Microscopic mechanisms of glasslike lattice thermal conductivity in tetragonal α−CsCu5Se3

Understanding and predicting the anharmonic lattice dynamics of complex strongly anharmonic crystals has long been a significant challenge in condensed matter physics and materials physics. Here, we study the underlying physical principles of the ultralow glasslike ${\ensuremath{\kappa}}_{\mathrm{L}}$ in tetragonal $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CsCu}}_{5}{\mathrm{Se}}_{3}$ by combining self-consistent phonon theory and the unified thermal transport theory of crystal and glass in the framework of the Peierls-Boltzmann transport equation and first-principles anharmonic lattice dynamics. We consider the impacts of both phonon broadening by three-phonon (3ph) interaction and phonon frequency anharmonic renormalization by quartic anharmonicity on the contributions from the particlelike phonon wave packet and wavelike phonon channel transport. The strong anharmonic renormalization leads to continuous temperature-induced hardening of the acoustic modes and low-lying optical modes, which is mainly associated with the tetrahedrally coordinated rattlinglike vibrations of the Cu3 atoms. This hardening effect reduces the phonon linewidths in the low-energy area via decreasing the available 3ph scattering phase space, thus partially counteracting the decreased impact on ${\ensuremath{\kappa}}_{\mathrm{L}}$ by increasing the phonon population and yielding a weak temperature-dependent ${\ensuremath{\kappa}}_{\mathrm{L}}$. Furthermore, the strong phonon broadening results in the coherent thermal transport channel that stems from the coherent coupling between different vibration eigenstates. Upon heating, the off-diagonal terms in the heat-flux operator become quite significant and dominate the phonon thermal transport process, suggesting the qualitative breakdown of the traditional phonon gas model in complex disordered crystals. In this paper, we provide the physical microscopic mechanisms of the glasslike ${\ensuremath{\kappa}}_{\mathrm{L}}$ in complex host-guest systems and highlight the influence of the lattice anharmonicity on thermal conductivity.