Floating zone growth and high temperature hardness of rare-earth hexaboride crystals: LaB6, CeB6, PrB6, NdB6, and SmB6

Article abstract of DOI:10.1006/jssc.2000.8842

Single crystals of rare-earth hexaborides, LaB₆, CeB₆, PrB₆, NdB₆, and SmB₆, were prepared by the floating zone method. Their crystal quality increased as the atomic number of the rare-earth metals in

Full text of DOI:10.1006/jssc.2000.8842

Journal of Solid State Chemistry 154, 238}241 (2000)
doi:10.1006/jssc.2000.8842, available online at http://www.idealibrary.com on
Floating Zone Growth and High Temperature Hardness of Rare-Earth
Hexaboride Crystals: LaB6, CeB6, PrB6, NdB6, and SmB6
S. Otaniꢀ
National Institute for Research in Inorganic Materials, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
and
H. Nakagawa, Y. Nishi, and N. Kieda
Shonan Institute of Technology, 1-1-25, nishikaigan Tsujido, Fujisawa, Kanagawa 252-8511, Japan
Received September 9, 1999; in revised form December 28, 1999; accepted January 2, 2000
2. EXPERIMENTAL
Single crystals of rare-earth hexaborides, LaB6, CeB6, PrB6,
NdB6, and SmB6, were prepared by the 6oating zone method.
Their crystal quality increased as the atomic number of the
rare-earth metals increased. The di4erence in the quality was
explained from the high-temperature hardness and the growth
temperature. ( 2000 Academic Press
Commercial powders of LaB and CeB were used as
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starting materials. The other hexaborides, PrB , NdB , and
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SmB , were synthesized by heating mixtures of their oxide
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and boron powders in vacuum according to the following
formula:
Key Words: 6oating zone growth; crystal; hardness; LaB6;
CeB6; PrB6; NdB6; SmB6.
REO #(6#x) BPREB #xBO!,
V
ꢁ
where RE is Pr, Nd, or Sm. After crushing and pulverization
of the products, the powder was isostatically pressed
(200 MPa) into a rod in a rubber bag and then heated under
vacuum at 17003C for 30 min.
The crystal was prepared at a pressure of 0.5 MPa of an
ambient argon gas by the RF heated #oating zone method.
The growth rate, the lower shaft, was 1 cm/h. Only the
growing crystal was rotated at a rate of 6 rpm. The obtained
crystals were several mm in diameter and 6 cm long. The
details were described elsewhere (1).
1. INTRODUCTION
The #oating zone method is well suited to preparing large
refractory crystals, but subgrain boundaries are formed by
thermal stress due to a steep temperature gradient (1). Re-
cently, high-quality refractory crystals, free of the bound-
aries, were prepared by controlling composition of a molten
zone (the traveling solvent #oating zone method) (2}4). The
improvement in quality was explained by increase in the
mechanical strength due to decrease in the growth temper-
ature (5). Therefore, measurements of the mechanical prop-
erties are expected to be useful for estimating the crystal
quality and getting useful information on improvement in
the crystal quality.
Etching patterns were observed under a microscope after
a mirror-like polished plane was etched in a HNO aqueous
ꢂ
solution (1:3) for several minutes at room temperature.
Microhardness was measured in vacuum from room tem-
perature to 11003C, using a diamond Vickers indenter (5).
The loads were 500 g during 10 s. Hardness was obtained by
measuring dimensions of the indentations on the photo-
graphs.
LaB and CeB crystals are used as an electron source of
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high brightness and longevity. Improvement in the crystal
quality is expected to increase the reliability of electron emis-
sion and to decrease the cost of making the emitters. In this
report, the crystal quality of LaB , CeB , PrB , NdB , and
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SmB , which melt congruently among the rare-earth hexa-
3. RESULTS AND DISCUSSION
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borides, is estimated from their high-temperature hardness.
LaB phase melts congruently at 27153C, as shown in
ꢁ
Fig. 1 (6). The crystals prepared at a congruent composition
ꢀ To whom correspondence should be addressed. Fax: #(81)-298-58-
5652. E-mail: otani@nirim.go.jp.
contained subgrain boundaries, as shown in Fig. 3a. How-
238
0022-4596/00 $35.00
Copyright ( 2000 by Academic Press
All rights of reproduction in any form reserved.

