Temperature dependences of the parameters of atoms in the crystal structure of the intermediate-valence semiconductor SmB6: investigation by high-resolution powder neutron diffraction

Abstract

The crystal structure of intermediate-valence (IV) samarium hexaboride has been studied on single-crystal double-isotope samples 154Sm11B6 by X-ray diffractometry at room temperature and by high-resolution powder neutron diffraction in the temperature range 23 K<or=T<or=300 K. The X-ray experiment revealed the occurrence of vacancies at the boron site, larger thermal vibrations of the Sm atom than of La in the isostructural non-IV material LaB6 and an aspherical charge distribution around the Sm nucleus. The neutron diffraction experiment confirmed the anomalous temperature dependence of the lattice parameter and revealed both a peculiar temperature dependence of the anisotropic thermal vibrations of the boron atom and a temperature-dependent change in the ratio of the isotropic thermal parameters of the Sm and B atoms. Thermal vibrations of the Sm ion can be satisfactorily described by the Einstein model with characteristic temperature Theta E approximately=120 K within the whole temperature range. The data obtained are discussed in terms of the influence of the fluctuating valence of the Sm ion on the structural parameters of atoms.