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

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

“…The peak position was fixed at 2.527 Å, a distance which gave the best fit to the Tl–O peak for the sample with 40 mol % Tl 2 O. This value is in excellent agreement with the average Tl–O distance in crystalline Tl 8 Ge 5 O 14 (2.525 Å) and in reasonable agreement with the Tl–O distance for a [TlO 3 ] unit with equal bond lengths (2.578 Å), as calculated from the Tl–O bond-valence parameter. ,, The width of the peak was fixed at 0.158 Å because this value was found to give a good fit for all compositions. Although, by fixing these values and allowing only the area of the peak to vary, the Tl–O distribution may not be modeled fully, this approach does measure changes in the Tl–O peak with composition and allows the results for different compositions to be directly compared.…”

“…The formation of Tl 0 (i.e., thallium metal) would result in a reduced thallium content in the glass that is undetectable by mass loss measurements but would reduce the density of the clear glass. The density of the 40 mol % Tl 2 O glass is slightly less than expected from the line of best fit shown in Figure a, but the indicated difference in composition is ∼1 mol % and will have only a small effect on the analysis …”

“…In fact, as will be shown, these two aspects of the structure are closely related. In a limited preliminary report, we have shown that for a germanate glass with 30 mol % Tl 2 O the value of n GeO is 4.44 ± 0.02, very much higher than for an alkali germanate glass with the corresponding composition (30 mol % Cs 2 O), for which the coordination number is 4.08 ± 0.03 …”

“…The revised compositions, assuming the mass loss is all Tl 2 O, are given in Table for comparison, but the deviation from the nominal composition is so small that they are ignored in the subsequent data analysis. The effect that varying composition has on the analysis of neutron diffraction data for these glasses has been discussed previously …”

“…It is very useful to compare the correlation functions, T ( r ), measured for thallium germanate glasses with those measured for cesium germanate glasses of the same GeO 2 content; this comparison shows how the effect of the modifier on the Ge–O coordination differs between the two systems . Figure shows this comparison for thallium germanate glasses with compositions between 5 and 30 mol % Tl 2 O ( T ( r ) for 20 mol % Tl 2 O is compared with T ( r ) for 21 mol % Cs 2 O because this is the closest cesium germanate composition which has been studied; all other comparisons are for glasses with exactly the same nominal GeO 2 content).…”