Influence of Lone-Pair Cations on the Germanate Anomaly in Glass

Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl 2 O–GeO 2, containing up to 40 mol % Tl 2 O, as a means of investigating the influence of lone-pair cations on the germanate anomaly. As observed previously in alkali germanate glasses, the average Ge–O coordination number, n GeO, is found to rise above four as Tl 2 O is added to the glass. However, whereas for alkali germanates n GeO has its maximum value (∼4.36 ± 0.03) at ∼19 mol % R 2 O (e.g., R = Cs), for thallium germanates it continues to rise until 30 mol % Tl 2 O, with a higher maximum value of 4.44 ± 0.02. For low Tl 2 O content, most thallium cations are on modifier sites with a high coordination number (6 or greater). As the Tl 2 O content increases, glass former [TlO 3 ] sites become increasingly common, and it is predicted from an extrapolation of the results that a glass with a composition of 50 mol % Tl 2 O would be composed entirely of [TlO 3 ] and [GeO 4 ] units. It is shown that the presence of [TlO 3 ] units allows higher coordinated Ge units to share an oxygen, and this is why n GeO continues to rise beyond the composition for which it is a maximum in alkali germanates. There is thus an interplay between the germanate anomaly and the environment of the lone-pair cation—an effect which does not occur in alkali germanates.