CoTeO₄ – a wide-bandgap material adopting the dirutile structure type

Abstract

High-quality crystals of CoTeO₄ were grown by application of chemical vapor transport reactions in closed silica ampoules, starting from polycrystalline material in a temperature gradient 640 °C → 580 °C with TeCl₄ as transport agent. Crystal structure analysis of CoTeO₄ from single crystal X-ray data revealed a dirutile-type structure with CoII and TeVI atoms at crystallographically distinct sites, each with point group symmetry 1̄. The statistical significance and accuracy of the previously reported structural model based on powder data with the ordered arrangement of Co and Te cations was noticeably improved. CoTeO₄ does not undergo a structural phase transition upon heating, but decomposes stepwise (Co₂Te₃O₈ as intermediate phase) to Co₃TeO₆ as the only crystalline phase stable above 770 °C. Temperature-dependent magnetic susceptibility and dielectric measurements suggest antiferromagnetic ordering at ∼50 K. Optical absorption spectroscopy and computational studies reveal wide-band semiconductive behavior for CoTeO₄. The experimentally determined band gap of ∼2.42 eV is also found for CdS, which is frequently used in photovoltaic systems but is hazardous to the environment. Hence, CoTeO₄ might be a possible candidate to replace CdS in this regard.