High Proton Conductivity over a Wide Intermediate-Temperature Range in Benitoite-Type KMg _{1– x} Li _x H _x (PO ₃ ) ₃ · y H ₂ O with a Three-Dimensional Water Network within a Rigid Open Framework

A major challenge in the development of energy conversion devices utilizing hydrogen redox reactions lies in overcoming material limitations by designing compounds that concurrently exhibit both high ionic conductivity and thermal stability in the intermediate-temperature range of 150–500 °C. In this study, we synthesized the benitoite-type KMg 1– x Li x H x (PO 3 ) 3 · y H 2 O, composed of KO 6 and MgO 6 octahedra and P 3 O 9 ring units stacked along the z -axis. From the crystal structure analysis, three oxygen sites corresponding to crystallization water molecules were identified, revealing the formation of a three-dimensional water molecular network within the open framework. The water molecule occupying the O(4) site is rigidly retained within the crystal structure by coordination bonding to K + . Furthermore, since this water molecule is located adjacent to the O(2) site of the MgO 6 octahedron, where the introduced protons are presumed to reside, it is considered to play a crucial role in facilitating proton transfer from the framework to the water molecules. KMg 1– x Li x H x (PO 3 ) 3 · y H 2 O exhibits excellent thermal stability up to 800 °C. Moreover, it exhibited high proton conductivity of 10 –3 S cm –1 from room temperature to 500 °C, achieving a practical level of 10 –2 S cm –1 in the intermediate-temperature range of 150–400 °C, indicating its strong potential as a solid electrolyte for intermediate-temperature applications.