Site-independent strong phonon-vacancy scattering in high-temperature ceramics ZrB2 and HfB2

Abstract

Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity $({\ensuremath{\kappa}}_{l})$ of ${\mathrm{ZrB}}_{2}$ and ${\mathrm{HfB}}_{2}$ are reported. These defects challenge the conventional understanding that associates larger impacts to bigger defects. We find the underlying reason to be a strong local perturbation caused by boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case of metal vacancy. We show that these behaviors originate from a mixed metallic and covalent bonding nature in the metal diborides. The thermal transport calculations are performed in a complete ab initio framework based on Boltzmann transport equation and density functional theory. Phonon-vacancy scattering is calculated using ab initio Green's function approach. Effects of natural isotopes and grain boundaries on ${\ensuremath{\kappa}}_{l}$ are also systematically investigated, however we find an influential role of vacancies to explain large variations seen in the experiments. We further report a two-order of magnitude difference between the amorphous and pure-crystal limits. Our results outline significant material design aspects for these multifunctional high-temperature ceramics.