Diffusion Monte Carlo Study on Relative Stabilities of Boron Nitride Polymorphs

One of the most fundamental properties of boron nitride (BN), the phase diagram, is still under debate both theoretically and experimentally even though BN compounds have well-known industrial applications. The determination of the most stable BN's polymorph (hexagonal, rhombohedral, wurtzite, and zinc-blende) is difficult with ab-initio methods because subtle vdW interactions are relevant in some of the polymorphs. This makes quantitative calculations challenging. As a result, despite significant theoretical research, there is no consensus yet on the most stable structure. There are several contradicting theoretical reports. We applied one of the state-of-the-art ab-initio methods, fixed-node diffusion Monte Carlo (FNDMC), to the four known BN's polymorphs. We also performed phonon calculations to investigate thermal stability. Our FNDMC calculations showed that hexagonal BN is the most stable among the four polymorphs at 0 K. Our calculations of the contribution of phonons to the free energy confirmed that this is the case even at 300K. We compared our results to previous sophisticated studies such as MBD, RPA, RPAx, and CCSD(T). We found that RPAx and CCSD(T) are consistent with our FNDMC, whereas MBD and RPA are not.