He L L, Akaishi M, Horiuchi S
National Institute for Research in Inorganic Materials, Tuskuba, Ibaraki, Japan.
Microsc Res Tech. 1998 Feb 15;40(4):243-50. doi: 10.1002/(SICI)1097-0029(19980215)40:4<243::AID-JEMT1>3.0.CO;2-T.
Structural evolution during the phase transition from h (hexagonal)- to c (cubic)-boron nitrides (BN) under high pressure (6.5-7.7 GPa) at high temperature (1,700-2,150 degrees C) was examined by using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). At the initial stage of the evolution, some starting h-BN plates were strongly folded, while others were slightly bent. As a result, a strong texture was formed. HRTEM revealed that the interplanar distance between sp2 sheets became slightly shortened and they were slightly sheared to each other during the folding and bending. As a result, m (monoclinic)-BN was formed near the folding plane with lattice parameters; a = 0.433 nm, b = 0.250 nm, c = 0.32-0.33 nm, and beta = 90-92 degrees. In a succeeding stage, the value of beta increased to 92-95 degrees. c-BN grains appeared with nano-scale twins and sometimes partly included wurtzite-type BN. They started to grow with secondary twins at higher temperature. EELS analysis revealed that the band structure of sp2 sheets changed during the transition from h-BN to m-BN; the density of state for the pi* bond became prominently high in m-BN as compared to that in h-BN.
利用高分辨率透射电子显微镜（HRTEM）和电子能量损失谱（EELS）研究了在高温（1700 - 2150℃）高压（6.5 - 7.7 GPa）下从h（六方）- 到c（立方）- 氮化硼（BN）相变过程中的结构演变。在演变的初始阶段，一些起始的h - BN片层强烈折叠，而另一些则轻微弯曲。结果，形成了强烈的织构。HRTEM显示，sp2片层之间的面间距略有缩短，并且在折叠和弯曲过程中它们彼此略有剪切。结果，在折叠平面附近形成了m（单斜）- BN，其晶格参数为：a = 0.433 nm，b = 0.250 nm，c = 0.32 - 0.33 nm，β = 90 - 92°。在随后的阶段，β值增加到92 - 95°。c - BN晶粒出现纳米级孪晶，有时部分包含纤锌矿型BN。它们在较高温度下开始以二次孪晶生长。EELS分析表明，在从h - BN转变为m - BN的过程中，sp2片层的能带结构发生了变化；与h - BN相比，m - BN中π*键的态密度显著升高。
