Gallium phosphate (GaPO
4 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 ...
. GaPO
4 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 form ...
, 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 tab ...
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
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 Stress (mechanics), mechanical s ...
. GaPO
4 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 ...
in
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, GaPO
4 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
GaPO
4 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 of ...
, 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
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 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
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
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
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 GaPO
4.
Further exceptional properties of GaPO
4 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 curren ...
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
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