Electrical properties of (Cs1−xAx)Al0.33Te1.67O6 (A = K and Rb) mixed valence pyrochlores

The preparation and electronic properties of Cs1−xAxAl0.33Te1.67O6 (A = K, Rb, and Cs) are reported. Replacing Cs with smaller Rb and K reduces cell parameters of the compounds but does not affect the overall structure. Electronic conductivities of all samples were measured and explained based on the band conduction model. Although XPS Te3d5/2 spectra indicate that all samples contain similar amount of Te4+/Te6+ mixed valency, their conductivities are varied from about 0.1 S cm in CsAl0.33Te1.67O6 to 3 × 10−5 S cm in RbAl0.33Te1.67O6 and 3 × 10−7 S cm in KAl0.33Te1.67O6 at 300 K. To explain such large differences, the band structure diagrams are proposed based on the UV-Vis spectra and XPS spectra at valence band region. When the obtained activation energies of conduction and the proposed band diagram are considered, it is concluded that AAl0.33Te1.67O6 (A = K, Rb, and Cs) are n-type semiconductors. The defect levels in these samples originate from Te4+ whose energy level relative to the conduction band minimum is different from samples to samples. Such differences are affected by Cs content in the structure as Cs seems to lower the band gap energy and increase the valence band maximum. It is the position of these defect levels that determines the electronic conductivity of the compounds.