Origin of Tetragonal Distortion In New Lead-Free Perovskite Compounds of Na_1/2Bi_1/2VO₃ and K_1/2Bi_1/2VO₃

Tetragonal perovskite oxides have emerged as an ideal platform for the rational design and targeted fabrication of high-performance piezoelectric materials, benefiting from their distinctive structural features that facilitate the construction of advanced piezoelectric systems with morphotropic phase boundary (MPB) structures. As a well-recognized core structural characteristic in piezoelectric ceramics, MPB can effectively modulate lattice distortion states and optimize domain switching behaviors, thereby remarkably improving the electromechanical response of piezoelectric ceramic materials. Nevertheless, the limited number of exploitable tetragonal perovskite parent materials greatly constrains the innovative design and in-depth development of high-efficiency, environmentally friendly lead-free piezoelectric systems. Recently, two novel lead-free tetragonal perovskite compounds, Na_1/2Bi_1/2VO₃ (c/a = 1.085) and K_1/2Bi_1/2VO₃ (c/a = 1.054), have been successfully discovered. Impressively, their tetragonal distortion degrees are highly matched with that of the classic high-performance piezoelectric parent material PbTiO₃ (c/a = 1.064). The moderate c/a ratios of the two vanadium-based perovskites endow them with excellent potential as qualified matrix phases to construct MPB structures with rhombohedral perovskites, opening a novel and feasible pathway for exploring advanced lead-free piezoelectric materials. However, the intrinsic microscopic physical mechanism responsible for the tetragonal distortion of Na_1/2Bi_1/2VO₃ and K_1/2Bi_1/2VO₃ remains ambiguous, severely restricting their further structural modification and performance optimization. In this work, the two target lead-free tetragonal perovskites were fabricated via a high-pressure and high-temperature synthetic route. Their crystal structures were systematically characterized, and the essential origin of tetragonal lattice distortion was deeply explored. Combined synchrotron X-ray absorption spectroscopy measurements and first-principles calculations confirm that the d¹ electronic configuration of V⁴⁺ ions dominates the tetragonal distortion of the two vanadium-based perovskites. This study clarifies the microscopic lattice tetragonality mechanism of the as-prepared perovskites and provides a reliable design strategy for developing novel tetragonal perovskites and high-performance lead-free piezoelectric materials through V⁴⁺ ion electronic state modulation.