Raman spectroscopy study of sillenites. I. Comparison between Bi-12(Si,Mn)O-20 single crystals

The Raman spectrum of Bi-12(Bi0.53Mn0.47)O-20 Single crystal with sillenite structure was measured and compared with Raman spectra of two other sillen ites, Bi12SiO20 and Bi-12(Si0.98Mn0.02)O-20 The peaks observed in the spect ra of these sillenites are identified on the basis of calculated vibrationa l modes of a cluster of 33 atoms forming the set necessary to generate the crystal structure by translation symmetry operators. The results show that the peaks in the spectral range 250-350 cm(-1), which result from the modes of symmetry A and are dominated by vibrations of O1 and O2 atoms, are very sensitive to the geometry of the BiOn polyhedra. The vibrational modes of the tetrahedral units are strongly influenced by the Bi-O framework. The ob served splitting of the anti-symmetric tetrahedral mode in sillenites can b e provoked by imperfections in Bi-occupied positions. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.