Gallium phosphate (GaPO₄ or gallium orthophosphate) is a colorless trigonal crystal with a hardness of 5.5 on the

Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

. GaPO₄ is isotypic with quartz, possessing very similar properties, but the silicon atoms are alternately substituted with

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...

and phosphorus, thereby doubling the piezoelectric effect. GaPO₄ has many advantages over quartz for technical applications, like a higher

electromechanical coupling coefficient The electromechanical coupling coefficient is a numerical measure of the conversion Energy conversion efficiency, efficiency between electrical and Acoustics, acoustic energy in piezoelectric materials. Qualitatively the electromechanical coupling ...

resonators A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...

, 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 (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 Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

( 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 A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually act ...

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 A microbalance is an instrument capable of making precise measurements of weight of objects of relatively small mass: of the order of a million parts of a gram. In comparison, a standard analytical balance is 100 times less sensitive; i.e. it is ...

Literature

External links

data sheet
{{Phosphates Gallium compounds Phosphates Piezoelectric materials