Novel Electronically Conducting Tellurium Oxides

Tellurium oxides seldom show measurable electronic conductivity. Tellurium oxides that appear to have Te$^{5+}$ contain Te$^{4+}$ and Te$^{6+}$ in two distinct crystallographic sites and are electronic insulators. Here we report on the synthesis and characterization of several new tellurium rich oxides of the general formula, CsM$_{x}$Te$_{2-x}$O$_{6}$, crystallizing in modified pyrochlore structure. Most of the compounds reported here are black in color with some exhibiting good electronic conductivities (2 S/cm) and Seebeck measurements indicate all are n-type. The observation of high electronic conductivities in compounds like CsGe$_{0.5}$Te$_{1.5}$O$_{6}$, CsAl$_{0.33}$Te$_{1.67}$O$_{6 }$confirms that observed conductivity is arising from doping of electrons into the empty 5s orbitals of Te$^{6+}$. This reduction is apparently accompanied with some small deviation from the ideal formula: oxygen content and/or ratio of cations on octahedral sites. This is in consistent with single-crystal X-ray as well as powder neutron diffraction structure refinements and the observed sign of the Seebeck coefficient. To our knowledge, this is a first observance of high electrical conductivity in mixed valent tellurium oxides.