Anharmonicity-induced phonon hardening and anomalous thermal transport in ScZn

Phonon anharmonicity plays a crucial role in the lattice dynamics and lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ of solids. Here, by combining first-principles calculations with the linearized Boltzmann transport equation, we unveil that phonon anharmonicity can make an unexpected contribution to ${\ensuremath{\kappa}}_{L}$ and thereby lead to anomalous thermal transport behavior in the intermetallic compound ScZn, a recently theoretically predicted topological phonon material. We explicitly consider phonon renormalization, four-phonon (4ph), and phonon-electron scattering to predict the ${\ensuremath{\kappa}}_{L}$ of ScZn. The results show that anharmonic phonon renormalization due to quartic anharmonicity gives rise to significant phonon hardening, especially for heat-carrying acoustic phonons. We also reveal that anharmonic effects largely decrease the fourth-order force constants, causing the dramatic suppression of 4ph scattering on ${\ensuremath{\kappa}}_{L}$. The incorporation of renormalization-induced phonon hardening and the decrease of quartic force constants makes a consequence of an almost twofold increase in the ${\ensuremath{\kappa}}_{L}$ compared to the conventional harmonic treatment, from 3.9 to 7.6 W/mK at room temperature. This is enabled by the suppressed phonon scattering phase space and weakened fourth-order lattice anharmonicity due to the temperature-induced phonon renormalization effect. Moreover, it is found that the electronic contribution ${\ensuremath{\kappa}}_{e}$ always plays a dominating role on the thermal conductivity; however, the lattice contribution is still considerable, leading to the substantial deviation of Lorenz number from the Sommerfeld value. This work provides a fundamental understanding of the thermal transport in ScZn and highlights the importance of anharmonic effects on the phonon energies and thermal conductivity of metals.