Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

NE5C/2005G006

In-situ observation of formation of Si clathrates
Motoharu IMAI*1
National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan

Experimental
High Pressures were applied using the multianvil highpressure apparatus MAX80, installed in the beam line PFAR NE5C. Samples were prepared by arc-melting of an
8:30 molar mixture of orthorhombic BaSi2 and Si. The
powdered samples were loaded in a h-BN capsule.
Pressure was evaluated from the lattice constant of an
NaCl internal pressure marker. The sample wascompressed up to 4.3 GPa at room temperature thenheated up to 1270 K at this pressure. X-ray diffraction
patterns were measured during this process.
Results and Discussion
Figure 1 shows x-ray diffraction patterns of8BaSi2+30Si observed during heating up to 1270 K at
about 4.3 GPa. At room temperature at 4.3 GPa, thesample consists of orthorhombic BaSi2 and Si. On heating

186

Users' Reportclclclc
BN+Sic
BN+Sitcclcl cltc

1070 Kclcl cl cl cl clcl clcl cl clBNcl

Intens ity (arb. units )Introduction
Type-I Si clathrates, whose general chemical formula isA8Si46, have a unique structure. In the compounds, Siatoms form Si20 dodecahedra and Si24 tetrakaidehedra, andthese polyhedra are linked by shared faces. The A atoms
are located in the center of the polyhedra. A search on thesuperconducting materials has been conducted in Si
clathrates since discovery of superconducting fullerides
because these polyhedra are isostructural with thefullerene C20 and C24. Yamanaka and his coworkers
synthesized the superconducting Si clathrate (Na,Ba)8Si46
by removing
Na atoms from Na2BaSi4 at hightemperatures under vacuum [1]. They, furthermore,
succeeded in a synthesis of Ba8Si46 from a mixture of Si
and orthorhombic BaSi2 at pressures ranging from 3 to
5GPa and temperatures ranging from 1073 to 1673K [2,3].
On the other hand, BaSi2 has been reported to undergo
two structural phase transitions, the orthorhombic-totrigonal, and the trigonal-to-cubic transitions, in thispressure-temperature region [4]. It is, therefore,
interesting to see which kind of phases (theorothorhombic, the trigonal or the cubic) of BaSi2 isresponsible for the formation of Ba8Si46 from the mixtureof BaSi2 and Si at high pressures and high temperatures.
In this research, we have tried to observe the formationof Ba8Si46 from the mixture of BaSi2 and Si in-situ at highpressures and high temperatures.

cc c970 K
c+Sic ctt t+c Sit 870 Ktt t Sit 770 K

BN+Si tt

BN+Sio40

oooo60Si 670 Ko80100

Photon Energy (keV)
up to 870K, only BaSi2 undergoes two structural phasetransitions: the orthorhombic-to-trigonal and the trigonalto-cubic transitions. Si undergoes no structural transitionat these temperatures. On further heating, the cubic BaSi2and Si start to react and the Si clathrate Ba8Si46 appears at970 K. The formation of Ba8Si46 is completed at 1070 K.
Fig. 1 X-ray diffraction patterns of 8BaSi2+30Si
observed during heating up to 1270 K at about 4.3 GPa.
The symbols “o”, “t”, “c” and “cl” represent reflections
from the orthorhombic BaSi2, the trigonal BaSi2, the cubic
BaSi2, and the Si clathrate Ba8Si46.

References[1] H. Kawaji et al., Phys. Rev. Lett. 74, 1427 (1995).
[2] S. Yamanaka et al., Inorg. Chem. 39, 56 (2000).
[3] H. Fukuoka et al., J. Phys. Chem. Solids 65, 333(2004).
[4] M. Imai et al., Phys. Rev. B58, 11922 (1998).
IMAI.Motoharu@nims.go.jp