Abstract:
The anharmonic contributions of order A6 to the Helmholtz free
energy for a crystal in which every atom is on a site of inversion
symmetry, have been evaluated The cor~esponding diagrams in the
various orders of the perturbation theory have been presented The
validity of the expressions given is for high temperatures. Numerical
calculations for the diagrams which contribute to the free energy have
been worked out for a nearest-n~ighbour central-force model of a facecentered
cubic lattice in the high-temperature limit and in the leading
term and the Ludwig approximations. The accuracy of the Ludwig approximation
in evaluating the Brillouin-zone sums has been investigated.
Expansion for all diagrams in the high-temperature limit has been
carried out The contribution to the specific heat involves a linear
as well as cubic term~ We have applied Lennard-Jones, Morse and
Exponential 6 types of potentials. A comparison between the contribution
to the free energy of order A6 to that of order A4 has been made.
