Lattice anharmonicity of alkali metals

Frequencies and eigenvectors are obtained for alkali metals by using the force constants calculated by Shyu and Gaspari. These are used to calculate the cubic and quartic anharmonic contributions to the specific heat in the high-temperature limit with nearest-neighbor central-force interaction, in which the effect of thermal expansion is included. Calculated values of the anharmonic coefficient A, defined by Cᵥ3Nk=1+AT, are in good agreement with the experimental results. The values of A are positive for alkali metals. Calculated dispersion curves lie higher than the experimental ones. The sum Σi=1³ωᵢ²(→k) is independent of →k for sodium, potassium, and rubidium, but not for lithium. It is concluded that a potential which may be adequate in estimating the effects of anharmonicity may not be essentially suitable to describe the harmonic properties.