Influence of Lone-Pair Cations on the Germanate Anomaly in Glass

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• 作者：Emma R. Barney ; Alex C. Hannon ; Nattapol Laorodphan ; Diane Holland
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：August 4, 2011
• 年：2011
• 卷：115
• 期：30
• 页码：14997-15007
• 全文大小：1077K
• 年卷期：v.115,no.30(August 4, 2011)
• ISSN：1932-7455

文摘

Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl₂O鈥揋eO₂, containing up to 40 mol % Tl₂O, 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鈥揙 coordination number, n_GeO, is found to rise above four as Tl₂O is added to the glass. However, whereas for alkali germanates n_GeO has its maximum value (4.36 卤 0.03) at 19 mol % R₂O (e.g., R = Cs), for thallium germanates it continues to rise until 30 mol % Tl₂O, with a higher maximum value of 4.44 卤 0.02. For low Tl₂O content, most thallium cations are on modifier sites with a high coordination number (6 or greater). As the Tl₂O content increases, glass former [TlO₃] sites become increasingly common, and it is predicted from an extrapolation of the results that a glass with a composition of 50 mol % Tl₂O would be composed entirely of [TlO₃] and [GeO₄] units. It is shown that the presence of [TlO₃] 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鈥攁n effect which does not occur in alkali germanates.