A Sr-Ga Oxy-Hydroxide with High Thermal Stability: Unraveling Its Characteristic Hydrogen-Bond Network

Oxy-hydroxides represent potential proton carriers for solid acid catalysts and proton conductors owing to their hydroxide-rich compositions. However, their applications in high-temperature environments are limited due to thermal instability associated with dehydration at moderate to high temperatures. Therefore, the development of oxy-hydroxides with enhanced thermal stability is of critical importance. Herein, we report the discovery of a strontium-gallium oxy-hydroxide, Sr₂Ga₃O₆(OH), with exceptional thermal stability. The Sr-Ga oxy-hydroxide was successfully synthesized via an unconventional synthesis route, "vapor hydroxidation", involving high-temperature heat treatment in highly concentrated water vapor. Structural characterization employing X-ray diffraction, neutron diffraction, and transmission electron microscopy revealed that the Sr-Ga oxy-hydroxide crystallizes in a trigonal structure (R3̅ space group) with lattice parameters a = 18.1904(2) Å and c = 7.2693(1) Å. Notably, OH^- anions are nonuniformly distributed within the crystal structure and are confined to a narrow space between two strontium sites. Thermogravimetry combined with desorption gas analysis indicated that OH^- anions are retained in the crystal structure up to approximately 850 °C. In situ infrared spectroscopy upon heating demonstrated proton redistribution via multilinked hydrogen bonds at elevated temperatures, which likely contributes to the excellent thermal stability.