Single-crystal IR spectroscopic investigation on fresnoite, Sr-fresnoite and Ge-fresnoite

Single crystals of fresnoite (Ba2TiSi2O8), the structural analogue Sr-fresn oite (Sr2TiSi2O8) and the structurally similar Ge-fresnoite (Ba2TiGe2O8) we re investigated by FTIR-reflectance- (IRR-) and Raman-spectroscopy. The IRR -spectra were subjected to a complete dispersion analysis in order to evalu ate the <(<epsilon>)over cap>-tensor in the range of 100-2000 cm(-1). The r esults were checked by Raman-spectroscopy, Ba-fresnoite was shown to contai n a small amount (about 1-2%) of four co-ordinated Ti, represented by an os cillator at 749 cm(-1). Al-mode at 858 and 871 cm(-1) are not mixed TO-LO-m odes but simply pure TO and LO, respectively. Raman microscopy showed that the Sr-fresnoite single crystal contained small amounts of rutil, which cou ld not be detected in the IRR-spectra. Because of the great similarity of t he E parallel toc-spectra of Ba-, Sr- and Ge-fresnoite, the Ai-mode represe nting the oscillation of the short Ti-O-bond of the Ge-fresnoite has been a ssigned to a vibration at 797 cm(-1) (C) 2001 Elsevier Science B.V. All rig hts reserved.