Phase-transition Mechanism Of Boron Nitride From First Principles At High Pressure

High-pressure may change existing state of materials effectively and make materials show exotic behaviors that wouldn't appear at ambient pressure, thus, it opened up a new dimension for the research of Condensed Matter Physics.At high pressures, distances between the atoms in materials are expected to decrease significantly and electron orbits of adjacent atoms would overlap. These might induce the change of electronic properties and even so structural phase transitions. Accordingly, every aspect of materials'properties would change significantly. Electron energy-loss near edge structure (ELNES) provides us useful information on electronic structure and chemical bonding around objective atoms in materials. It can be used to identify phases in complex systems, and even to study phase transformation.Due to moderate computational load, wide range of application, higher accuracy, and direct comparison with experimental results, the first-principle method based on density functional theory has become one of most important approaches in the field of computational materials. With this method, based on the studies of the ELNES, electronic and energy properties for h-BN and w-BN, we obtained the original result:h-BN is an anisotropic layered compound and represents an interesting quasi-two-dimensional insulator. Since the widely used in the industry, it have motivated detailed theoretical and experimental studies for a long time. To simulate the spectra better, we explored the core-hole effect on the ELNES. Comparison with the experimental result, the best fitted spectra is obtained with the ground state model for the B K-edge and the final state model for the N K-edge. So in this paper, the ELNES at the B K-edge is calculated directly from the ground state theory and the N K-edge is simulated from the final state theory. It is known that at high pressure, h-BN directly transforms into a hexagonal close-packed polymorph, w-BN. While the transition pressures and their chemical trends have been extensively studied, little is so far known about the transformation mechanism. Previously, two possible phase-transition mechanisms have been proposed: chair deformation, which only involves the direct bonding between the layers, and boat deformation, which requires a relative rotation and displacement between the layers. Meng et al. performed accurately inelastic X-ray scattering experiments on h-BN to probe the chemical bonding changes at the phase transition of h-BN ? w-BN in 2004 and they have proposed a phase-transition mechanism that layers get close together, each N atom in a hexagonal layer buckles down in the [0001] direction and forms bonds with a B atom directly below it in an adjacent layer. Although different mechanisms were proposed, the lack of direct theoretical evidences precludes us from the full understanding of the proposed phase transition mechanism. In the present work, we have extensively explored the ELNES to reveal the phase transition mechanism of h-BN ? w-BN. We simulated two cases: momentum transfer (q) parallel or perpendicular to the c axis for B atom and N atom. Not only peak's intensity but also peak's position, our results are in excellent agreement with the experimental data to support the phase transition path proposed by Meng et al..To further confirm this phase-transition path, we have presented the enthalpy curves with the displacementζof N atoms at different pressures. At zero pressure, two energy minima exist atζ= 0.00 (h-BN) andζ= 0.125 (w-BN), with an energy barrier of 0.347 eV/f.u. located atζ= 0.06. With increasing pressure, the energy barrier gradually decreases and eventually disappears above 24 GPa, signifying the transition to w-BN. This nicely illustrates the physical mechanism of h-BN w-BN transition.To observe directly the change of the chemical bonding in the transition, we have examined the charge density in the (1120) plane cutting through atoms along the transformation path. Initially, it can be clearly seen that for h-BN, the strong directional bonding between adjacent coplanar atoms shows charge accumulation closer to N atoms, and interplanar bonding is evidenced very weak. With the displacement of N atoms along the transition path, each N atom gradually bonds to a B atom in the adjacent plane and at the transition sp2 hybridization in h-BN evolves into sp3 hybridization in w-BN.