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, whic h included samples with different dergees of ordering, with various ra tios of the concentrations of the hexagonal and rhombohedral phase, wa s prepared by chemical vapor deposition. This study revealed the essen tial influence of the microstructure of the pyrocrystal on the reflect ion and transmission spectra. It is shown that to identify the normal oscillations of the hexagonal phase, crystal-optical effects and effec ts associated with phonon scattering must be taken into account. The n ormal oscillations of the rhombohedral phase of BN were identified dir ectly from experimental spectra of pyrocrystals which are similar in t heir properties to a single crystal. On the basis of the results obtai ned 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 co ntradictions between the experimental data and shows that in addition to sp(2) hybridized electrons, sp hybridized electrons participate in the formation of interatomic bonds in anisotropic BN. (C) 1998 America n Institute of Physics. [S1063-7826(98)00309-3].