FZ GROWTH AND HARDNESS OF HEXABORIDE CRYSTALS
239
FIG. 1. (a) Phase diagram of the La}B system (6) and (b) the traveling solvent #oating zone method by which the high-quality crystals, free of
boundaries, are prepared (2, 3).
ever, formation of the boundaries was suppressed when the slope, which suggests that the hexaborides have similar
growth temperature was decreased by 2003C due to control- mechanical properties. The hardness of the borides de-
ling the zone composition to La- or B-excessive composi- creased as the atomic number increased. For example, the
tion (the traveling solvent #oating zone method), as shown hardness of LaB , CeB , PrB , NdB , and SmB was 1160,
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1090, 1080, 1010, and 940 kg/mm , respectively, at 9003C.
in Fig. 1 (2, 3). Therefore, in this report, regarding the LaB
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crystal growth as a standard that indicates relation between That is, the hardness decreased by 6, 7, 13, and 19%,
the crystal quality and the hardness, the crystal quality of respectively, compared with that of LaB , as shown in
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the other rare-earth hexaborides was estimated from their Table 1.
high-temperature hardness.
On the other hand, their growth temperatures, which
Hardness was measured on the (100), (110), and (111) were estimated from the heating power (7), decreased as the
planes of the hexaboride crystals of LaB , CeB , PrB , atomic number increased, as shown in Table 1. Their
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NdB , and SmB . The hardness on the (100) planes was growth temperatures decreased by 140, 210, 250, and 3703C,
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measured with good reproducibility because of the clear-cut respectively, compared with the LaB crystal growth at a
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square shape of the indentations. Therefore, the hardness of congruent-melting composition, 27153C (6). The decrease in
the boride crystals was compared on the (100) planes, as the growth temperature corresponds to an increase in the
shown in Fig. 2. The temperature dependence of the hard- hardness by 19, 30, 37, and 60% in the cases of CeB , PrB ,
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ness consisted of two linear parts with di!erent slopes in all NdB , and SmB , respectively, as estimated from the tem-
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the samples. In the high temperature range, where the crys- perature dependence of the hardness in Fig. 2.
tal quality is determined due to plastic deformation (4), the Therefore, the crystals of CeB , PrB , NdB , and SmB
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hardness of all the hexaborides decreased with the same were estimated to have higher hardness by 10% ("19}9),
20% ("30}10), 23% ("37}14), and 41% ("60}19), re-
spectively, at their growth temperatures, compared with
that of LaB crystals prepared at a congruent composition,
ꢁ
as shown in Table 1. In the case of the LaB crystal, the
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decrease in the growth temperature by 2003C leads to an
increase in hardness by 28% and the disappearance of
subgrain boundaries (2, 3). Therefore, the quality of CeB
crystals are expected to be a little better than that of the
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LaB crystals, but the PrB and SmB crystals are expected
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to be of much better quality.
Figure 3 shows the etching patterns of the (100) planes.
The crystal quality increased as the atomic number of the
rare-earth metal increased. LaB crystals prepared at
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a congruent composition had many subgrain boundaries,
FIG. 2. Vickers hardness on the (100) planes.
demonstrated as lines in Fig. 3a. The CeB crystal contained
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240
OTANI ET AL.
FIG. 3. Etching patterns of the (100) planes of (a) LaB , (b) CeB , (c) PrB , (d) NdB , and (e) SmB crystals. The size of the photographed area is
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4;3 mm.
4. SUMMARY
fewer boundaries than the LaB crystal. Few boundaries
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were contained in the PrB , NdB and SmB crystals.
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The crystal quality of the rare-earth hexaborides was
estimated from the high-temperature hardness and the
Therefore, the crystal quality of the hexaborides was well
consistent with the estimation from the hardness.
TABLE 1
Growth Temperature and Vickers Hardness
Growth temp.
Hardness (kg/mmꢃ)
Increase in hardness at the growth
temperature (%)
3C
*¹
*H? (%)
(at 9003C)
*H@ (%)
LaB
(boundary-free)
2715
2515
2575
2505
2465
2345
(0)
(1160)
(1160)
(1090)
(1080)
(1010)
(940)
ꢁ
(!200)
(!140)
(!210)
(!250)
(!370)
#28
#19
#30
#37
#60
0
!6
#28
#13
#23
#24
#41
CeB
ꢁ
PrB
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!7
NdB
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!13
!19
SmB
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? Increase in hardness estimated from decrease in the growth temperature (see Fig. 2).
@ Decrease in hardness of the borides, compared with LaB (see Fig. 2).
ꢁ

FZ GROWTH AND HARDNESS OF HEXABORIDE CRYSTALS
241
REFERENCES
growth temperature. The hardness at their growth temper-
ature was well consistent with the crystal quality. The hard-
ness measurement, simple method of examining
mechanical properties, was found to be useful for estimating
crystal quality in the case of a series of structurally similar
compounds.
1. S. Otani and Y. Ishizawa, Prog. Crystal Growth Charact. 23, 153 (1991).
2. S. Otani and Y. Ishizawa, J. Crystal Growth 118, 461 (1992).
3. S. Otani, S. Honma, Y. Yajima, and Y. Ishizawa, J. Crystal Growth 126,
466 (1993).
4. S. Otani and Y. Ishizawa, J. Crystal Growth 140, 451 (1994).
5. S. Otani, T. Tanaka, and Y. Ishizawa, J. Alloys & Compd. 202, L25 (1993).
6. T. B. Massalski (Ed.) &&Binary Alloy Phase Diagram,'' 2nd ed. ASM,
Ohio, 1990.
a
ACKNOWLEDGMENTS
The authors express thanks to Mr. T. Ohsawa, NIRIM, for the technical 7. D. K. Donald, Rev. Sci. Instr. 32, 811 (1961).
supports. 8. A. G. Atkins and D. Tabor, Proc. R. Soc. A 292, 441 (1966).