Direct Observation of Sm Valence Fluctuation in a Heavy Fermion Compound SmOs4Sb12 via 149Sm Synchrotron-Radiation-Based Mössbauer Spectroscopy

149Sm synchrotron-radiation-based Mossbauer spectroscopy was performed in a heavy fermion compound SmOs4Sb12. A small recoil-free fraction due to the low Einstein temperature at the Sm site inhibits the observation of the spectra over a wide temperature region. However, successful estimation of the Einstein temperature at the Sm site by exploiting the temperature dependence of the recoil-free fraction makes it possible to distinguish between the isomer shift values from the contribution of the second-order Doppler shift of the center shift. A comparison between the present work and the results of previous X-ray absorption spectroscopy [Mizumaki et al., J. Phys. Soc. Jpn. 76, 053706 (2007).] demonstrates that the Sm valence fluctuates on the order of 10 MHz in SmOs4Sb12.