X-ray study with synchrotron radiation for P- and Sb-based skutterudite compounds at high pressures

Using synchrotron radiation, the x-ray diffraction of P- and Sb-based skutterudite compounds has systematically been studied at room temperature and high pressures. Bulk moduli of binary skutterudites CoX 3 (X = P and Sb), filled skutterudites CeT 4 X 12 (T = Fe, Ru and Os), LaRu 4 X 12 and PrRu 4 X 12 are obtained from the volume versus pressure curves fitted by a Birch equation of state. Bulk moduli are 150–225 GPa for P-based skutterudites and 80–115 GPa for Sb-based skutterudites. The bulk modulus of the phosphides is about two times that of the corresponding antimonides. The antimonides are very compressible compared with the phosphides. The Grüneisen constants (γ) for skutterudite compounds are estimated from the compressibility and its volume derivative in the Debye approximation. The bulk modulus of CoX 3 and CeT 4 X 12 increases with increasing lattice constant, attaining a maximum for the Ru compounds. Bulk moduli for the Ru-based skutterudites are larger than those of the corresponding Fe and Os compounds. This may be closely related to the interesting physical properties in the Ru-based skutterudites. The bulk modulus of the binary skutterudites is smaller than that of the filled skutterudites. The cell volume for CeT 4 X 12 (T = Fe, Ru and Os; X = P and Sb) decreases monotonically with increasing pressure. The valence state of Ce ion in these compounds does not change at high pressures.