  With the constraint from Onsager reciprocity relations, here we generalize
the Klemens model for phonon-phonon Umklapp scattering from isotropic to
anisotropic. Combined with the anisotropic Debye dispersion, this anisotropic
Klemens model leads to analytical expressions for heat transfer along both
ab-plane (k_ab) and c-axis (k_c), suitable for both layered and chainlike
materials with any anisotropy ratio of the dispersion and the scattering. The
model is justified by comparison with experimental k_ab and k_c of bulk
graphite at high temperatures, as well as the thickness-dependent k_c of
graphite thin films at room temperature.
