RAMAN EFFECT OF BORON-CARBIDE (B4.3C TO B10.37C)

Careful FT-Raman investigations were performed on boron carbide with c hemical compositions between B4.3C at the carbon-rich limit of the hom ogeneity range and B10.37C not far from the boron-rich limit yielding highly resolved and reliable spectra. Contrary to the results of other authors (Tallant et al.), it is evident that the symmetry selection r ules of IR and Raman active phonons hold in the case of boron carbide in spite of the considerable structure distortions. Hence boron carbid e remains essentially crystalline, at least close to the carbon-rich l imit. The most prominent Raman doublet at 270/320 and 869/928 cm-1 ban ds are attributed to a total symmetrical vibration of pentagonal pyram ids of icosahedra relative to the end atoms of the chain; the Raman ba nd at 1065 cm-1 belongs to the inter-icosahedral B-B bond. From the re sonance frequencies, the force constants of the intra-icosahedral bond (0.4 mdyne angstrom-1), of the icosahedron-chain bond (2.2 mdyne angs trom-1) and of the intericosahedral B-B bond (2.4 mdyne angstrom-1) we re determined. They agree quite well with the theoretical calculations of Beckel et al. Towards the boron-rich limit of the homogeneity rang e the Raman spectra are considerably influenced by electronic transiti ons.