JOURNAL OF SOLID STATE CHEMISTRY 133, 198—200 (1997)
ARTICLE NO. SC977427
Single Crystal Growth and Properties of Incongruently Melting
TbB6, DyB6, HoB6, and YB6
K. Takahashi* and S. Kunii
Department of Physics, Tohoku University, Aramaki, Aoba-ku, Sendai 980-77, Japan; and *Institute for Material Research, Tohoku University,
Katahira, Aoba-ku, Sendai 980-77, Japan
Received April 7, 1997; accepted April 10, 1997
reactions were performed at about 1650°C for 2 h, removing
the vaporized BO in vacuum. An induction furnace was
used with a graphite susceptor surrounded outside by a car-
Details of the single crystal growth of incongruently melting
TbB6, DyB6, HoB6, and YB6 using a crucible-free vertical float-
ing zone method are reported. Magnetic susceptibility is reported bon felt for temperature insulation. To avoid graphite con-
and discussed. It is suggested that antiferro-quadrupolar ordering
occurs in DyB6 between 30 and 25.6 K. ( 1997 Academic Press
tamination we used a boron-nitride pipe surrounding the
powder rod. By the above procedure, however, we always
got a mixture of three compounds, REB , REB , and REB
,
ꢁ
ꢀ
ꢅꢃ
due to the incongruently melting property. Based on diffu-
sion and reaction kinetics, the intensity ratio among the
three types of diffraction lines is changeable depending on
the reaction temperature. Therefore we can arrange the
overall composition of the mixed compounds at P on the
liquidus curve of Fig. 1. The powder of the mixed com-
pounds was pressed hydrostatically into a rod of 10 mm `
(diameter);100 mm at 2.5 kg/mmꢃ using a special rubber
scak and then sintered at 1700°C for 2 h in the same surface.
Using the sintered rod obtained above, the incongruently
melting rare-earth hexaborides have been grown by a cru-
cible-free vertical floating zone method under 1 MPa press-
urized high-purity (99.9995%) argon. The single crystal
growing speed was 4 mm/h. Figure 2 shows the cross section
of the growing crystal; the total length of it is about 10 cm.
There is a surface skin (&1 mm thick) indicated in Fig. 2
INTRODUCTION
Rare-earth hexaborides with a simple cubic CaB -type
structure have been attracting much attention because of
ꢀ
their variety of electronic and magnetic properties. Recent
development of the study is due to a single crystal growth
technique that overcomes the very high melting temperature
(&2500°C). Heavy rare-earth hexaborides melt incon-
gruently in a condition which makes it difficult to grow big
single crystals. We have, however, succeeded in growing big
single crystals of them by a crucible-free vertical floating
zone method. The details of sample preparation are similar
to those described in Ref. (1) except for the starting composi-
tion of materials and the growing speed. The purpose of the
present paper is to show (a) the details of the single crystal
growth procedure and (b) the results of magnetic suscep-
tibility measurements.
that is always formed when growing REB crystals. Only in-
ꢀ
side is the single crystal of REB . REB and REB com-
ꢀ
ꢁ
ꢅꢃ
pletely disappeared and only REB remains. In Fig. 1, REB
ꢀ
ꢀ
SAMPLE PREPARATION AND CRYSTAL GROWTH
is not a line compound; that is, REB has some width of
composition. However, Fig. 1 represents the situation under
ꢀ
Incongruently melting rare-earth hexaborides were first
obtained in powder form by the following reactions.
Tb O #31BP4TbB #7BO and RE O #15B P
normal pressure (1 atm), whereas the rare-earth element is
always defective (see B/REB "85.7% ("6/7)&89% in the
ꢀ
DyB case). In our floating zone procedure, we use 1 MPa
ꢁ ꢂ
ꢀ
ꢃ ꢄ
ꢀ
2REB #3BO (RE"Dy, Ho, and Y). Rare-earth oxide
of pressurized argon, preventing rare-earth defects. This is
ꢀ
powders (99.99%) were purchased from Japan Yttrium
confirmed by lattice parameter (a ) measurements in which
ꢆ
Company. For the borothermal reduction, we used an
amorphous submicron boron powder of 99.999% purity,
purchased from Aldrich Chemical Company (USA). In the
case of 11-boron-enriched samples for neutron measure-
ments, 99.5 at.% enriched ꢅꢅB was purchased from Isotec
(USA). Mixed powders of oxide and boron were pressed
(a ) is consistent with stoichiometric REB (3).
ꢆ
ꢀ
RESULTS AND DISCUSSIONS
To know the magnetic ground state, magnetic susceptibil-
into a square rod (10;10;200 mmꢄ) at 2.5 kg/mmꢃ. The ity and specific heat measurements have been carried out.
198
0022-4596/97 $25.00
Copyright ( 1997 by Academic Press
All rights of reproduction in any form reserved.
Takahashi
