polycrystalline rod. The molten zone in such a state could float stably in the rf coil and the process of growth from the polycrystalline rod to the single crystal boule was easily done. After the first pass, apparently there was no shrinkage on the melt as was the case during the first pass and tile molten zone remained too large. This caused an instability of the molten zone and in the worst case, the zone dropped off. This difficulty
Gas outlet Drive shaft
Fig. 2. Schematic diagram of sintering furnace. BN holder
was decreased by the friction between the powder and the die. Then the rubber sheet was removed and athe 2 under friccompact condition was pressed again 300 kg/cm tion-free (fig. 1). at Finally, the compact was pressed again by a hydrostatic press (1 ton/cm2) in order to obtain uniform density. This procedure prevents the pellets to bend during sintering. The pellets of LaB 6 were sintered using a graphite susceptor heated by induction heating under an argon gas atmosphere for 0.5 hr at 2000 °C.The obtained
View direction Polycrystalline Upper crystal rod
.::
Drive Molten zone RE coil ~
I 0
00 00
direction
/Mirror
Lower crystal
‘ “
Polycrystalline rod BN holder
~polycrystalline rods had about 73% of the theoretical density. A schematic diagram of the sintering furnace is shown in fig. 2.
Drive shaft Fig. 3. Process of multi float zone passage.
T Tanaka et al.
/ Growth of high purity LaB6 single crystals
which occurred after the first pass could be avoided by setting the drive speed of the upper boule to be less than the growing speed of the lower crystal, so that both the molten zone and the cross section of the lower crystal became small as compared with the cross section of the upper boule. In this way repeated zone passes can be made possible. However, if the diameter of the upper crystal is changed, this also can cause the failure of the multiple zone passages. Therefore, the polycrystalline rod for the first pass needs to be uniform and straight so that the diameter of the crystal obtained at the first pass is almost constant. When these conditions are completely satisfied, the multi-float zone passages can be achieved continuously. The crystal growing system is shown schematically in fig. 3.
chemical analysis. Furthermore, in order to check if the ratios fluctuate from the stoichiometry along the growth axis, the concentration of lanthanum ion has been measured by electron probe microanalysis. The X-ray intensity from the lanthanum ion was about 8.5 X 102 c/sec and the total counts of 20 sec fluctuate only less than 3% from one point to another along the growth axis. Thus any appreciable deviation from the stoichiometry has not been observed. Impurities have been qualitatively analysed by emission spectrography which revealed the presence of Co, Si, Mn, Cr, Fe, Mo, Ti, Zr and Ni from the raw material, but only Si, Mg and Fe were detected in the single float zone passed crystals. For the triply passed crystals, no impurities were detected by emission spectrography. Carbon impurities were detected to be 0.6 wt% from the raw material but could not be detected in any crystals where the detection limit of carbon was 0.04 wt%. The residual resistance ratio R/R0, which is defined approximately by the ratio of room temperature resistance R to liquid helium temperature resistance R0, was also used to characterize the crystals. The R/R0 ratio of singly passed crystals was about 20 and is comparable with those reported previously [2, 41. On the other hand, the R/R0 ratio for the triply passed crystals had a maximum value of 450 and a minimum of 200. Thus the triple passage of the molten zone increases the purity more than one order of magnitude
3. Results Two kinds of LaB6 crystals have been synthesized. One is a crystal passed once by the molten zone and the other is a triply passed one. One of these crystals is shown in fig. 4 where the size of the crystal is about 7 mm diam. X 60 mm. LaB6 is well known to have a non-stoichiometry region [61.Those crystals were chemically analysed and the results show that the ratio of La/B is the stoichiometric ratio of 1/6 within the accuracy of the
~
195
iT
—
—
___
-----t’J
II IItt~~~ll ~
15
16
17
118
19
210
21
Fig. 4. Single crystal of LaB6 whose dimension is 7 mm diaiii. X 60 mm.
t96
T. Tanaka et at
/ Growth of/ugh purity LaB6 single
crystals
0.5
Hg. 5. Ilerg -Barrell pholograpliy of I all~hs 12111 dilfr.iction.
~i 0.2 U
as compared with the single passage crystals. From the results of these analyses, it is shown that the purity of the crystals obtained now was markedly improved by the multi-float zone passage. After the triple passage of the molten zone, the boules of LaB6 became single crystalline which occasionally contained several crystallites of different orientations. Seed crystals for the selection of a preferred growth axis were not used. The existence of an easy axis of crystal growth was not found. The further observation by Berg—Barrett photographs (fig. 5) has shown that this LaB6 crystal con- 3. sists of sub-grains whose dimensions are about 1 mm The sub-grains observed in fig. 5 are misorientated by about several seconds to each other. The diffractions from the sob-grains of the large misorientation are not recorded on fig. 5. The maximum misorientation of the sob-grains measured by back-Laoe photographs was about a few degrees, and was observed mainly at the outer side of the crystal bonles. The sub-grains were not eliminated by slowing down the speed of crystal growth from 20 mm/hr to 10 mm/hr.
