Site independent strong phonon-vacancy scattering in high temperature ceramics ZrB$_2$ and HfB$_2$

Abstract: Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity ($\kappa_l$) of ZrB$_2$ and 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 the boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case metal vacancies. We show that these behaviours originate from a mixed metallic and covalent bonding nature in the metal diborides. The thermal transport calculations are performed in a complete \textit{ab initio} framework based on Boltzmann transport equation and density functional theory. Phonon-vacancy scattering is calculated using \textit{ab initio} Green's function approach. Effects of natural isotopes and grain boundaries on $\kappa_l$ 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 multi-functional high temperature ceramics.