Arrangement of water molecules and high proton conductivity of tunnel structure phosphates, KMg 1- x H 2 x (PO 3 ) 3 · y H 2 O.

A fast proton conductor was investigated in a mixed-valence system of phosphates with a combination of large cations (K + ) and small cations (Mg 2+ ), which resulted in a new phase with a tunnel structure suitable for proton conduction. KMg 1- x H 2 x (PO 3 ) 3 · y H 2 O was synthesized by a coprecipitation method. A solid solution formed in the range of x = 0-0.18 in KMg 1- x H 2 x (PO 3 ) 3 · y H 2 O. The structure of the new proton conductor was determined using neutron and X-ray diffraction measurements. KMg 1- x H 2 x (PO 3 ) 3 · y H 2 O has a tunnel framework composed of face-shared (KO 6 ) and (MgO 6 ) chains, and PO 4 tetrahedral chains along the c -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 4 tetrahedra by hydrogen bonds and form a chain along the c -axis in the tunnel, which would provide an environment for fast proton conduction associated with water molecules. The KMg 1- x H 2 x (PO 3 ) 3 · y H 2 O sample with x = 0.18 exhibited high proton conductivity of 4.5 × 10 -3 S cm -1 at 150 °C and 7.0 × 10 -3 S cm -1 at 200 °C in a dry Ar gas flow and maintained the total conductivity above 10 -3 S cm -1 for 60 h at 150 °C under N 2 gas atmosphere.