It has been believed for some time that anharmonicity of lattice vibrations is responsible for the continued rise of the specific heats of certain solids in the classical temperature region. A general analysis is here carried out of the linear chain model interacting through a Morse potential. All the major thermal properties, such as specific heats, thermal expansion, and compressibility are derived, and a tentative comparison is made with observed properties of the alkali metals.

It is found that the linear chain then exists in two widely different states with properties characteristic of condensed and gas-like phases; these two states are separated by a relatively narrow transition region in temperature where the specific heat passes through a rather sharp maximum (except at high pressures when the maximum ultimately disappears).

• Received 1 June 1954

DOI:https://doi.org/10.1103/PhysRev.96.57