Crystal structure and thermoelectric properties of clathrate, Ba8Ni3.5Si42.0: Small cage volume and large disorder of the guest atom

Abstract

Samples with the type-I clathrate composition Ba8NixSi46−x have been synthesized and their structure and thermoelectric properties characterized. Microprobe analysis indicates the Ni incorporation to be 2.62≤x≤3.53. The x=3.5 phase crystallizes in the type-I clathrate structure (space group: Pm-3n) with a lattice parameter of 10.2813(3)Å. The refined composition was Ba8Ni3.5Si42.0, with small vacancies, 0.4 and 0.5 atoms per formula unit, at the 2a and 6c sites, respectively. The position of the Ba2 atom in the large cage was modeled using a 4-fold split position (24j site), displaced 0.18Å from the cage center (6d site). The volume of the large cage is calculated to be 146Å3, smaller than other clathrates with similar cation displacement. The sample shows n-type behavior with a maximum of −50μV/K at 823K above which the Seebeck coefficient decreases, suggesting mixed carriers. Lattice thermal conductivity, κl, is 55mW/K above 600K.