Pressure-enhanced tetravalent Hf4+ ion conduction in HfNb(PO4)3

High pressure, high temperature ac impedance measurements of a recently identified tetravalent cation (Hf4+) conductor, HfNb(PO4)3, have been carried out at 400 °C up to 4.5 GPa and at 1.5 GPa up to 800 °C. The results show a pressure-enhanced effect on Hf4+ ion conduction. At 400 °C, the conductivity increases with pressure from 1 atm value of 1.13×10−5 (Ω cm)−1 to a maximum value of 1.76×10−4 (Ω cm)−1 at ∼1.5 GPa. At 1.5 GPa, the ionic conductivity increases with the increasing temperature, showing two regions with different activation energies, i.e. 0.67 eV for T<500 °C and 1.34 eV for T>500 °C. In comparison with the 1 atm data, the application of high pressure clearly enhances the ionic conductivity by a factor of ∼10 times at 300 °C, ∼40 times at 600 °C and ∼160 times at 700 °C.