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 (similar to 4.36 +/- 0.03) at similar to 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