Origin of preferred orientation in an isotropic material: High pressure synthesis of bc8-Si

High pressure experiments and ab initio calculations are used to investigate unexpected crystallographic preferred orientation in the bc8 phase of silicon formed under non-hydrostatic conditions. Microstructural characterization in two orthogonal directions reveals that the preferred orientation is only visible when the sample is viewed perpendicular to the compression axis. Curiously, the elastic constants of bc8-Si are almost perfectly isotropic, making it counter-intuitive that preferred crystallographic orientation is observed. This conundrum is resolved by tracking the phase transformation pathway and computing the three-dimensional Young's modulus. We find the preferred orientation most likely originates from the highly anisotropic simple-hexagonal phase and is passed on to subsequent daughter phases via displacive phase transformations. Our investigation of preferred orientation in bc8-Si complements other high pressure studies where preferred orientation in silicon phases is often observed but not explained.