Electronic band structure and conduction mechanism of mixed valence Te4+/Te6+

While many post-transition metals could form mixed valence oxides with high conductivities leading to various interesting applications, oxides of tellurium are usually insulators. The only known mixed valence tellurium oxides that show appreciable electronic conductivities are the $\mathrm{Cs}{(M,\mathrm{Te})}_{2}{\mathrm{O}}_{6}$ series. However, experimental and theoretical investigations of the compounds are scarce. To study the effects of $M$ metal substitution of tellurium on the structural and electronic properties of $\mathrm{Cs}{(M,\mathrm{Te})}_{2}{\mathrm{O}}_{6}$ mixed valence compounds, a series of oxides with the general formula $\mathrm{CsM}{\mathrm{n}}_{x}\mathrm{T}{\mathrm{e}}_{2\text{\ensuremath{-}}x}{\mathrm{O}}_{6}$ have been prepared and characterized. Substituting Te in $\mathrm{CsT}{\mathrm{e}}_{2}{\mathrm{O}}_{6}$ with as small as 2.5% Mn $(x=0.05)$ changes the structure from rhombohedral to cubic symmetry. Based on the x-ray photoelectron spectroscopy and x-ray absorption near edge structure studies, we found that Mn in the samples with $x\ensuremath{\le}0.33$ is in +3 states, while samples with $x>0.33$ contain a mixture of $\mathrm{M}{{\mathrm{n}}^{3}}^{+}$ and $\mathrm{M}{{\mathrm{n}}^{4}}^{+}$. Detailed studies indicate that there are two substitution schemes depending on the $x$ values. The switching between these two schemes occurs at around $x=0.30\ensuremath{-}0.33$, which results in a change in the trend of electronic conductivity. To describe the electronic properties, simple schematic band structure diagrams of the samples are proposed based on experimental results and density functional theory calculations. ${\mathrm{CsMn}}_{\mathrm{x}}{\mathrm{Te}}_{2\text{\ensuremath{-}}x}{\mathrm{O}}_{6}(x<0.33)$ contains $\mathrm{T}{{\mathrm{e}}^{4}}^{+}/\mathrm{T}{{\mathrm{e}}^{6}}^{+}$ mixed valency where both species are in the same crystallographic sites. Yet, the Te $5s$ band is split into two parts with $\mathrm{T}{{\mathrm{e}}^{4}}^{+}$ contributing to the valence band maximum and $\mathrm{T}{{\mathrm{e}}^{6}}^{+}$ contributing to the conduction band minimum. Such a splitting indicates that they are distinguishable because of a local distortion in the lattice. We found that energy levels of $\mathrm{T}{{\mathrm{e}}^{4}}^{+}5s$ states relative to the conduction band exhibit a strong correlation with the unit cell parameters. When the cell parameter is small, $\mathrm{T}{{\mathrm{e}}^{4}}^{+}$ is destabilized in the lattice causing its energy level to increase. While metallic conduction has not been achieved in this series of compounds, the detailed investigation in this work deepens the understanding about their electronic structures and sheds light on the possibility of designing type III mixed valence tellurium oxides.