Gallium phosphate
# Gallium phosphate

Gallium phosphate $(GaPO_4$ or gallium orthophosphate) is a colorless trigonal crystal with a hardness of 5.5 on the Mohs scale. $GaPO_4$ is isotypic with quartz, possessing very similar properties, but the silicon atoms are alternately substituted with gallium and phosphorus, thereby doubling the piezoelectric effect. $GaPO_4$ has many advantages over quartz for technical applications, like a higher electromechanical coupling coefficient in resonators, due to this doubling. Contrary to quartz, $GaPO_4$ is not found in nature. Therefore, a hydrothermal process must be used to synthesize the crystal.

## Modifications

$GaPO_4$ possesses, in contrast to quartz, no α-β phase transition, thus the low temperature structure (structure like α-quartz) of $GaPO_4$ 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 $GaPO_4$ shows the arrangement of tetrahedrons consisting of $GaO_4$ and $PO_4$ that are slightly tilted. Because of the helical arrangement of these tetrahedrons, two modifications of $GaPO_4$ exist with different optical rotation (left and right).

## Sources

$GaPO_4$ does not occur in nature; thus it must be grown synthetically. Presently, only one company in Austria produces these crystals commercially.

## History and technical importance

Pressure sensors based on quartz have to be cooled with water for applications at higher temperatures (above 300°C). Starting in 1994 it was possible to substitute these big sensors with miniaturized, non cooled ones, based on $GaPO_4.$ Further exceptional properties of $GaPO_4$ 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, $GaPO_4$ is very suitable for piezoelectric pressure sensors at high temperatures and for high temperature microbalance.

## Literature

* Gautschi, Gustav (2013-06-29). Piezoelectric Sensorics: Force Strain Pressure Acceleration and Acoustic Emission Sensors Materials and Amplifiers. ISBN 978-3-662-04732-3.
