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 piezo electric 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.

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Modifications

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

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

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

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

G. Gautschi: „Piezoelectric Sensorics“, published by Springer