Density of alkali germanate glasses related to structure Y.D. Yiannopoulos, C.P.E. Varsamis, E.I. Kamitsos * National Hellenic Research Foundation, Theoretical and Physical Chemistry Institute, 48 Vass. Constantinou Ave., Athens 11635, Greece Abstract The structural origin of the `germanate anomaly' eect remains a matter of controversy. We employ in this study a quantitative description of the germanate network structure to simulate the experimental density of potassium germ- anate glasses with K 2 O contents up to 40 mol%. The results show that the short range order structure, including the change of germanium co-ordination number from four to six, cannot reproduce the maximum in density. The `germanate anomaly' eect seems to involve both short and medium range order structural changes as metal oxide is added to GeO 2 . Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Alkali germanate glasses, xM 2 O 1 xGeO 2 M alkali metal), are characterised by the pres- ence of extrema in the composition dependence of physical properties such as density, refractive in- dex and glass transition temperature [1±3]. This behaviour is widely known as the `germanate anomaly eect', in comparison with the monotonic property±composition relations exhibited by alkali silicate glasses. The structural origin of the `germanate anomaly' continues to be a matter of controversy in the literature. Originally, the anomalous composition dependence of physical properties was attributed to the change of Ge co- ordination number from N Ge 4 to N Ge 6 by creation of additional Ge±O±Ge bridging bonds which cross link the germanate network [1±3]. Early spectroscopic studies [4±9] supported the formation of octahedral germanate units Geé 2 6 é bridging oxygen atom) in the composition range 0 6 x 6 0:20, and the subsequent formation of non-bridging oxygen NBO) atoms in germanate tetrahedra Q i , with 4 i NBOs per tetrahedron, at higher metal oxide contents. This structural picture was questioned by Henderson and Fleet [10], who interpreted the Raman spectra of sodium germa- nate glasses without invoking a change in Ge co- ordination number. Instead, they proposed the formation of germanate tetrahedra with NBOs as the only structural modi®cation mechanism, and attributed the `germanate anomaly' eect to the saturation of the glass network with three-mem- bered rings of germanate tetrahedra. Recent investigations of alkali germanate glas- ses with EXAFS [11,12], XPS [13], Raman and infrared [14,15], and combined X-ray and neutron diraction [16] techniques suggest the co-existence of both modi®cation mechanisms, i.e. creation of Geé 2 6 octahedra and Q i tetrahedra with NBOs, throughout a wide composition range. Other studies, including NMR [17], neutron diraction [18] and molecular dynamics [19] seem to converge towards a sequential activation of the two main modi®cation mechanisms of the glass structure. Journal of Non-Crystalline Solids 293±295 2001) 244±249 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +30-1 7273 828; fax: +30-1 7273 794. E-mail address: eikam@eie.gr E.I. Kamitsos). 0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 0 1 ) 0 0 6 7 7 - 9