Structural evolution in boron nitrides during the hexagonal-cubic phase transition under high pressure at high temperature
✍ Scribed by He, Lian-Long; Akaishi, Minoru; Horiuchi, Shigeo
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 827 KB
- Volume
- 40
- Category
- Article
- ISSN
- 1059-910X
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✦ Synopsis
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°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 sp 2 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 ϭ 0.433 nm, b ϭ 0.250 nm, c ϭ 0.32-0.33 nm, and  ϭ 90-92°. In a succeeding stage, the value of  increased to 92-95°. 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 sp 2 sheets changed during the transition from h-BN to m-BN; the density of state for the * bond became prominently high in m-BN as compared to that in h-BN.