Gallium phosphate (GaPO<sub>4</sub> or gallium orthophosphate) is a colorless trigonal crystal with a hardness of 5.5 on the Mohs scale. GaPO<sub>4</sub> is isotypic with quartz, possessing very similar properties, but the silicon atoms are alternately substituted with gallium and phosphorus, thereby doubling the piezoelectric effect. GaPO<sub>4</sub> has many advantages over quartz for technical applications, like a higher electromechanical coupling coefficient in resonators, due to this doubling. Contrary to quartz, GaPO<sub>4</sub> is not found in nature. Therefore, a hydrothermal process must be used to synthesize the crystal.
GaPO<sub>4</sub> possesses, in contrast to quartz, no ñ-ò phase transition, thus the low temperature structure (structure like ñ-quartz) of GaPO<sub>4</sub> is stable up to 970ðC, as are most of its other physical properties. Around 970ðC another phase transition occurs which changes the low quartz structure into another structure similar with cristobalite.
The specific structure of GaPO<sub>4</sub> shows the arrangement of tetrahedrons consisting of GaO<sub>4</sub> and PO<sub>4</sub> that are slightly tilted. Because of the helical arrangement of these tetrahedrons, two modifications of GaPO<sub>4</sub> exist with different optical rotation (left and right).
GaPO<sub>4</sub> does not occur in nature; thus it must be grown synthetically. Presently, only one company in Austria produces these crystals commercially.
Pressure sensors based on quartz have to be cooled with water for applications at higher temperatures (above 300ðC). Starting in 1994 it was possible to substitute these big sensors with miniaturized, non cooled ones, based on GaPO<sub>4</sub>. Further exceptional properties of GaPO<sub>4</sub> for applications at high temperatures include its nearly temperature independent piezo effect and excellent electrical insulation up to 900ðC. For bulk resonator applications, this crystal exhibits temperature compensated cuts of up to 500ðC while having Q factors comparable with quartz. Due to these material properties, GaPO<sub>4</sub> is very suitable for piezoelectric pressure sensors at high temperatures and for high temperature microbalance.