Anharmonic Effects in Heat Capacity of Solids

Heat capacity is a measure of how much energy heat carriers carry; therefore, it directly impacts performance and efficiency of various energy applications such as thermal storage and heat exchangers. For dielectric solids, heat capacity is most typically described by a purely harmonic oscillator model. This coupled with how these weakly interact anharmonically determine lattice thermal conductivity in various calculation schemes. In this work, we explicitly characterize individual mode contributions to heat capacity in some simple crystals such as argon and silicon and find that in contrast to common assumptions based on harmonic oscillators, each mode is strongly anharmonic especially at high temperatures, leading to the breakdown of the equipartition theorem. We discuss implications of our findings in the context of thermal transport in solids.