Advanced techniques for extracting structural information from neutron diffraction of glasses

• Main Author: Noor, Faizani M.
• Published: University of Warwick 2016
• Subjects: 539.7; QC Physics
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.698758

The structures of lithium and lead tellurite glasses (Li2O-TeO2 and PbO-TeO2) are studied using the combinations of neutron diffraction, isotope substitution neutron diffraction, and synchrotron X-ray diffraction. Additional complementary methods such as density measurement, thermal analysis and Raman spectroscopy show that the tellurite networks in lithium and lead tellurite glasses exhibit similar behaviour as a function of composition. From the diffraction data, real-space interatomic information on the glasses was extracted. The local environments of Te remain largely unchanged in both glasses if the second content is less than 15 mol%, as reported earlier for K2O-TeO2. For Li2O or PbO contents greater than 15 mol%, however, the average Te-O coordination number changes with composition and differently for the two oxides. A TeO2 Model, which has been successfully applied to K2O-TeO2 glasses, is extended to Li+ and Pb2+. By comparing the total correlation functions of the glasses to relevant crystal phases, the short-range parameters of the crystals can be used to optimise the model for Li+ and Pb2+ and explain the value of the average coordination number of Te. The environments of Li+ and Pb2+ were extracted using isotope substitution neutron diffraction (Li) and complementary neutron–X-ray diffraction (Pb). In the glasses studied (10, 15, 20, 25 and 30 mol% Li2O and 10, 12.5, 15, 17.5 and 20 mol% PbO), both Li+ and Pb2+ behave as modifiers with the average nLiO = 4 – 5 and nPbO = 8 with the distances rLiO and rPbO comparable to crystal phases of similar composition. The structures of lithium and potassium borogermanate glasses are studied using the combinations of neutron diffraction, isotope substitution neutron diffraction, and 11B MAS NMR. From the complementary methods such as density measurement, and Raman spectroscopy, changes are seen to occur in both borate and germanate networks. From 11B NMR, the average B-O coordination number, nBO, in lithium borogermanate glasses is different from nBO in potassium borogermanate glasses of the same stoichiometry and resembles the behaviour of Li+ and K+ in in borosilicate glasses, as described by the Zhong’s lithium borosilicate model and Dell and Bray’s sodium borosilicate model. From neutron diffraction data (null isotope neutron diffraction for lithium borogermanate), the average Ge-O coordination number nGeO is extracted and, like nBO, nGeO in lithium borogermanate is different from potassium borogermanate. In the former, nGeO increases as a function of Li2O, whilst in the latter, nGeO shows a maximum at about 20% K2O. For Li+, the average Li-O coordination number nLiO can be obtained using the difference technique (using diffraction data from samples made with natural and null-scattering lithium isotopes). For the limited samples examined, nLiO was found to have a value of 4 to 5 with two distinct Li-O distances which can be assigned to Li-OB and Li-OGe distances.