Local Atomic Structure of Molybdenum-Based Vacancy-Ordered Perovskites: A Direct Evidence of Oxyhalide Formation and Selective Doping

Doping in lead-free vacancy-ordered perovskites (A2BX6) 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 Cs2MoOxCl6−x (1.3 < x < 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 (Cs2AgBiCl6) via Ag+/Bi3+ codoping revealed selective dopant incorporation. While Bi3+ 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+ incorporation is fundamentally hindered due to the unfavorably large ionic radius of Ag+. At higher dopant concentrations, the system undergoes phase segregation into discrete Cs2MoOxCl6−x and Cs2AgBiCl6 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.