Disordered state in first-order phase transitions: Hexagonal-to-cubic and cubic-to-hexagonal transitions in boron nitride

The microscopic mechanism of diffusion-type solid-solid first-order transitions was studied in boron nitride. This substance exhibits an extreme case of transitions between very different lattices, hexagonal (graphite-like) and cubic (diamond-like) phases, accompanied by a large change in the volume. A boundary between the hexagonal and cubic phases was produced by local heating of either a cBN or hBN sample in a diamond-anvil cell under hydrostatic pressure with the aid of a laser so that the sample was partially transformed to another phase. The boundary was studied with the aid of micro-Raman and high-resolution electron microscopy. In the transient region between the two phases a mixture of highly fragmented disordered nanocrystallites and the amorphous state of BN was found. BN nanotubes growing from the amorphous state were also discovered in this region.