Local atomic structure of molybdenum-based vacancy-ordered perovskites: A direct evidence of oxyhalide formation and selective doping

Abstract

Doping in lead-free vacancy-ordered perovskites (A sub(2)BX sub(6)) is a primary strategy for modulating their optoelectronic properties; however, the precise local atomic arrangements and the feasibility of doping-induced structural transformations remain poorly understood, as global structural techniques often fail to capture local variations. In this work, utilizing extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculations, we probe the local atomic structure of the Mo-based vacancy-ordered perovskite and demonstrate that the material preferentially adopts an oxyhalide configuration, identified as Cs sub(2)MoO sub(x)Cl sub(6-x) (1.3 less than x less than 1.5), which possesses a significantly more favorable formation energy than the pure halide phase. Attempts to drive a structural transition toward a double perovskite (Cs sub(2)AgBiCl sub(6)) via Ag sup(+)/Bi sup(3+) codoping revealed selective dopant incorporation. While Bi sup(3+) successfully substitutes for Mo within the octahedral environment-a process confirmed by EXAFS and supported by a high positive Bader charge transfer (+2.73 e)-Ag sup(+) incorporation is fundamentally hindered due to the unfavorably large ionic radius of Ag sup(+). At higher dopant concentrations, the system undergoes phase segregation into discrete Cs sub(2)MoO sub(x)Cl sub(6-x) and Cs sub(2)AgBiCl sub(6) phases rather than doping. This study establishes that understanding the local atomic coordination is indispensable for rational dopant engineering in low-dimensional halide systems where bulk structural techniques are insufficient.