Crystal structure and thermoelectric properties of clathrate, Ba _8Ni _(3.5)Si _(42.0): Small cage volume and large disorder of the guest atom

摘要

Samples with the type-I clathrate composition Ba _8Ni _xSi _(46-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) ?. The refined composition was Ba _8Ni _(3.5)Si _(42.0), 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 0.18 ? from the cage center (6d site). The volume of the large cage is calculated to be 146 ? ~3, smaller than other clathrates with similar cation displacement. The sample shows n-type behavior with a maximum of -50 μV/K at 823 K above which the Seebeck coefficient decreases, suggesting mixed carriers. Lattice thermal conductivity, κ _l, is 55 mW/K above 600 K.