Alkali environments in tellurite glasses

Neutron diffraction measurements are reported for five binary alkali tellurite glasses, chi M2O center dot(100 - chi)TeO2 (containing 10 and 20 mol% K2O, 10 and 19 mol% Na2O, and 20 mol% (Li2O)-Li-7), together with Na-23 MAS NMR measurements for the sodium containing glasses. Differences between neutron correlation functions are used to extract information about the local environments of lithium and sodium. The Na-O bond length is 237(1) angstrom and the average Na-O coordination number, n(NaO), decreases from 5.2(2) for chi = 10 mol% Na2O to 4.6(1) for chi = 19 mol% Na2O. The average Li-O coordination number, n(LiO), is 3.9(1) for the glass with chi = 20 mol% Li2O and the Li-O bond length is 2.078(2) angstrom. As chi increases from 10 to 19 mol% Na2O, the Na-23 MAS NMR peak moves downfield, confirming an earlier report of a correlation of peak position with sodium coordination number. The close agreement of the maximum in the Te-O bond distribution for sodium and potassium tellurite glasses of the same composition, coupled with the extraction of reasonable alkali coordination numbers using isostoichiometric differences, gives strong evidence that the tellurium environment in alkali tellurites is independent of the size of the modifier cation used. (C) 2015 Elsevier B.V. All rights reserved.