Effects of high and very high pressure

Today, in diamond cells, pressures in excess of 10 GPa are commonly achieved. By very high pressures, we mean those in excess of 100 MPa (1,000 bar); it should be noted that the relationships in this article apply at atmospheric pressure. Numerous phenomena have been discovered and studied in this field. Polymorphic and electronic transitions have been observed, as have the appearance of metallic and sometimes superconducting phases at low temperatures. In the field of synthesis, the most important developments have been the synthesis of diamond from graphite and the synthesis of cubic boron nitride. High pressures are also important in geophysics and astrophysics. The Earth and planets are very high-pressure environments, where numerous transitions can occur in the constituent elements.

Matter reacts to pressure in ways that are often unusual, and which we need to be aware of, as asymmetries and irregularities of shape and structure can initiate reactions in parts subjected to pressures commonly reached. Matter may be in liquid or gaseous form at low pressure, but is generally only in solid form at high pressure. A metal can be compressed to such an extent that its volume is reduced by 40% of its normal value. A solid rod of relatively soft metal undergoes constriction towards the section of least resistance, and the longitudinal results of the pressure can cause it to break, as happens on tensile test specimens. The mechanical effects exerted by pressure on a wall often manifest themselves on the opposite side; a tube tears from the outside when a fluid is compressed inside it. Compressed outwards, on the other hand (and the case is particularly clear with a tube of vitreous material), it fills up with splinters of its own substance.

Figure 1 shows the pressure-temperature diagram for argon with its three gas-liquid-solid phases. We only consider properties in the hatched regions corresponding to p > 100 MPa, i.e. those covering the dense fluid, the solid and the fluid-solid transition. We therefore exclude the properties of the dilute gas and the critical region.

The unit of pressure is the Pascal (Pa) or the Newton /(meter ² ) (N /m ² ), although other units have also been used in the past, such as the kilogram-force/square centimeter (kgf /cm ² ), the atmosphere (atm) or the bar (bar). Some conversion factors are given in table 1 .