New Insights into the Mechanism for Growing Cubic Boron Nitride Crystals under High Pressure

Cubic boron nitride (cBN) with superhardness exhibits many exceptional properties rivaling diamond and holds great promise for diverse applications such as advanced cutting tools and high-performance electronic devices, where high-quality single crystals are often highly demanded. However, the synthesis of its large single crystals has been a long-standing challenge, mainly due to a more complex reaction process involved in the growth of cBN when compared with that of diamond, making its growth mechanism largely elusive. In this work, we present a comprehensive investigation on the growth of cBN crystals through a seed method under high pressure using specially designed growth strategies with Li3N as a transport agent, leading to the discovery of the crucial role that the atomic transport rate plays in growing its crystals. The seed crystals are revealed to undergo a reversible cubic → hexagonal → cubic transition sequence at high temperature during a two-step compression process. In addition, the phase diagrams for the formation of cBN in different transport agents are also explored. These findings provide new insights into the mechanism for growing large cBN crystals.