Rb ₂ NaTlX ₆ (X = Br, Cl): A Novel, Direct‐Bandgap Double Perovskite with Superior Mechanical, Optical, and Thermoelectric Properties

The present study investigates the structural, dynamical, mechanical, electronic, optical, and thermoelectric properties of the double perovskites Rb 2 NaTlX 6 (X = Br, Cl) using density functional theory. Phonon dispersion calculations confirm that the compounds are mechanically robust and dynamically stable. Analysis of the electronic band structure reveals direct band gaps in both Rb 2 NaTlBr 6 and Rb 2 NaTlCl 6 . These gaps increase significantly when the modified Becke–Johnson approximation (mBJ) is used instead of the standard PBE−generalized gradient approximation (GGA) approximation. For Rb 2 NaTlBr 6 , for example, the band gap rises from 0.76 eV (GGA) to 1.64 eV (mBJ) and then to 2.95 eV with the inclusion of spin‐orbit coupling (SOC). Similarly, Rb 2 NaTlCl 6 's band gap increases from 1.70 eV (GGA) to 4.46 eV (mBJ) and then slightly decreases to 4.43 eV when mBJ‐GGA + SOC is included. The compounds also demonstrate strong optical absorption in the visible spectrum and favorable thermoelectric performance, suggesting their potential for use in energy conversion applications.