Elastic Constants of Cubic Boron Phosphide and Boron Arsenide

We report the room temperature elastic constants of boron phosphide (BP) and boron arsenide (BAs) single crystals derived from Brillouin frequencies measured by picosecond interferometry. The synthesis of BP and BAs with thermal conductivity as high as 540 and 1000 W m(-1) K-1, respectively, has made them promising materials for thermal management. Accurate measurements of elastic constants are needed to assess the accuracy of computational modeling of the lattice dynamics. The crystals are cut and polished in different orientations to access waves traveling along different directions. The surface normal orientations of the crystals are determined using electron backscattering diffraction. We studied the Brillouin frequencies of quasilongitudinal waves in five different orientations of BP and BAs crystals. Quasishear waves were observed in two orientations of BP and one orientation of BAs. The propagation directions and acoustic velocities are used to construct Christoffel equations which are then solved for the elastic constants. We report C-11, C-12, and C-44 values of 354 +/- 5 GPa, 83 +/- 15 GPa, and 190 +/- 8 GPa for BP and 291 +/- 5 GPa, 76 +/- 13 GPa, and 173 +/- 6 GPa for BAs. The measured elastic constants for BAs differ by less than 5% and 17% from calculated elastic constants obtained through local density approximation and Perdew-Burke-Ernzerhof density functional calculations, respectively. In most cases, the measured elastic constants are larger than the calculated elastic constants.