Effect of Elastic Deformations on Direct Polymorphic Transformations in BN under Pressure

This paper gives an investigation of the effect of shear deformation on direct transitions in BN under pressure. The transition of a hexagonal (graphite-like) phase h-BN into a cubic ("sphalerite") c-BN through the intermediate rhombohedral phase r-BN, at a pressure P = 3.0 GPa, is shown. With shear deformation applied, the mechanism of h-BN transformation into the hexagonal (wurtzite) phase w-BN changes, and the following planes are parallel in the phases: for P = 5.0 GPa and deformation epsilon = 0 degrees (001)(h)parallel to (001)(w); and for P = 5.0 GPa and deformation epsilon congruent to 7.2 (001)(h)parallel to (100)(w). The shear deformation at 6-17 GPa leads to the reverse transition of w-BN to h-BN and the observed orientation relationship is different from the one occurring at the thermal transformation of w-BN to h-BN (T congruent to 1600 K) at a pressure P = 0.1 MPa.