Structural studies of thallium containing germanate and borate glasses and crystalline phases

The structure of glasses and devitrified phases, in Tl2O⋅GeO2 and Tl2O⋅B2O3 systems, have been investigated using solid-state nuclear magnetic resonance (NMR), neutron diffraction and Raman spectroscopy. Gas pycnometry, thermal analysis and powder X-ray diffraction (XRD) have also been used to characterise and measure properties of glasses and devitrified phases. The crystallisation study showed that metastable crystalline phases were obtained from lowtemperature heat-treatment. X-ray diffraction of devitrification products, obtained at higher temperature, indicates crystallisation of the corresponding, more stable, stoichiometric compounds from the glass but with one or more second phases. Thallium NMR studies reveal that various chemical environments of thallium are present in the heat-treated samples. For the Tl2Ge4O9 sample, whose crystal structure is known, magic angle spinning NMR reveals the presence of one thallium site, the [Ge3O9] ring structure was observed using Raman spectroscopy. A germanate anomaly is present in the thallium germanate glasses indicated by a minimum value of molar volume at x ~ 0.15. Neutron diffraction and Raman spectroscopy confirm the change in the coordination number of germanium from [GeO4] to [GeO6 or 5]. Tl solid-state NMR suggests that at least two thallium sites are also present in the glass, low and high coordinated thallium. Tl-NMR spectra are broad and shifted mainly due to the chemical shift dispersion and chemical shift anisotropy interaction, respectively. T2 measurements confirmed that dipole-dipole and pseudodipole or exchange interactions are stronger in high thallium content glasses. A 203Tl enrichment study determined that the broadening due to the exchange interaction is ~ 10 kHz. Only three crystalline thallium borate phases were obtained, either singly or in combination with each other. Density data, X-ray diffraction patterns and solid-state NMR results of the three phases formed support what structural information exists in the literature. Chemical shift interaction and chemical shift dispersion contribute to the line broadening and lineshape of Tl NMR of TlB5O8 and TlB3O5 containing sample. Dipolar interaction and the Tl…Tl exchange interaction are stronger in the Tl2B4O7 sample. The chemical shift anisotropy parameters of TlB5O8 and TlB3O5 are presented. A borate anomaly is present in thallium borate glasses with a minimum value of molar volume at x ~ 0.28. Neutron diffraction and solid-state NMR spectroscopy confirm the change in the coordination number of boron to oxygen atoms from [BO3] to [BO4]. N4 values obtained from both techniques are very close and are larger than found for alkali borate glasses. Raman spectroscopy also reveals the changes in boron-oxygen superstructural units. 205Tl solid-state NMR suggests that at least two thallium sites are present in the glass, low and high coordinated thallium. Tl-NMR spectra are broad and shifted mainly due to the chemical shift dispersion and chemical shift anisotropy interaction, respectively. T2 measurement confirmed that dipole-dipole and exchange interactions occur in glasses throughout the range of studied compositions

Toward a structural model for the aluminum tellurite glass system

Neutron diffraction, 27Al MAS NMR, and 27Al Double Quantum MAS NMR results are presented and analyzed to determine the local environments of the cations in a series of aluminum tellurite glasses. Total scattering results show that, within a maximum Te–O distance of 2.36 Å, tellurium exhibits a mix of [TeO3E] and [TeO4E] environments (E = electron lone-pair), with a linear reduction in the average tellurium–oxygen coordination number as Al2O3 is added to the glass. This is accompanied by a linear decrease in the average aluminum–oxygen coordination number as [AlO4] units form at the expense of [AlO6] units, while the fraction of [AlO5] units remains roughly constant. A consideration of the bonding requirements of the five structural units in the glass, [TeO3E], [TeO4E], [AlO4], [AlO5], and [AlO6], has allowed a direct quantitative relationship between tellurium–oxygen and aluminum–oxygen coordination numbers to be derived for the first time, and this has been successfully extended to the boron tellurite system. Double Quantum 27Al MAS NMR indicates that, in contrast to previous reports, the shortest Al...Al separations are significantly smaller (∼3.2 Å) than expected for a uniform distribution and there is a preference for [AlO6]–[AlO6] and [AlO4]–[AlO4] corner sharing polyhedra. These associations support a new structural model which successfully applies the principle of charge balance to describe the interaction of tellurium and aluminum and identifies and explains the clustering of [AlOn] polyhedra in the glass and their preferred associations. [AlO6] and [TeO4E] units dominate the network in TeO2-rich glasses and [AlO4]− units form to stabilize the [TeO3E]+ units as alumina is added to the glass

A neutron diffraction and Tl-205 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, n(Ge-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 n(Ge-O) declines for caesium germanate glasses as the amount of modifier increases above 18 mol% Cs2O, n(Ge-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 Tl-205 NMR measurements which indicate the presence of two thallium environments. (C) 2010 Elsevier B.V. All rights reserved

Influence of lone-pair cations on the germanate anomaly in glass

Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl(2)O-GeO(2), containing up to 40 mol % Tl(2)O, 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 Tl(2)O is added to the glass. However, whereas for alkali germanates n(GeO) has its maximum value (similar to 4.36 +/- 0.03) at similar to 19 mol % R(2)O (e.g., R = Cs), for thallium germanates it continues to rise until 30 mol % Tl(2)O, with a higher maximum value of 4.44 +/- 0.02. For low Tl(2)O content, most thallium cations are on modifier sites with a high coordination number (6 or greater). As the Tl(2)O content increases, glass former [TlO(3)] sites become increasingly common, and it is predicted from an extrapolation of the results that a glass with a composition of 50 mol % Tl(2)O would be composed entirely of [TlO(3)] and [GeO(4)] units. It is shown that the presence of [TlO(3)] 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