Synthesis and the physical properties of layered copper oxytellurides Sr2TMCu2Te2O2 (TM = Mn, Co, Zn)
We synthesized a new series of layered copper oxytellurides Sr2TMCu2Te2O2 with variation in transition metal (TM = Mn, Co, and Zn) elements. These compounds are the first example having alternately stacked anti-fluorite Cu2Te2 and perovskite-like TM-oxide layers. Owing to the longer ionic radius of Te compared to those of Se and S, they exhibit larger lattice parameters than isostructural sulfides and selenides. First principles band structure calculation for the Zn compound as a representative reveals a semiconducting direct band gap of ∼1.7 eV and the Co compound shows a comparable band gap in the diffuse reflectance measurement result. From the thermoelectric property study, we determined a power factor of ∼70 μW m−1 K−2 and a figure of merit ZT of ∼0.045 at 770 K for the Co compound, which encourages further improvement in thermoelectric response by applying various enhancement methods. In terms of the magnetic properties, a signature of antiferromagnetic order is observed in the Co and Mn compounds. The present results not only highlight the structural flexibility of the system to tune the physical properties but also suggest that the replacement of the blocking layer can be a new way to improve the thermoelectric performance of layered copper oxychalcogenides.