Benchmarking Scheelite Oxides XTcO 4 (X = Li, K, Rb, Cs) via First-Principles Calculations: Stability, Band Gaps, and Thermoelectric Performance for Sustainable Energy Technologies

This study investigates the structural, optoelectronic, and thermoelectric properties of scheelite oxides XTcO 4 (X = Li, K, Rb, Cs) using first-principles ab-initio calculations. The results reveal that CsTcO 4 is the most stable compound among the studied oxides, with the lowest ground state energy. Within the PBE+U framework, the calculated direct band gaps of LiTcO 4 , KTcO 4 , RbTcO 4 , and CsTcO 4 are 2.30, 2.34, 2.27, and 2.37 eV, respectively, indicating their potential for visible/near UV photovoltaic applications. These materials exhibit significant photon absorption in the visible and UV regions, with XTcO 4 (X = Li, K, Rb, Cs) showing superior thermoelectric performance. The analysis of thermoelectric properties confirms the thermal performance of these compounds, while KTcO 4 and CsTcO 4 exhibit positive Seebeck coefficients indicating p-type semiconducting behavior. Conversely, the Seebeck coefficient (S) values for the LiTcO 4 and RbTcO 4 compounds decrease with increasing temperature, suggesting a transition in the material’s semiconductor type from p-type to n-type behavior. The calculated electronic, optical, and thermoelectric properties of CsTcO 4 indicate interesting intrinsic characteristics from a fundamental materials-science perspective. However, the radioactive and artificial nature of technetium presents substantial practical limitations for experimental handling, large-scale synthesis, and implementation in energy-conversion devices. Therefore, the present results should be regarded as a theoretical assessment of the intrinsic properties and structure–property relationships of CsTcO 4 rather than as evidence of immediate technological applicability. This study provides valuable insights into the properties of scheelite oxides, offering benchmark data for future research and development in green energy technologies.