Site independent strong phonon-vacancy scattering in high temperature ceramics ZrB and HfB
Abstract
Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity (κl) of ZrB2
and HfB2 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 κl 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 purecrystal limits. Our results outline significant material design aspects for these multifunctional high-temperature
ceramics.