Ionic conduction and negative thermal expansion of (H3O)Zr2(PO4)3

(NH4)Zr2(PO4)3 powder synthesized by hydrothermal reaction was heat-treated at 600 °C to obtain HZr2(PO4)3 powder, which was then autoclaved in deionized water at 150 °C to prepare (H3O)Zr2(PO4)3 powder. Thermal analysis showed that (H3O)Zr2(PO4)3 was stable up to around 120 °C, and high-temperature X-ray measurements revealed that it was in the (H3O)Zr2(PO4)3 state under 150 °C, coexisted with (H3O)Zr2(PO4)3 and HZr2(PO4)3 between 200 and 400 °C, and was in the HZr2(PO4)3 state above 500 °C. Ionic conductivity measurements were performed on (H3O)Zr2(PO4)3 from 50 to 100 °C while continuously applying a pressure of 100 MPa. The ionic conductivity at 100 °C was found to be 6.7×10?7 S?cm?1, and the apparent activation energy for ionic conduction calculated from the slope of the Arrhenius plot was 23.0 kJ･mol?1. The lattice constants of (H3O)Zr2(PO4)3 were refined using high-temperature XRD measurements in air from 30 to 100 °C and Rietveld analysis. It was confirmed that both the a- and c-axis of the lattice constants contract as the temperature increases, making it a material that exhibits low or negative thermal expansion.