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Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl2O–GeO2, containing up to 40 mol % Tl2O, 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 Tl2O is added to the glass. However, whereas for alkali germanates n GeO has its maximum value (∼4.36 ± 0.03) at ∼19 mol % R2O (e.g., R = Cs), for thallium germanates it continues to rise until 30 mol % Tl2O, with a higher maximum value of 4.44 ± 0.02. For low Tl2O content, most thallium cations are on modifier sites with a high coordination number (6 or greater). As the Tl2O content increases, glass former [TlO3] sites become increasingly common, and it is predicted from an extrapolation of the results that a glass with a composition of 50 mol % Tl2O would be composed entirely of [TlO3] and [GeO4] units. It is shown that the presence of [TlO3] 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.

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