High-pressure chemistry

(physical chemistry) The study of chemical reactions and phenomena that occur at pressures exceeding 10,000 bars (a bar is nearly equivalent to a kilogram per square centimeter), mainly concerned with the properties of the solid state.

Chemistry at very high pressures, arbitrarily chosen to be above 10⁴ bars (1 gigapascal), and mainly concerned with solid and liquid states. At 25°C (77°F) and 10⁴ bars (1 GPa), nearly all ordinary gases are liquid or solid, and only a few liquids are not frozen; thus most high-pressure chemistry involves either higher temperatures, at which chemical reactions can occur at appreciable rates, or studies of internal arrangements in solids.

From 1 bar (10² kilopascals) to about 10⁵ bars (10 GPa), normal low-pressure chemical behavior prevails, and only minor departures from the usual valence and coordination rules are found. However, many interesting changes in materials can be effected in this pressure range as atoms are forced into new bonding arrangements. From 10⁵ to 10⁹ bars (10 to 10⁵ GPa), the energy added by compression becomes comparable with chemical bond energies, so that outer-shell electronic orbits are distorted and atoms and molecules change in character. A general tendency toward more metallic behavior is observed as the electrons become less strongly fixed to particular atoms, and chemical bonds may be broken. Upward of about 10⁹ bars (10⁵ GPa), the delocalization of electrons is extensive, and the material consists of a mixture of ions and electrons, so that chemical bonds are of little importance. The boundaries on these three pressure ranges are, of course, only approximate, and show some variation according to the temperature and the atoms involved.

The simplest effect of high pressure is the closer compression of atoms. The noble gases and alkali metals are quite compressible, whereas most oxides and the stronger metals are considerably stiffer. However, at a pressure exceeding about 10⁵ bars (10 GPa), most of the easily compressed electronic clouds are tightened up, and the compressibilities of most substances approach each other.

Substances which consist of large molecules are easily stiffened or frozen by high pressures. The mobility of the molecules is sharply decreased by a sort of interlocking and tangling effect; thus for the substance to be sheared, chemical bonds must be broken, a process which requires considerable energy. This stiffening phenomenon limits the study of most reactions of organic molecules to low pressures because they are rather large and “freeze” easily, but yet are usually not stable enough to withstand the temperatures necessary for liquefaction or intermolecular reactions.