Significant four-phonon scattering and its heat transfer implications in crystalline Ge2Sb2Te5

We systematically demonstrate the temperature-dependent thermal transport properties in crystalline ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ via first-principles density functional theory informed linearized Boltzmann transport equation. The investigation, covering a wide temperature range (30--600 K), reports the emergence of an unusual optical-phonon-dominated thermal transport in crystalline ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Further, a significant contribution of four-phonon scattering is recorded which markedly alters the lattice thermal conductivity. Therefore, the combined effect of cubic and quartic phonon anharmonicity is seen to navigate the underlying physical mechanism and open up intriguing phononic interactions in ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ at high temperature. Irrespective of three- and four-phonon processes, umklapp is seen to prevail over normal scattering events. Consequently, four-phonon scattering is found to notably reduce the lattice thermal conductivity of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ to $28%$ at room temperature and $42%$ at higher temperature. This quartic anharmonicity further manifests in the breakdown of ${T}^{\ensuremath{-}1}$ scaling of thermal conductivity and challenges the idea of a universal lower bound to phononic thermal diffusivity at high temperature. The faster decay of thermal diffusivity compared to ${T}^{\ensuremath{-}1}$ is rationalized encompassing the quartic anharmonicity via a modified timescale. These results invoke better understanding and precision to the theoretical prediction of thermal transport properties of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Concomitantly, this also triggers the possibility to explore the manifestations of the lower bound of thermal diffusivity in materials possessing pronounced four-phonon scattering.