Solid
one of the four states of aggregation of matter, the others being liquid, gas, and plasma. Substances in the solid state, or solids, are distinguished by stability of shape and by the nature of the thermal motion of their atoms, which perform small oscillations about equilibrium positions. In addition to the crystalline state of solids (seeCRYSTALS), there exists the amorphous state, which includes the vitreous, or glassy, state. Crystals are characterized by long-range order in the arrangement of atoms. In amorphous solids, long-range order is absent (seeLONG-RANGE AND SHORT-RANGE ORDER).
The laws of classical physics are applicable to most solids. According to these laws, the lowest energy state of a system of atomic particles (atoms, ions, or molecules) is associated with a periodic arrangement of identical groups of particles—that is, with a crystalline structure. From the thermodynamic point of view, therefore, the amorphous state is not an equilibrium state and should, over a period of time, crystallize. Under ordinary conditions, however, this time may be so great that the amorphous solid does not exhibit its nonequilibrium nature and is practically stable. The difference between a crystalline solid and a liquid is qualitative: the crystal has, and the liquid lacks, long-range order in the arrangement of atoms. The difference between an amorphous solid and a liquid is only quantitative: an amorphous solid can be regarded as a liquid with a very high viscosity, which may often be assumed to be infinitely large.
The concept of a solid, like that of a liquid, is an idealization. It would be more accurate to speak of the solid and liquid properties of a condensed medium. From the standpoint of elastic properties, for example, a substance with a nonzero static shear modulus θ should be considered a solid; for a liquid, θ = 0. With respect to plastic properties, a substance that is irreversibly de-formable only under a finite applied stress above a certain threshold value should be considered a solid; for liquids, even highly viscous, resin-type liquids, the threshold stress of irreversible deformation is zero.
All substances in nature solidify at atmospheric pressure and a temperature T > 0°K, with the exception of He, which at atmospheric pressure remains liquid down to T = 0°K. A pressure of 24 atmospheres (atm), at T = 1.5°K, is necessary for the crystallization of He. This unique property of He is explained in the quantum theory of solids and liquids (seeHELIUM and QUANTUM FLUID).
A special state of matter that is intermediate between a crystal and a quantum fluid has been detected in the investigation of solid solutions of helium isotopes under pressure. This state is called a quantum crystal. In ordinary crystals, the wave properties of the atoms lead to the existence of lattice vibrations at T = 0°K. In quantum fluids, these properties completely destroy the crystal structure. In quantum crystals, the wave properties of the atoms preserve the distinctness of the lattice sites but allow the atoms to move from site to site.
Solids are the principal materials used by man. Various properties of solids have been made use of in all the devices created by man, from the flint implements of Neanderthal man to modern machines and mechanisms. In the early stages of the development of civilization, such directly perceptible mechanical properties of solids as hardness, weight, plasticity, elasticity, and brittle-ness were made use of, and solids were employed only as structural materials. Modern technology makes use of a tremendous range of physical properties of solids—such as electrical, magnetic, and thermal properties—that are generally inaccessible to direct human perception and can be detected only in laboratory investigations.