A neutron diffraction and 205Tl NMR study of the thallium germanate glass system

Neutron diffraction data, measured for two thallium germanate glass compositions, are presented and compared with previously published data for caesium germanate glasses. The measured coordination number, nGe–O, for the 10 mol% Tl2O thallium germanate glass is in good agreement with the average coordination number measured for the equivalent caesium germanate. However, while nGe–O declines for caesium germanate glasses as the amount of modifier increases above 18 mol% Cs2O, nGe–O for the 30 mol% Tl2O thallium germanate glass remains high with, on average, 4.40 ± 0.03 oxygen neighbours per germanium. The difference in behaviour of the germanium coordination, compared to caesium germanate glasses, arises from a change in the average thallium environment, as the role of the thallium changes from that of a modifier (similar to an alkali), to a role where some of the thallium atoms act as network formers. This is supported by 205Tl NMR measurements which indicate the presence of two thallium environments.