Stabilizing the metastable superhard material wurtzite boron nitride by three-dimensional networks of planar defects

Abstract

Wurtzite boron nitride (w-BN) is a metastable superhard material that is a high-pressure polymorph of BN. Clarifying how the metastable high-pressure material can be stabilized at atmospheric pressure is a challenging issue of fundamental scientific importance and promising technological value. Here, we fabricate millimeter-size w-BN bulk crystals via the hexagonal-to-wurtzite phase transformation at high pressure and high temperature. By combining transmission electron microscopy and ab initio molecular dynamics simulations, we reveal a stabilization mechanism for w-BN, i.e., the metastable high-pressure phase can be stabilized by 3D networks of planar defects which are constructed by a high density of intersecting (0001) stacking faults and {1010} inversion domain boundaries. The 3D networks of planar defects segment the w-BN bulk crystal into numerous nanometer-size prismatic domains with the reverse crystallographic polarities. Our findings unambiguously demonstrate the retarding effect of crystal defects on the phase transformations of metastable materials, which is in contrast to the common knowledge that the crystal defects in materials will facilitate the occurrence of phase transformations. metastable phase | superhard material | planar defect | electron microscopy W urtzite boron nitride (w-BN) has attracted intense interest due to its outstanding properties and potential applications. w-BN is a fascinating superhard material with a hardness next to diamond (1-3), rendering it a candidate material to replace diamond. w-BN is also a promising III-V group wide-bandgap material for advanced electronic devices because it has many properties superior to GaN and AlN, such as a wider band gap, higher thermal conductivity, and larger spontaneous polarization (4, 5). Since the first synthesis of w-BN in the 1960s (6), extensive efforts have been devoted to the fabrication of high-quality w-BN samples (7-11), the in-depth understanding of phase transformations between BN polymorphs (12-16), and the theoretical investigation of deformation mechanisms of w-BN (17). Unfortunately, such efforts are greatly hampered by the technical difficulties in stabilizing w-BN at atmospheric pressure. The w-BN phase obtained by the direct conversion of hexagonal BN (h-BN) at high pressure tends to recover to the h-BN phase after releasing pressure, and only a small amount of w-BN can maintain its structure (12,15,18,19). The prevailing synthesis method of w-BN is the shock compression of h-BN, which can only produce w-BN fine powders with micrometer grain size, and other BN impurities are readily introduced (7, 13, 20, 21). Synthesis of w-BN bulk crystals is challenging due to lack of efficient approaches for stabilizing the metastable high-pressure phase. Since w-BN is a thermodynamically metastable phase, w-BN single crystals cannot be fabricated by a dissolution and precipitation process. When BN crystals were recrystallized by solvent system, only h-BN or cubic BN (c-BN) crystals were grown at their stable pressure and temper...

Pressure dependence of the interlayer and intralayer E2g Raman-active modes of hexagonal BN up to the wurtzite phase transition

“…The Raman spectrum of Fig. 3 is consistent with the Raman spectrum of a millimeter-sized w-BN bulk crystal obtained by a high-pressure, high-temperature transformation recently reported [49]. The w-BN frequencies calculated by Ohba et al [43] using LDA ab initio methods are indicated on Fig.…”

Bias-Enhanced Formation of Metastable and Multiphase Boron Nitride Coating in Microwave Plasma Chemical Vapor Deposition

“…Chen et al [ 23 ] have discovered a stabilization mechanism for w -BN (produced utilizing the high pressure/high temperature approach) based on 3D networks of planar defects that are formed by dense network of intersecting stacking faults (ISF) and inversion domain boundaries (IDB). When the two orthogonal planar defects penetrate each other, an ISF–IDB junction is constructed to create a partial dislocation [ 23 , 56 ]. In GaN thin film, the empirical potential calculation model has shown that the ISF–IDB junction has low formation energy [ 56 ].…”

“…Boron nitride is not found naturally; it has to be produced from boron and nitrogen precursors. Different methods and experimental conditions have been used to synthesize BN from a variety of boron and nitrogen precursors [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. Preparation of boron nitride (BN) was reported from high pressure/high temperature synthesis using diamond anvil cell/laser heating.…”

“…At ordinary temperatures and pressures h -BN is the stable structure of BN and c -BN at high temperatures and pressures. The wurtzite phase of BN is metastable at all pressures and temperatures and its stabilization is challenging because of the very low kinetic barrier for the transformation back to ambient phases [ 4 , 23 ]. Among the different allotropes of boron nitride, c -BN and w -BN are superhard in nature [ 27 ].…”

Materials science research of boron nitride obtained at high pressure and high temperature

“…9) A detailed observation of the fine structure of this wBN revealed an extremely large amount of inversion domain boundaries, the origin of which is attributed to stacking faults introduced during the initial stages of pressurization of hBN. 10) The change in color reflects this change in fine structure. The inversion domain boundaries could be an inhibition factor (energy barrier) in a reversible transition from wBN to original hBN, and is characteristic of a binary system.…”