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 degreesC up to 4.5 GPa and at 1.5 GPa up to 800 degreesC. The results show a pressure-enhanced effect on Hf4+ ion conduction. At 400 degreesC, the conductivity increases with pressure from 1 atm value of 1.13 x 10(-5) (Omega cm)(-1) to a maximum value of 1.76 x 10(-4) (Omega cm)(-1) at similar to 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 degreesC and 1.34 eV for T > 500 degreesC. In comparison with the I atm data, the application of high pressure clearly enhances the ionic conductivity by a factor of similar to 10 times at 300 degreesC, similar to 40 times at 600 degreesC and similar to 160 times at 700 degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.