SOLID-SOLUTIONS OF SILICON IN BORON-CARBIDE-TYPE CRYSTALS

The optical absorption spectrum of silicon-doped boron carbide in the spectral range of the absorption edge and its low-energy tail, obtaine d from transmission measurements between 0.25 and 4 eV, is compared wi th that of undoped boron carbide (B4.3C) with comparably low distortio ns. Silicon is proved to be an effective dopant of boron carbide, beca use the edge absorption spectrum and the plasma edge are considerably changed. However, the conduction remains p-type for the sample investi gated. The IR phonon spectrum of silicon-doped boron carbide is simila r to that of boron-rich boron carbide, which contains a considerable a mount of chainless unit cells. It is shown that the Si atoms occupy th ese cells, forming a two-atom chain. From a comparison of the Raman sp ectrum with those of similar structures and theoretical calculations, it can be estimated that the force field constant in such chains varie s linearly with the distance between the atoms.