A-site cation engineering of lead-free M2NiCl6 (M = Na, K, Cs) vacancy-ordered double perovskites: first-principles insights into stability, band-gap tuning, excitonic behavior, and optical response

Abstract

The growing global energy demand and the environmental consequences of fossil-fuel consumption underscore the urgent need to develop sustainable, efficient, and environmentally benign energy materials. Lead-free vacancy-ordered double halide perovskites M2NiCl6 (M = Na, K, Cs) were systematically investigated using first-principles calculations to evaluate their structural stability, mechanical reliability, thermophysical behavior, electronic structure, and optical response. The optimized compounds crystallize in the cubic Fm3̄m phase with stable [NiCl6]2− octahedral frameworks. Negative formation and binding energies confirm their thermodynamic stability, while tolerance and octahedral factors support their structural feasibility. Elastic analysis verifies mechanical stability for all compounds, with K2NiCl6 showing ductile behavior and the highest fracture toughness, whereas Cs2NiCl6 exhibits superior stiffness, hardness, and dimensional stability. The low elastic moduli and thermal conductivities indicate mechanically soft and thermally insulating characteristics. Electronic band-structure calculations reveal indirect semiconducting behavior, with TB-mBJ band gaps of 1.883, 1.792, and 1.083 eV for Na2NiCl6, K2NiCl6, and Cs2NiCl6, respectively. PDOS analysis shows that Ni-3d and Cl-3p states dominate the band edges, while enhanced Cs-4p hybridization narrows the band gap. Strong visible-to-near-infrared absorption, low reflectivity, and tunable optical conductivity suggest promising potential for photovoltaic, photodetector, and optoelectronic applications.