Raman and optical reflection spectra of germanate and silicate glasses A. Trukhin a, * , B. Capoen b a Institute of Solid State Physics, University of Latvia, 8 Kengaraga Street, LV-1063 Riga, Latvia b Laboratoire de Physique des Lasers, Universite ´ des Sciences et Technologies de Lille, France Received 11 May 2005; received in revised form 22 August 2005 Available online 25 October 2005 Abstract Germanate and phosphosilicate glasses made in oxygen surplus conditions were studied by Raman and optical reflection methods. We found that the optical reflection spectra of the germanate glasses are quite similar to the one those of a GeO 2 crystal with the a-quartz structure. The reflection of phosphosilicate glasses is very close to silica glass-related spectra. Hence, the determining influence of the tetrahedral structure on reflection spectra is revealed. The Raman spectra of germanate samples are similar to those reported the one known in the literature. Octahedral entities, namely bands similar to stishovite vibration modes, were difficult to detect in phosphosilicate glasses through Raman spectroscopy. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.43.F; 78.30; 78.55.ÀHx; 42.70.Ce 1. Introduction The Glassy state could be presented as a set of chaotic density fluctuations corresponding to the polymorphism of the crystalline state of a material. In the case of germa- nia and silica glasses, there is a major structural motif is the tetrahedron. Tetrahedral structure allows a wide range of density fluctuations by the creation of rings of different dimensions [1]. In the case of the corresponding crystals the properties of electronic states are not very sensitive to the density, provided that the main structural motif is the tetrahedron. Despite a density difference between a-quartz and coesite, the positions of the intrinsic absorption thresh- olds coincide [2]. Moreover, the intrinsic optical reflection spectra of silica glass and a-quartz exhibit some similar fea- tures, although their bands have slightly different intensities and widths [3]. There is an increasing interest in the possible existence of octahedra as minority motives in the short-range order of germanate and silicate glasses [4]. Experimental evi- dences of the existence of the rutile-type structure was obtained in the case of germania glass [4]. Two positions of the intrinsic absorption threshold [4] were observed, one at 6 eV coinciding with that of the a-quartz-like struc- tured GeO 2 crystal, another at 4.5 eV coinciding with that of the rutile-like structured GeO 2 crystal. It is now well recognized that alkali plays a role of catalyst for the crea- tion of octahedral structures in the case of germanium dioxide [5,6]. The threshold at 4.5 eV could be obtained on strongly oxidized samples, while in samples melted under normal or reducing conditions, the absorption of oxygen deficient centers obscures the intrinsic threshold position [7,8]. The situation with silicon dioxide is far from clear. The intrinsic absorption threshold of stishovite single mono- crystal is situated at 8.75 eV [2] being higher than that for a-quartz. This shows different structures of the electronic states related to tetrahedral and octahedral structures in sil- icon dioxide. In silica glass, which has an absorption threshold energy of 8.2 eV for the purest samples [8], a pos- sible threshold related to the sixfold-coordinated silicon cannot be observed. So, the existence of motives with 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.017 * Corresponding author. Tel.: +371 7260 686; fax: +371 7132 778. E-mail address: truhins@latnet.lv (A. Trukhin). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 351 (2005) 3640–3643