  We systematically demonstrate the temperature-dependent thermal transport
properties in crystalline Ge$_2$Sb$_2$Te$_5$via first-principles density
functional theory-informed linearized Boltzmann transport equation. The
investigation, covering a wide temperature range (30 K-600 K), reports the
emergence of an unusual optical phonon-dominated thermal transport in
crystalline Ge$_2$Sb$_2$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 Ge$_2$Sb$_2$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 Ge$_2$Sb$_2$Te$_5$ to 28
$\%$ at room temperature and 42 $\%$ at higher temperature. This quartic
anharmonicity further manifests in the breakdown of T$^{-1}$ 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$^{-1}$ is rationalized encompassing the quartic
anharmonicity via a modified time scale. These results invoke better
understanding and precision to the theoretical prediction of thermal transport
properties of Ge$_2$Sb$_2$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.
