Contents

Gallium phosphate

Gallium phosphate (GaPO₄ or gallium orthophosphate) 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 coefficientinresonators, 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[edit]

GaPO₄ possesses, in contrast to quartz, no α-β phase transition, thus the low temperature structure (structure like α-quartz) 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[edit]

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 (left and right).

Sources[edit]

GaPO₄ 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[edit]

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 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[edit]

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

External links[edit]

• data sheet