Abstract
Better variational lower bounds are derived as approximate solutions of a linearized Boltzmann equation for phonons. The role of the off‐diagonal part of the linearized phonon collision operator is considered in the frame of B displaced Planck distribution for phonons. A result derived here can be expressed in terms of the Debye term and the Ziman‐limit conductivity result‐showing that the Debye term is the “zeroth approximation” of that variational lower bound.