Raman and photoluminescence investigations have been performed on single crystals of undoped and doped (Cr and Pr) bismuth silicon oxide (Bi12SiO20, BSO). X-ray diffraction studies reveal the phase purity of the materials. BSO crystallizes in cubic structure having space group 123 and lattice parameter; a = 10.14 Angstrom. The effects of Cr and Pr doping on the micro-Raman spectra of the single crystals have been investigated in detail. The vibrational frequencies of the groups of atoms, namely, O(2)Bi3, Si(O(3)Bi3)(4) and O(1)Bi3, in the BSO crystals shift towards lower wavenumber due to dopant contributions. The expansion of Si(O-(3))(4) tetrahedran has been correlated to its flexibility to change the tetrahedral angle, i.e. 107.47degrees. The photoluminescence (PL) study indicates bathochromic shift of the absorption edge in Pr-doped BSO compared to pure and Cr-doped crystals and appropriately corroborates the observed vibrational properties. (C) 2002 Elsevier Science B.V. All rights reserved.