Incorporation of PbF2 into Heavy Metal Oxide Borate Glasses. Structural Studies by Solid State NMR

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• 作者：Roger Gomes Fernandes ; Jinjun Ren ; Andrea S. S. de Camargo ; Antonio Carlos Hernandes ; Hellmut Eckert
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：March 15, 2012
• 年：2012
• 卷：116
• 期：10
• 页码：6434-6445
• 全文大小：565K
• 年卷期：v.116,no.10(March 15, 2012)
• ISSN：1932-7455

文摘

A series of heavy metal oxide (HMO) glasses with composition 26.66B<sub>2sub>O<sub>3sub>-16GeO<sub>2sub>-4 Bi<sub>2sub>O<sub>3sub>-(53.33-x)PbO-xPbF<sub>2sub> (0 鈮?x 鈮?40) were prepared and characterized with respect to their bulk (glass transition and crystallization temperatures, densities, molar volumes) and spectroscopic properties. Homogeneous glasses are formed up to x = 30, while crystallization of 尾-PbF<sub>2sub> takes place at higher contents. Substitution of PbO by PbF<sub>2sub> shifts the optical band gap toward higher energies, thereby extending the UV transmission window significantly toward higher frequencies. Raman and infrared absorption spectra can be interpreted in conjunction with published reference data. Using <sup>11sup>B and <sup>19sup>F high-resolution solid state NMR as well as <sup>11sup>B/<sup>19sup>F double resonance methodologies, we develop a quantitative structural description of this material. The fraction of four-coordinate boron is found to be moderately higher compared to that in glasses with the same PbO/B<sub>2sub>O<sub>3sub> ratios, suggesting some participation of PbF<sub>2sub> in the network transformation process. This suggestion is confirmed by the <sup>19sup>F NMR spectra. While the majority of the fluoride ions is present as ionic fluoride, 20% of the fluorine inventory acts as a network modifier, resulting in the formation of four-coordinate BO<sub>3/2sub>F<sup>鈥?/sup> units. These units can be identified by <sup>19sup>F{<sup>11sup>B} rotational echo double resonance and <sup>11sup>B{<sup>19sup>F} cross-polarization magic angle spinning (CPMAS) data. These results provide the first unambiguous evidence of B鈥揊 bonding in a PbF<sub>2sub>-modified glass system. The majority of the fluoride ions are found in a lead-dominated environment. <sup>19sup>F鈥?sup>19sup>F homonuclear dipolar second moments measured by spin echo decay spectroscopy are quantitatively consistent with a model in which these ions are randomly distributed within the network modifier subdomain consisting of PbO, Bi<sub>2sub>O<sub>3sub>, and PbF<sub>2sub>. This model, which implies both the features of atomic scale mixing with the network former borate species and some degree of fluoride ion clustering, is consistent with all of the experimental data obtained on these glasses.