Arrangement of water molecules and high proton conductivity of tunnel structure phosphates, KMg(1−x)H(2x)(PO(3))(3)·yH(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(2x)(PO(3))(3)·yH(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(2x)(PO(3))(3)·yH(2)O. The structure of the new proton conductor was determined using neutron and X-ray diffraction measurements. KMg(1−x)H(2x)(PO(3))(3)·yH(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(2x)(PO(3))(3)·yH(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.