Cazorla Claudio, Gould Tim
School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia.
Sci Adv. 2019 Jan 18;5(1):eaau5832. doi: 10.1126/sciadv.aau5832. eCollection 2019 Jan.
Boron nitride (BN) is a material with outstanding technological promise due to its exceptional thermochemical stability, structural, electronic, and thermal conductivity properties, and extreme hardness. Yet, the relative thermodynamic stability of its most common polymorphs (diamond-like cubic and graphite-like hexagonal) has not been resolved satisfactorily because of the crucial role played by kinetic factors in the formation of BN phases at high temperatures and pressures (experiments) and by competing bonding and electrostatic and many-body dispersion forces in BN cohesion (theory). This lack of understanding hampers the development of potential technological applications and challenges the boundaries of fundamental science. Here, we use high-level first-principles theories that correctly reproduce all important electronic interactions (the adiabatic-connection fluctuation-dissipation theorem in the random phase approximation) to estimate with unprecedented accuracy the energy differences between BN polymorphs and thus overcome the accuracy hurdle that hindered previous theoretical studies. We show that the ground-state phase of BN is cubic and that the frequently observed hexagonal polymorph becomes entropically stabilized over the cubic at temperatures slightly above ambient conditions ( = 335 ± 30 K). We also reveal a low-symmetry monoclinic phase that is extremely competitive with the other low-energy polymorphs and that could explain the origins of the experimentally observed "compressed h-BN" phase. Our theoretical findings therefore should stimulate new experimental efforts in bulk BN and promote the use of high-level theories in modeling of technologically relevant van der Waals materials.
氮化硼（BN）是一种具有卓越技术前景的材料，因其具有特殊的热化学稳定性、结构、电子和热导率特性以及极高的硬度。然而，其最常见的多晶型物（类金刚石立方和类石墨六方）的相对热力学稳定性尚未得到令人满意的解决，这是因为动力学因素在高温高压下（实验中）BN相的形成过程中以及在BN凝聚过程中的竞争键合、静电和多体色散力（理论上）都起着关键作用。这种认识上的不足阻碍了潜在技术应用的发展，并挑战了基础科学的边界。在此，我们使用能够正确再现所有重要电子相互作用的高级第一性原理理论（随机相位近似中的绝热连接涨落耗散定理），以前所未有的精度估计BN多晶型物之间的能量差，从而克服了阻碍先前理论研究的精度障碍。我们表明，BN的基态相是立方相，并且在略高于环境条件的温度下（= 335 ± 30 K），经常观察到的六方多晶型物在熵上比立方相更稳定。我们还揭示了一种低对称单斜相，它与其他低能量多晶型物极具竞争力，并且可以解释实验中观察到的“压缩h-BN”相的起源。因此，我们的理论发现应该会激发对块状BN的新实验研究，并促进在对技术相关的范德华材料进行建模时使用高级理论。
