Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN
Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN
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DOI：
10.1143/jjap.42.1694
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发表时间：
2003-04
影响因子：
1.5
通讯作者：
A. Yoshiasa;Y. Murai;O. Ohtaka;T. Katsura
中科院分区：
文献类型：
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作者：
A. Yoshiasa;Y. Murai;O. Ohtaka;T. Katsura
采用Kawai型高压装置合成了六方金刚石（hDIA）和纤锌矿型氮化硼（wBN）粉末，并通过Reitveld精修对其结构的基本细节进行了研究，以探讨其热力学稳定性和转变机制。产物的X-射线衍射图由hDIA和立方金刚石（cDIA）的混合物与堆垛层错很好地解释。在800 ~ 1400°C退火的产物中，hDIA和cDIA的质量分数为50：50，在1600°C退火的产物中，hDIA和cDIA的质量分数为20：80。高于1600°C的温度似乎有利于cDIA的形成或诱导hDIA转化为cDIA。结构精修表明，减少和增加在hDIA和wBN中的C-C和B-N键的基底和顶端距离，分别通过降低对称性从立方到六方引入。由于纤锌矿型化合物的相对稳定性在很大程度上取决于四面体键角的畸变，因此对hDIA和wBN中与理想四面体的偏差进行了改进以讨论其稳定性。通过与以往模拟结果的比较，讨论了从石墨和类石墨BN到hDIA和wBN的转变机制。根据合成hDIA和陨石中发现的天然hDIA的相似特征，提出了陨石中hDIA的形成机制。
Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the essential details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. X-ray diffraction profiles of the products were well explained by a mixture of hDIA and cubic diamond (cDIA) with stacking faults. The mass fraction of hDIA and cDIA was 50:50 for the products annealed between 800 and 1400°C and it became 20:80 for the product annealed at 1600°C. Temperatures higher than 1600°C seem to favor the formation of cDIA or to induce the conversion from hDIA to cDIA. Structure refinement revealed that a decrease and an increase in the basal and apical distances of C–C and B–N bonds in hDIA and wBN, respectively, are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds largely depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both hDIA and wBN was refined to discuss their stability. The transition mechanism from graphite and graphite-like BN to hDIA and wBN is discussed by comparing the present results and those of previous simulation studies. Based on analogous features observed in the synthetic hDIA and lonsdaleite (natural hDIA found in meteorites), the formation mechanism of hDIA in meteorites is proposed.