We have investigated elastic properties of the Ce-based filled skutterudite antimonides CeRu4Sb12 and CeOs4Sb12 by means of ultrasonic measurements. CeRu4Sb12 shows a slight increase around 130 K in the temperature dependence of the elastic constants C11, (C11−C12)∕2, and C44. No apparent lattice softening toward low temperature expected by a quadrupolar response of the 4f-electronic ground state of the Ce ion was observed at low temperatures. On the other hand, CeOs4Sb12 shows a pronounced lattice softening toward low temperature in the longitudinal C11 as a function of temperature below about 15 K, while a slight lattice softening was observed in the transverse C44 below about 1.5 K, in contrast to no softening in CeRu4Sb12 at low temperatures. Furthermore, CeOs4Sb12 shows a steep drop around a phase-transition temperature of Ts=0.9 K in both C11 and C44. The lattice softening observed in C11 below about 15 K cannot be explained reasonably only by the crystalline electric-field (CEF) effect. Instead, it is most likely to be responsible for a coupling between elastic strain and a relevant renormalized quasiparticle band with a small energy gap in the vicinity of the Fermi level. The elastic properties and the 4f ground state of Ce ions in CeRu4Sb12 and CeOs4Sb12 are discussed in terms of the CEF effect and the band structure in the vicinity of Fermi level.
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