4. Electronic properties of LaB6 Hall coefficients of some crystals have been measored. The values of the Hall coefficients are to be independent of the crystal purity and are —4.5 X 10—12 V cm/A G. This value corresponds to the assumption that one carrier exists per unit cell and this means that one lanthanum ion offers one electron as a carrier. The present result agrees with the prediction by Longuet-Higgins et al. [71. The crystal purity of LaB6 was markedly improved by the multi-float zone passes and the effect of im-
D1 0.D5 0.02 0,01
5
10
20
50
100
300
Temperature I It I Fig. 6. Electrical resistivity of LaB6, open circles show experimental data and solid line is calculated curve.
purities on the electrical resistivities of those crystals whichthe gives the residual resistance is much suppressed. Thus intrinsic conduction property of LaB 6 can be revealed at a wider temperature range. En fig. 6 the electrical resistivity versus temperature curve is shown for the crystal whose residual resistance ratio is 450. Electrical resistivities of metals are usually expressed by the summation of two terms [81.One is a constant term ascribed to impurity scattering and the other is the Bloch—Gruneisen term which is the expression of the scattering of electrons by acoustic phonons, respectively. The electrical resistivity of LaB6 shown in fig. 6 can not be fitted by the summatioii of these two terms, and therefore a further scattering mechanism should be considered. A proposed mechanism is the polar phonon mode scattering of electrons, because LaB6 has some properties of ionic crystals [71and it suggests the existence of polar phonon scattering which does not exist in the usual metals. Thus tIle solid line in fig. 6 is calculated as the resistivity to the summation of the impurity scattering term, the acoustic phonon scattering term and the polar phonon scattering term. It seems that the calculated curve agrees quite well with the experinlental data. The characteristic temperatures of the acoustic phonon mode and the polar phonon mode which give the best fit in this cal-
T Tanaka etat
/ Growth of IliAd? purity LaB6 single crystals
eulation are 245 K and 920K, respectively [9]. The value of 245 K for the acoustic phonon mode is in good agreement with the Debye temperatures reported [2,10]. On the other hand, there are no data for the characteristic temperature for the polar mode of LaB6. It should be noted that the measurement of this value is left as an unsolved problem.
197
Acknowledgements The authors wish to express their gratitude to Drs. 0. Fukunaga, Y. Ishizawa and K. Uchida for their valuable advice. They also thank Mrs. M. Kobayashi for the chemical analysis, Mr. A. Ono for the EPMA measurement and Mr. S. Momma for the Berg—Barrett photography. References
5. Conclusions [11 T. Niemyski and F. Kierzek-Pecohd, J. Crystal Growth High purity LaB6 single crystals have been obtained by the multi-float zone passes. The process was achieved by controlling the drive speed of the upper crystal to be slower than that of the growing crystal. The triply passed crystals had a higher purity than the singly passed ones of at least one order of magnitude, and the ratio of La/B was held in the stoichiometrie ratio in every crystal. The crystal boules became single crystalline and consisted of sub-grains. These sub-grains were inisorientated by small angles to each other. The electrical resistivity measurement for the highest purity crystal revealed the characteristic feature of the conduction property of LaB6, i.e., in the scattering mechanisms of LaB6 the polar phonon mode scattering plays an important role in addition to the impurity scattering and the acoustic phonon scattering.
3/4 (1968) 162.
121 J. Etourneau, J. Mereurio, R. Naslain and P. HagenmUller, J. Solid State Chem. 2 (1970) 332.
131 Yu.B. Paderno, ES. Garf, T. Niemyski and I. Pracka, Poroshkovaya Met. No. 10 (82) (1969) 55. 141 Yu.B. Paderno, VI. Novikov and ES. Garf, Poroshkovaya Met. No. 11(83) (1969) 70.
151 B.J. Curtis and H. Graffenberger, Mater. Rcs. Bull. 1 (1966) 27.
161 G.V. Sanhsonov, L.Ya. Markovskii, A.F. Zhigach and 171 [81 191 1101
MG. Valyashko, Boron, Its Compounds and Alloys (Acad. Sci. Ukrainian S.S.R., Kiev, 1960). H.C. Longuet-Iliggins and M. de V. Roberts, Proc. Roy. Sue. (London) 224 (1954) 336. A.F1. Wilson, Theory of Metals, 2nd ed. (Cambridge Univ. Press, 1965). T. Tanaka, Y. Ishizawa, T. Akahane, S. Kawai and E. Bannai, detail process of the calculation to be published. J. Mercurio, J. Etourneau, R. Naslain and J. Bornnerot, Compt. Rend. (Paris) B 268 (1969) 1766.