ANHARMONIC INTERACTION OF LOW-FREQUENCY TRANSVERSE-WAVES IN CRYSTALS

We evaluate the relaxation time of a normal process of three-phonon of the type T + T = L for low frequency by using an approximation based on the continuum approximation. We find that the relaxation time satis fies Herring's rule. We also compare the relative strengths among thre e different types of normal processes, T + T = L, L + T = L and T + L = L for a typical case.