Designing stable bromide perovskites for energy conversion: d-orbital engineering and molecular dynamics study of K2MBr6 (M = Sn, Ti, Pd, Pt)

Abstract

Lead-free vacancy-ordered double perovskites K₂MBr₆ (M = Sn, Ti, Pd, Pt) were systematically investigated using density functional theory (DFT) to assess their structural stability, electronic behavior, optical activity, and photocatalytic potential for energy-conversion applications. Structural optimization using the WC-GGA functional confirms that all compounds crystallize in the cubic Fm-3m phase with negative formation energies, dynamically stable phonon spectra, and robust thermal behavior supported by ab-initio molecular dynamics simulations at 300 K. Electronic properties obtained using the TB-mBJ potential with spin–orbit coupling reveal a transition from direct band gaps for K₂SnBr₆ and K₂TiBr₆ (Γ→Γ) to indirect band gaps for K₂PdBr₆ and K₂PtBr₆ (X→X), reflecting the influence of d-orbital contributions across the series. Optical calculations indicate strong absorption coefficients in the visible region, linked to Br-p and M-d electronic transitions. Mechanical analysis confirms elastic stability and moderate ductility for all compositions. Photocatalytic feasibility was evaluated using absolute band-edge positions derived from the Mulliken electronegativity model, showing that all compounds possess sufficiently high VBM levels to drive the oxygen evolution reaction, while their CBM positions lie above the H⁺/H₂ reduction potential, indicating that they function as oxidation-active semiconductors suitable for photoanodes or Z-scheme configurations. These results position K₂MBr₆ perovskites as promising lead-free materials for optoelectronic and photocatalytic applications, with tunable properties governed by d-orbital engineering at the B-site cation.