Ultrasound measurements on the skutterudite compound SmOs4P12

We have investigated the elastic properties of the Sm-based filled skutterudite compound SmOs 4 P 12 for the first time. The measurements were performed on polycrystalline samples prepared at high temperatures and high pressures by using a wedge-type, cubic anvil, high-pressure apparatus. A clear and steep decrease was observed at an antiferromagnetic transition temperature of 4.5 K in the temperature dependence of both the longitudinal and the transverse elastic constants. Absence of a precursor behavior reminds us of the multipolar ordering, possibly octupolar ordering, observed in the isostructural system SmRu 4 P 12 . The transition is robust against an external magnetic field. We argue that the low-lying degenerate levels derived from the 4f-multiplet ground state of the Sm ion is split by the crystalline electric field in SmOs 4 P 12 . Furthermore, we discuss the phononic properties, which include the ionic degrees of freedom for the rattling motion, and compare them with the values for other isostructural systems.