Terminal Oxygens in Amorphous TeO₂

Abstract

Understanding the structure of single-component glasses is essential for developing structural models of more complex multicomponent glasses. Currently, such models for tellurite systems are purely qualitative. This study presents neutron diffraction and Raman spectroscopy measurements of the structure of pure amorphous TeO 2 , showing that it is formed from a combination of two-thirds [TeO 4 ] pseudo-trigonal bipyramids and onethird [TeO 3 ] trigonal pyramids with a terminal oxygen. This is in contrast to all crystalline polymorphs of TeO 2 , which are formed solely from the four-coordinated units. Using this result, a quantitative model has been developed that successfully predicts the average Te−O coordination number, n TeO , for a series of potassium tellurite glasses, xK 2 O•(100 − x)TeO 2 . The observed n TeO is constant up to 15 mol % K 2 O due to the presence of terminal oxygen atoms in the tellurite network.

Alkali environments in tellurite glasses

“…showed that there is a step-change in and PQ at about 15 mol% Na2O, close to the composition where a change in Te environment is proposed by our structural model for tellurites [19]. By means of an empirical relation derived by Koller et al, [38] they used their values of to estimate the average coordination number of sodium in their sodium tellurite glasses, giving nNaO = 5.8 and 5.5 for the glasses containing 10 and 20 mol% Na2O respectively.…”

“…The total correlation functions, (Figure 4), exhibit two peak maxima at ~1.9 and 2.8 Å, which may be assigned to Te-O bonds and O…O distances respectively and thus arise from the Te-O network. It is important to note that the distribution of Te-O bond lengths in tellurite glasses [19] extends over a wide range of interatomic distances (say ~1.8 -2.4 Å). The expected positions for the M-O contributions to are indicated by arrows in Figure 4;…”

“…However, this will only be the case if the Te-O bond distribution is independent of M. Several Raman studies of alkali tellurite glasses have shown that nTeO is relatively unaffected by the size of modifying cation [10,23] and a quantitative model [19], proposed by the Authors to predict the observed change in Te-O coordination number (measured by neutron diffraction) with potassium oxide content, is independent of the alkali coordination number, and hence alkali type. Therefore, the objectives of this study are two-fold; to extract information about the local environment of alkali ions in tellurite glasses and to investigate whether the Te-O environment in alkali tellurites is indeed independent of the modifier used.…”

“…The former are found in pure crystalline -TeO2 [15], while the latter have an arrangement of atoms similar to that present in M2TeO3 crystals [16][17][18]. The change in the local environment of a Te atom from [TeO4E] to [TeO3E] provides an additional NBO in the network, and hence reduces the total number of unfavourable M-BO bonds needed to fulfil the bond requirements of the M + ions [19].…”

“…A detailed knowledge of the behaviour of the M-O coordination is important for the development of a reliable model for the composition-dependence of the Te-O network in M2O-TeO2 glasses [19]. However, there are few direct observations of the local environment of an alkali ion in alkali tellurite glasses and the results of these studies are now summarised.…”