Structural Instability of Unfilled Skutterudite Compounds MX3 (M = Co, Rh, and Ir; X = As and Sb) under High Pressure

Powder X-ray diffraction patterns of the binary unfilled skutterudite compounds MX 3 ( M = Co, Rh, and Ir; X = As and Sb) have been studied under high pressures up to 60 GPa, using synchrotron radiation and a diamond anvil cell. The X-ray diffraction patterns of MX 3 above a critical pressure show anomalous reductions in the intensities of several Bragg peaks. Furthermore, the pressure-volume curves of MX 3 reveal a saturation behavior of the volume reduction with pressure. Upon pressure release, MX 3 exhibit a volume greater than those of pristine compounds. This anomalous behavior could be interpreted as a pressure-induced self-insertion reaction. Furthermore, the high-temperature and high-pressure phase diagram of RhSb 3 was obtained. The structural instability of MX 3 under high pressures is discussed.