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A4: Theory - bulk materials
A theoretical method combining ab initio phonon calculations and the Callaway model has been developed and applied to the evaluations of lattice thermal conductivity for over 10 materials with various chemical natures. A cutoff frequency of each acoustic mode, obtained directly from phonon dispersions, is introduced to extend the applicability of the traditional Callaway model. Mode properties, including heat capacity, group velocity, and phonon relaxation time are considered in our method, and the agreement between our predictions and experiments is fairly good. Our results show that the interactions between acoustic and optic phonons can be found in many compounds, not just the ones with crystalline voids. These interactions reduce the amount of heat-carrying acoustic modes, and serve as a new means of lattice thermal conductivity reduction.