Anomalous correlation between cage size and valence state in SmOs4Sb12

We have carried out the synchrotron powder diffraction and x-ray absorption experiments of a valence-fluctuated heavy fermion compound SmOs4Sb12. The size of the Sb cage which includes a Sm atom shows significant temperature dependence. This correlates with the temperature dependence of the Sm valence. However, the cage size shrinks with decrease of temperature although the averaged ionic radius becomes large. This anomalous temperature dependence suggests the correlation between the cage size and the hybridization of the 4f electron with the conduction electrons in SmOs4Sb12.