High-temperature, structural disorder, phase transitions, and piezoelectric properties of GaPO4

Gallium orthophosphate was studied at high temperature up to 1303 K by total neutron scattering and 1173 K by piezoelectric measurements. Rietveld refinements at 1223 K confirm the stability of the structural distortion in the alpha-quartz-type phase with an average tilt angle delta=18.8° at this temperature. In contrast, reverse Monte Carlo (RMC) refinements of total neutron scattering data indicate that, whereas the degree of structural disorder initially slowly varies over a very large temperature interval in the alpha-quartz-type phase, an increase in disorder is observed beginning above 1023 K. Piezoelectric measurements indicate that the quality factor (Q) of GaPO4 resonators remains stable up to this temperature above which the piezoelectric properties of the material degrade. This degradation can be correlated to the increase in structural disorder. RMC refinements indicate that the high-temperature beta-cristobalite-type phase at 1303 K is characterized by significant thermally induced disorder with oxygen atom density forming a continuous ring around the vector joining neighboring gallium and phosphorous atoms. Gallium phosphate may be expected to retain its piezoelectric properties up to within 200 K of the phase transition temperature and as a consequence be used in applications at temperatures slightly above 1000 K