Arrangement of water molecules and high proton conductivity of tunnel structure phosphates, KMg H (PO)·HO.

A fast proton conductor was investigated in a mixed-valence system of phosphates with a combination of large cations (K) and small cations (Mg), which resulted in a new phase with a tunnel structure suitable for proton conduction. KMg H (PO)·HO was synthesized by a coprecipitation method. A solid solution formed in the range of = 0-0.18 in KMg H (PO)·HO. The structure of the new proton conductor was determined using neutron and X-ray diffraction measurements. KMg H (PO)·HO has a tunnel framework composed of face-shared (KO) and (MgO) chains, and PO tetrahedral chains along the -direction by corner-sharing. Two oxygen sites of water molecules were detected in the one-dimensional tunnel, one of which exists as a coordination water of K sites. Multi-step dehydration was observed at 30 °C and 150 °C from thermogravimetric/differential thermal analysis measurements, which reflects the different coordination environments of the water of crystallization. Water molecules are connected to PO tetrahedra by hydrogen bonds and form a chain along the -axis in the tunnel, which would provide an environment for fast proton conduction associated with water molecules. The KMg H (PO)·HO sample with = 0.18 exhibited high proton conductivity of 4.5 × 10 S cm at 150 °C and 7.0 × 10 S cm at 200 °C in a dry Ar gas flow and maintained the total conductivity above 10 S cm for 60 h at 150 °C under N gas atmosphere.