Raman Spectra of Bismuth Oxyhalide Single Crystals

Oxyhalogenides of bismuth BiOX (X=Cl, Br and I) are very interesting materials which find various applications as X-ray luminescent screens, anti-Stokes (frequency upshift) converters, luminophors, and photoconducting analyzers of linear polarized radiation. Since each primitive cell consist of six monoatomic sites, the structure of the reduced representation of the 15 normal modes of vibration is: Γ= $2A_{1g}(Ra.)+B_{1g}(Ra.)+ 3E_{g}(Ra.)+ 2A_{2u}(i.r.)+ 2E_{u}(i.r.)$, in which the vibrations of $A_{1g}$, $B_{1g}$ and $E_{g}$ species are active in the Raman spectrum and those of $A_{2u}$ and $E_{u}$ species are active in the infrared (i.r.) spectrum. The Raman active modes are observed in frequency range 55 - 225 $cm^{-1}$, 50 - 185 $cm^{-1 }$, and 45 - 175 $cm^{-1}$ for the BiOCl, BiOBr and BiOI single crystals respectively.