First-Principles evaluation of Lead-Free X2NaIO6 (X=Sr, Ba) Double-Perovskite: For optoelectronic and solar cell applications

Abstract

The standard lead-free double perovskite X₂NaIO₆ has become a viable alternative for lead-based perovskites due to its remarkable optoelectronic capabilities, high stability, non-toxicity, and multifunctionality. This study is presented to evaluate the structural, electronic, optical, and mechanical properties of X₂NaIO₆ (X=Sr, Ba) by applying a first-principles method based on density functional theory (DFT). These investigations were carried out at the level of the generalized gradient approximation (GGA) for the exchange–correlation functional, parameterized by Perdew Burke and Erenzerhof (PBE) within the CASTEP code. The tolerance factors (τ) for Sr₂NaIO₆ and Ba₂NaIO₆ are 0.98 and 1.0, confirming the cubic nature of both compounds. Both compounds Sr₂NaIO₆ and Ba₂NaIO₆ have direct narrow bandgap values of 1.57 eV and 1.42 eV, respectively. We also presented a comprehensive study of their optical properties. Shear modulus (G), Poisson’s ratio (v), elastic coefficient (C_ij), anisotropy (A), bulk moduli (B), Young’s modulus (Y), and Pugh’s ratio (B/G) are computed. The combined electronic, optical, and mechanical investigations show that both materials are suitable for optoelectronic, photovoltaic, and solar cell applications.