Is there a new form of boron nitride with extreme hardness?

摘要

In the course of an investigation of boron nitride, its phase diagram and its synthesis at high pressure and high temperature some particles of BN of extreme hardness were extracted. Tests for hardness showed that the samples were actually harder than dimond. In ensuing checks the X-ray and electron diffraction patterns reveled that the material posses amorphous structure, i.e. no diffraction lines were observed. An attempt is made here to understand the structural nature of such a material which can explain the findings and to predict the likely hardness for such structures. We have defined hardness as the decrease in total energy as deformations of representative clusters of atoms ware simulated. Calculations have been tested previously for diamond and cubic boron nitride and now have been applied to calculate the expected hardness of the present models. We find that a promising generative model is a cage system consisting of truncated boron-nitrogen octahedrons connected to each other in a random arrangement over the four-and six-membered rings, thus leading to amorphous diffraction patterns. The model manifested hardness with surpassed both that of cBN and of diamond. It appears moreover, that the cages can be stiffened even further by the incorporation of foreign atoms in the model. The inclusion of calcium as such a foreign atom was tested and this is predicted to increases the hardness.