High-temperature, structural disorder, phase transitions, and piezoelectric properties ofGaPO4

Abstract

Gallium orthophosphate was studied at high temperature up to $1303\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by total neutron scattering and $1173\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by piezoelectric measurements. Rietveld refinements at $1223\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ confirm the stability of the structural distortion in the $\ensuremath{\alpha}$-quartz-type phase with an average tilt angle $\ensuremath{\delta}=18.8\ifmmode^\circ\else\textdegree\fi{}$ 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 $\ensuremath{\alpha}$-quartz-type phase, an increase in disorder is observed beginning above $1023\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Piezoelectric measurements indicate that the quality factor $(Q)$ of $\mathrm{Ga}\mathrm{P}{\mathrm{O}}_{4}$ 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 $\ensuremath{\beta}$-cristobalite-type phase at $1303\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of the phase transition temperature and as a consequence be used in applications at temperatures slightly above $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.