GaPO4 Stiffness and Piezoelectric Constants Measurements using the Combined Thickness Excitation and Lateral Field Technique

Gallium orthophosphate (GaPO₄) is now a commercially available piezoelectric crystal with great potential as high temperature gas, temperature and pressure sensors for a number of challenging harsh environment applications in the automotive, aerospace, gas and oil well industries. The higher piezoelectric coefficients of GaPO₄ with respect to quartz (about 1.2 times higher for e₁₁ 2.5 times higher for e₁₄) and the existence of temperature compensated orientations for bulk acoustic waves (BAW) and surface acoustic waves (SAW) are positive characteristics for communications and frequency control applications. In addition, SAW phase velocities along temperature compensated orientations circa 30% lower than that found on ST-X quartz have been predicted, which favors more compact devices. Naturally, the precise determination of any crystal orientation for applications in frequency control, communication, or sensing relies on the measurement of the acoustic properties of the material. In particular, special attention must be devoted to the room temperature measurements performed, otherwise the temperature coefficients, albeit precise, will not result in the correct identification of the temperature compensated orientations. In this work, the elastic and piezoelectric constants of GaPO₄ are measured using the combined thickness excitation and lateral field excitation methods, through which multiple harmonics are measured and verified for consistency. Piezocryst Advanced Sensorics GmbH, Graz, Austria, provided the five different plate orientations used in the measurements, namely X, Y, plusmn45deg Y-rotated, and Z cuts. Based on relative precision better than 10^-3 for the thickness measurements and 10^-4 for the frequency measurements, the resulting phase velocities obtained also have uncertainties in the 10^-3 range. The comparison between GaPO₄ BAW phase velocities previously- published in the literature and the results obtained from this work indicate discrepancies ranging from 0.1 to 3.2%. For instance, along the Y-cut maximum discrepancies around 30 m/s are observed for the quasi longitudinal and quasi fast shear waves, and discrepancies around 55 m/s for the pure shear wave. Consistent e₁₁ values have been extracted from X and Y cuts, which are within 10% of those reported in the literature. The complete set of stiffness and piezoelectric constants measured, the errors involved, and the comparison with the literature are given and discussed in the paper