Phonon theory of the entropy of liquid sodium

The Percus-Yevick phonon theory of liquids is reappraised and extended in the light of recent developments, and calculations are performed for the entropy of liquid sodium. The harmonic contribution is assumed to arise entirely from density fluctuations, the wavenumbers for which enlarge the usually defined Debye sphere from its original value of ksub(D) to the new value of k0 = 3sup(1/3)ksub(D). Most of the entropy comes from this extended region (ksub(D), k0). The anharmonic contribution arises mainly from the interaction of phonon pairs with large (approximately 2k0) wavenumber difference. The natural description of these effects is through the static structure factor, a(k). A connection is, thereby, in some measure made with the alternative effective hard sphere packing theory of entropy in which the emphasis is on the region around the principal peak of a(k). (author)