Cs2KAlY6 (Y = Cl, Br, I): A lead-free double perovskites for optoelectronic and photocatalytic applications

This research work investigates the structural, elastic and mechanical, electronic, optical, acoustic, and thermal properties of the cubic, lead-free double perovskites Cs₂KAlCl₆, Cs₂KAlBr₆, and Cs₂KAlI₆. The Full-potential linearized augmented plane wave (FP-LAPW) formulism based on density functional theory (DFT) is employed using WIEN2k simulation code. These perovskites strongly fulfill the Gold-Schmidt tolerance (t_G) and octahedral (μ_o) factor conditions. The negative values of formation enthalpy ($∆$H_f) demonstrate their thermodynamic stability. However, the Born’s stability criteria, the elastic and mechanical features, confirms its elastic and mechanical stability. The Cs₂KAlCl₆ and Cs₂KAlBr₆ are ductile, while Cs₂KAlI₆ is brittle, and all the three halide double perovskites exhibit anisotropic behavior, as indicated by their Pugh's & Poisson ratios, Cauchy-pressure and anisotropic factors. The acoustic behaviors of these compounds are explored through sound velocities estimations. Thermal performances are analyzed with Debye & melting temperature, and thermal expansion coefficient. It is evident from the electronic properties, these perovskites are direct band semiconductors having band gap values 4.38, 3.04, and 1.71 eV for Cs₂KAlCl₆, Cs₂KAlBr₆ and Cs₂KAlI₆ respectively. A stepwise tuning of the band gap is observed, driven by the increase in ionic size moving down the halogen group from Cl to Br to I, which gradually reduces the band gap. The threshold values of optical parameters for Cs₂KAlCl₆/(Cs₂KAlBr₆ and Cs₂KAlI₆) start from ultraviolet/(visible light) range and both go on maximum in the ultraviolet region. Hence, offer an open range to be used in optoelectronic and photovoltaic renewable energy applications. Moreover, the alignment of valence and conduction band edge potentials relative to the standard redox potentials of water splitting make these compounds a promising choice for hydrogen production through photocatalysis.