where a is the lattice constant a = n for a cubic lattice, and n is the atomic number density. Slack phonon conductivity model mainly considering acoustic phonon scattering (three-phonon interaction) is given as k p = k p , S = 3.1 × × 10 12 ⟨ ⟨ M ⟩ ⟩ V a 1 / 3 T D , ∞ ∞ 3 T ⟨ ⟨ γ γ G 2 ⟩ ⟩ N o 2 / 3 high temperatures ( T > 0.2 T D , phonon-phonon scattering only) , where ⟨ M ⟩ is the mean atomic weight of the atoms in the primitive cell, V a =1/ n is the average volume per atom, T D,∞ is the high-temperature Debye temperature, T is the temperature, N o is the number of atoms in the primitive cell, and ⟨γ G ⟩ is the mode-averaged square of the Grüneisen constant or parameter at high temperatures. This model is widely tested with pure nonmetallic crystals, and the overall agreement is good, even for complex crystals.