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 Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, possessing very similar properties, but the

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...

atoms are alternately substituted with gallium and

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

, thereby doubling the

piezoelectric effect Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...

. 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 Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

process must be used to synthesize the crystal.

Modifications

GaPO₄ possesses, in contrast to quartz, no α-β

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...

, 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 Cristobalite is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the ...

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 An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...

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