The study of the effects of high pressure on the properties of matter. Since most properties of matter are modified by pressure, the field of high-pressure physics encompasses virtually all branches of physics.
The “high” of high-pressure physics connotes experimental difficulty. At liquid-helium temperatures, pressures of several hundred bars are considered high. In general, however, the high-pressure range may be arbitrarily regarded as extending from about 1 kbar (100 MPa or 14,500 lb/in.2) upward to the present experimental limit. Prolonged static pressures in excess of 1 megabar (100 gigapascals or 1.45 × 107 lb/in.2) can be achieved in very small samples weighing about 1 microgram.
Transient pressures as high as about 107 bars (1000 GPa or 1.45 × 108 lb/in.2) have been attained in shock waves produced by high explosives or by projectile impact.
The major effects of high pressure on matter include diminution of volume, phase transitions, changes in electrical, optical, magnetic, and chemical properties, increases in viscosity of liquids, and increases in the strength of most solids. In general solids are less compressible than liquids, and the compressibility of both solids and liquids decreases with increasing pressure.
At high pressure many solids exhibit polymorphic phase changes, that is, a rearrangement of the atoms or molecules in the solid. There are no universally applicable rules governing the number of phase changes or the kind of phase change to be expected at high pressure, but there is a thermodynamic requirement that the phase that is stable at high pressure must have a smaller volume than the phase that is stable at low pressure. See Thermodynamic principles
Frequently, dramatic changes in physical properties result from phase changes. Ferromagnetic iron transforms to a paramagnetic form at pressures somewhat above 100 kbar (10 GPa or 1.45 × 106 lb/in.2). In the same pressure range, the semiconducting element germanium transforms into a metallic phase that has an electrical conductivity greater than a million times that of the semiconductor. Similar semiconductor-to-metal transitions at high pressure have been observed in the cases of silicon, indium arsenide, gallium antimonide, indium phosphide, aluminum antimonide, and gallium arsenide. See Semiconductor
Many phases that form at high pressure transform back to low-pressure phases as the pressure is released. However, some high-pressure phases may be retained in a metastable condition at low pressures, and some low-pressure phases can persist metastably at high pressure. Diamond, the high-pressure form of carbon, is thermodynamically unstable at room temperature and pressures below about 12 kbar (1.2 GPa or 1.74 × 105 lb/in.2). Nonetheless diamond persists indefinitely as a metastable phase at low temperatures; it transforms to the stable form, graphite, only when heated to temperature in excess of 1800°F (1000°C) at low pressure.