| Popis: |
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. |
