Abstract
A model calculation of lattice thermal conductivity is presented. The method uses an effective relaxation-time τq for the phonons in a phonon mode q taking into account departure from equilibrium of the other phonon modes which participate in both U- and N-processes. While low temperature results are in agreement with most earlier works, high temperature results are, on quantitative considerations, in some disagreement with those of some previous authors. In particular, it is shown that the effect of the off-diagonal U-processes is to give a negative contribution to the conductivity at high temperatures.