Y(HPO₄)(NO₃)(H₂O)·2H₂O: A Solvent-Responsive Inorganic Layered Material with Reversible Phase Transitions and Optically Active Proton Conductivity

A two-dimensional (2D) inorganic phosphate compound, Y(HPO4)(NO3)(H2O)·2H2O (YH2H), was synthesized via a slow evaporation method and structurally characterized by single-crystal X-ray diffraction. YH2H crystallizes in the orthorhombic space group, Pbcm and features a layered structure composed of edge-sharing YO8 polyhedra. The interlayer space is stabilized by two types of lattice water molecules, which form an extended hydrogen-bonding network critical to both structural integrity and functionality. Upon heating, YH2H undergoes a reversible two-step dehydration process, forming a contracted dehydrated phase (YH). Real-time in situ powder X-ray diffraction captured these sequential phase transitions. Notably, YH spontaneously rehydrates under ambient conditions, fully restoring the original framework. Methanol intercalation into the YH phase leads to the formation of an expanded intercalated phase (Y-MeOH), highlighting the material’s solvent-responsive interlayer tunability. The hydrogen-bond network in YH2H enables effective proton transport, yielding a proton conductivity of 2.28 × 10–4 S·cm–1 at 90 °C and 90% relative humidity. Unlike conventional proton-conducting materials, YH2H maintains conductivity under reduced or fluctuating humidity by autonomously reabsorbing water, eliminating the need for continuous external humidification. In addition to its structural and ionic responsiveness, YH2H exhibits diverse optical functionalities, including ultraviolet transparency, high birefringence, and ligand-centered phosphorescence. This multifunctionality, coupled with reversible interlayer modulation, establishes YH2H as a rare example of a fully inorganic, interlayer-engineerable 2D material with broad potential for solid-state ionic devices, photonics, and optoelectronic applications.