Ultrafast light-induced formation of a metastable hidden state in bismuth vanadate
Bismuth vanadate (BiVO$_4$) is a key photocatalyst for solar fuel applications, yet fundamental questions remain regarding the nature of photogenerated polaronic states and the lattice dynamics that govern its light-to-chemical pathways. Here, we use femtosecond optical pump-X-ray probe measurements to track the photoinduced electronic and structural dynamics in BiVO$_4$ across multiple length and time scales. Transient X-ray absorption spectroscopy captures sub-picosecond electron localization within VO$_4$ tetrahedra, consistent with small polaron formation, whereas time-resolved X-ray diffraction reveals a slower, multi-picosecond lattice reorganization into a hidden photoexcited state that is structurally distinct from both the monoclinic ground state and the high-temperature tetragonal phase. Supported by density functional theory, we show that hole-lattice interactions dynamically reduce the ground state monoclinic distortion, stabilizing the hidden state. Our results demonstrate that electron- and hole-lattice coupling jointly shape the excited state landscape, with implications for carrier transport, interfacial energetics, and light-to-chemical energy conversion pathways.