Normal lattice vibrations and the crystal structure of anisotropic modifications of boron nitride

A multicomponent infrared study of boron nitride (BN) pyrocrystals was performed in polarized light. A series of textured polycrystals, which included samples with different dergees of ordering, with various ratios of the concentrations of the hexagonal and rhombohedral phase, was prepared by chemical vapor deposition. This study revealed the essential influence of the microstructure of the pyrocrystal on the reflection and transmission spectra. It is shown that to identify the normal oscillations of the hexagonal phase, crystal-optical effects and effects associated with phonon scattering must be taken into account. The normal oscillations of the rhombohedral phase of BN were identified directly from experimental spectra of pyrocrystals which are similar in their properties to a single crystal. On the basis of the results obtained by us it is shown that ionic-covalent bonds exist between some of the atoms of neighboring layers in anisotropic modifications of BN. A refined model of the lattice structure eliminates an entire host of contradictions between the experimental data and shows that in addition to sp2 hybridized electrons, sp hybridized electrons participate in the formation of interatomic bonds in anisotropic BN.