Structural Studies and Electrical Properties of Cs/Al/Te/O Phases with the Pyrochlore Structure

A series of polycrystalline and single crystal cesium aluminum tellurates with the pyrochlore structure have been prepared and characterized. The variations in cell edge for the Cs/Al/Te/O phases range from 10.06 Å for the Al rich limit to 10.14 Å for the Te rich limit. Rietveld structural analyses based on both X-ray and neutron diffraction data were performed on 5 different compositions. Single crystals of 3 compositions were prepared and studied by X-ray diffraction. The anharmonic component of the thermal motion for Cs was small but became significant on replacing Cs with Rb. A maximum in the electrical conductivity of about 0.1 S/cm is found in the middle of this range close to the ideal composition of CsAl_1/3Te_5/3O₆. The conductivity is attributed to filled Te 5s states associated with Te⁴⁺ lying just below the conduction band based on empty Te 5s states associated with Te⁶⁺. The relatively large Te⁴⁺ ion is compressed by the lattice, and as this compression increases the filled 5s states approach the conduction band and thereby increases conductivity.