TEMPERATURE DEPENDENCES OF THE PARAMETERS OF ATOMS IN THE CRYSTAL-STRUCTURE OF THE INTERMEDIATE-VALENCE SEMICONDUCTOR SMB6 - INVESTIGATION BY HIGH-RESOLUTION POWDER NEUTRON-DIFFRACTION

The crystal structure of intermediate-valence (IV) samarium hexaboride has been studied on single-crystal double-isotope samples (SmB6)-Sm-1 54-B-11 by x-ray diffractometry at room temperature and by high-resolu tion powder neutron diffraction in the temperature range 23 K less-tha n-or-equal-to T less-than-or-equal-to 300 K. The x-ray experiment reve aled the occurrence of vacancies at the boron site, larger thermal vib rations 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 de pendence of the lattice parameter and revealed both a peculiar tempera ture dependence of the anisotropic thermal vibrations of the boron ato m and a temperature-dependent change in the ratio of the isotropic the rmal parameters of the Sm and B atoms. Thermal vibrations of the Sm io n can be satisfactorily described by the Einstein model with character istic temperature THETA(E) congruent-to 120 K within the whole tempera ture 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.