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

We present a comprehensive investigation of the low-temperature topochemical modification of the Ruddlesden-Popper-type (RP) indium oxyfluoride LaBaInO₃F₂ using sodium hydride (NaH). By varying NaH equivalents, we monitored phase evolution, anion exchange, and reduction pathways using combined structural, spectroscopic, elemental and the density functional theory (DFT) analysis. LaBaInO₃F₂ undergoes controlled anion-extraction, with hydride-for-fluoride substitution produces mixed-anion oxyfluoride-hydride phases, LaBaInO₃F_2-xH_y, and NaF as a stable byproduct. At moderate xNaH contents (x ≤ 1), the RP-framework incorporates hydride with minimal decomposition, supported by reoxidation experiments and DFT-calculated reaction enthalpies that confirm oxidizable hydride and subtle modification of the indium coordination environment. In contrast, higher xNaH contents (x > 1) result in the formation of secondary phases, indicating progressive lattice degradation. X-ray photoemission spectroscopy (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 In³⁺, which is known to be notoriously hard otherwise. Optical spectroscopy reveals bandgap narrowing and enhanced visible-light absorption in hydride-rich samples, however, DFT estimates that idealized hydride substitution widens the bandgap, consistent with the experimentally non-ideal reduction with secondary phases. Overall, these findings establish hydride-based topochemical strategies as a promising route for controlled anion exchange with implications for photocatalytic and energy applications.