Raman effect of boron carbide (B 4.3C to B 10.37C)

Abstract

Careful FT-Raman investigations were performed on boron carbide with chemical compositions between B 4.3C at the carbon-rich limit of the homogeneity range and B 10.37C 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 rules of IR and Raman active phonons hold in the case of boron carbide in spite of the considerable structure distortions. Hence boron carbide remains essentially crystalline, at least close to the carbon-rich limit. The most prominent Raman doublet at 270/320 and 869/928 cm −1 bands are attributed to a total symmetrical vibration of pentagonal pyramids of icosahedra relative to the end atoms of the chain; the Raman band at 1065 cm −1 belongs to the inter-icosahedral BB bond. From the resonance frequencies, the force constants of the intra-icosahedral bond (0.4 mdyne Å −1), of the icosahedron-chain bond (2.2 mdyne Å −1) and of the intericosahedral BB bond (2.4 mdyne Å −1) were determined. They agree quite well with the theoretical calculations of Beckel et al. Towards the boron-rich limit of the homogeneity range the Raman spectra are considerably influenced by electronic transitions.