Gallium phosphate


Gallium phosphate is a colorless trigonal crystal with a hardness of 5.5 on the Mohs scale. GaPO4 is isotypic with quartz, possessing very similar properties, but the silicon atoms are alternately substituted with gallium and phosphorus, thereby doubling the piezoelectric effect. GaPO4 has many advantages over quartz for technical applications, like a higher electromechanical coupling coefficient in resonators, due to this doubling.
Contrary to quartz, GaPO4 is not found in nature. Therefore, a hydrothermal process must be used to synthesize the crystal.

Modifications

GaPO4 possesses, in contrast to quartz, no α-β phase transition, thus the low temperature structure of GaPO4 is stable up to 970°C, as are most of its other physical properties. Around 970°C another phase transition occurs which changes the low quartz structure into another structure similar with cristobalite.

Structure

The specific structure of GaPO4 shows the arrangement of tetrahedrons consisting of GaO4 and PO4 that are slightly tilted. Because of the helical arrangement of these tetrahedrons, two modifications of GaPO4 exist with different optical rotation.

History and technical importance

based on quartz have to be cooled with water for applications at higher temperatures. Starting in 1994 it was possible to substitute these big sensors with miniaturized, non cooled ones, based on GaPO4.
Further exceptional properties of GaPO4 for applications at high temperatures include its nearly temperature independent piezo effect and excellent electrical insulation up to 900°C. For bulk resonator applications, this crystal exhibits temperature compensated cuts of up to 500°C while having Q factors comparable with quartz. Due to these material properties, GaPO4 is very suitable for piezoelectric pressure sensors at high temperatures and for high temperature microbalance.

Literature