Nonlinear effects in the phonon system of diamond crystal

TL;DR

This paper presents a theoretical study of diamond's thermodynamic properties, revealing that phonon-phonon interactions cause a linear increase in heat capacity with temperature, differing from typical crystal behavior.

Contribution

It introduces a generalized phonon model accounting for interactions, predicting unique temperature dependence of heat capacities in diamond.

Findings

Isochoric heat capacity increases linearly with temperature in diamond.

The model predicts deviations from the Dulong-Petit law at high temperatures.

Experimental observations align with the theoretical predictions.

Abstract

Thermodynamic properties of diamond are theoretically investigated on the ground of self-consistent description of a phonon gas in lattice, which generalizes the Debye model with taking into account the phonon-phonon interaction. It is shown that, at high temperatures, the theory predicts the linear in the temperature deviation of the isochoric heat capacity from the Dulong-Petit law. Unlike for the most crystals, where the decrease in the isochoric heat capacity is observed, our calculations for diamond predict the linear increase of the isochoric heat capacity with the temperature, viewed experimentally. The isobaric heat capacity of diamond, similar to other substances, linearly increases with the temperature.