Anion Exchange in LaBaInO3F2: Structural and Optical Effects of Low-Temperature Topochemical Modification

We present a comprehensive investigation of topochemical modification in the Ruddlesden–Popper-type (RP) indium-based oxyfluoride LaBaInO3F2, using low-temperature sodium hydride (NaH) treatment. By systematically varying NaH equivalents, we monitored the phase evolution, anion exchange, and reduction pathways using powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), diffused-reflectance spectroscopy (DRS), and elemental hydrogen analysis. The results reveal that LaBaInO3F2 undergoes a controlled anion-extraction, with hydride-for-fluoride substitution producing mixed-anion oxyhydride phases, LaBaInO3F2−xHy, and NaF as a stable by-product. At moderate NaH contents (x ≤ 1), the RP framework incorporates hydride with minimal decomposition, as confirmed by partial structural recovery upon re-oxidation, supporting the presence of oxidizable hydride and subtle modification of the indium coordination environment. In contrast, higher NaH equivalents (x > 1) result in the formation of secondary phases, indicating progressive lattice degradation. XPS indicates the possibility of subtle reduction of indium, with redox changes secondary to anion exchange. The executed study revealed a pathway how to stabilize hydride ions next to indium, which is known to be notoriously hard otherwise. Optical spectroscopy reveals bandgap narrowing and enhanced visible-light absorption in hydride-rich samples. Overall, these findings establish hydride-based topochemical strategies as a promising route for controlled anion exchange and electronic structure tuning in layered indium oxyfluorides, with implications for photocatalytic and energy applications.