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

TL;DR

This study investigates how vacancies affect phonon scattering and thermal conductivity in ZrB$_2$ and HfB$_2$, revealing site-independent strong scattering effects due to local perturbations caused by boron vacancies.

Contribution

It demonstrates that boron vacancies cause strong local perturbations affecting thermal transport, challenging conventional defect impact assumptions, with detailed ab initio analysis of phonon-vacancy interactions.

Findings

Boron vacancies induce strong local perturbations in interatomic force constants.

Metal and boron vacancies have similar effects on phonon scattering despite size differences.

Vacancies significantly influence thermal conductivity variations in high-temperature ceramics.

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.