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BAs was predicted to have an unusually high thermal conductivity at roomtemperature of 2000$\,$Wm$^{-1}$$\,$K$^{-1}$, comparable to that of diamond.However, the experimentally measured thermal conductivity of BAs singlecrystals is an order of magnitude lower. To identify the origin of this largeinconsistency, we investigated the lattice structure and potential defects inBAs single crystals at atomic scale using aberration-corrected scanningtransmission electron microscopy (STEM). Rather than finding a largeconcentration As vacancies ($V_\mathrm{As}$), as widely thought to dominate thethermal resistance in BAs crystals, our STEM results showed enhanced intensityof some B columns and reduced intensity of some As columns, suggesting thepresence of antisite defects with As$_\mathrm{B}$ (As-atom on B site) andB$_\mathrm{As}$ (B-atom on As site) with significant concentrations. Furthercalculations show that the antisite pair with As$_\mathrm{B}$ next toB$_\mathrm{As}$ is preferred energetically among the different types of pointdefects investigated, and confirm that such defects lower the thermalconductivity for BAs. Using a concentration of6.6$\pm$3$\times$10$^{20}$$\,$cm$^{-3}$ for the antisite pairs estimated fromSTEM images, thermal conductivity is estimated to be65-100$\,$Wm$^{-1}$$\,$K$^{-1}$, in reasonable agreement with our measuredvalue. Our study suggests that As$_\mathrm{B}$-B$_\mathrm{As}$ antisite pairsare the primary lattice defects suppressing thermal conductivity of BAs.Possible approaches are proposed for growth of high quality crystals or filmswith high thermal conductivity.

Antisite pairs suppress the thermal conductivity of BAs