Strain induced phase transitions, superior optoelectronics, and photocatalytic potential of Ba2GdNbO6: promising material for solar hydrogen production.

Strain engineering provides an effective route for tuning the multifunctional features of materials, thereby enhancing their potential for spintronic and energy-conversion applications. In this work, we investigate the effects of hydrostatic ([111]) strain on the structural, mechanical, magnetic, electronic, photocatalytic, and optical aspects of the Ba2GdNbO6 structure using first-principles calculations within GGA + U (generalized gradient approximation + Hubbard parameter) and GGA + U + SOC (spin-orbit coupling) frameworks. The negative formation enthalpies, elastic constants, and ab initio molecular dynamics simulations confirm the thermodynamic, mechanical, and thermal stability of the structures. Further, vibrational free-energy calculations reveal that tensile strain becomes thermodynamically favorable at elevated temperatures due to enhanced vibrational entropy. It is predicted that unstrained compound exhibits an indirect band gap (E g) of 3.27/3.26 eV with a stable ferromagnetic (FM) ground state within the GGA + U/GGA + U + SOC scheme. Interestingly, under compressive strain, a magnetic transition from the FM to antiferromagnetic (AFM) ground state occurs, accompanied by an electronic indirect-to-direct E g transition. In contrast, tensile strains preserve the FM ground state having lower indirect E g as is found in the unstrained one. Moreover, charge-density, electron localization function, and Bader analyses reveal enhanced metal-oxygen charge transfer under compression and reduced orbital overlap under tension while preserving predominantly ionic bonding. Notably, the system exhibits near ultraviolet light-harvesting, achieving absorption coefficients as high as 1.26/1.18/1.10 × 106 cm-1 at 3.26 eV, 1.48/1.44/1.42 × 106 cm-1 3.26 eV, and 1.36/1.37/1.36 × 106 cm-1 2.87 eV along the xx-/yy-/zz-axis for the -5%, 0%, and +5% strained systems, respectively, highlighting its excellent optical response. Even more strikingly, the band-edge potentials robustly straddle the water redox couples across the entire strain window with valence/conduction band edge potentials of 2.39/-0.88, 2.39/-0.87, and 2.18/-0.67 V (vs. normal hydrogen electrode) at -5%, 0%, and +5% strain, respectively, providing sufficient overpotential for spontaneous hydrogen and oxygen evolution reactions and establishing Ba2GdNbO6 as a strain-resilient photocatalyst for solar-driven water splitting.