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

Abstract

Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl2O–GeO2, 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–O 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 ± 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 ± 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an effect which does not occur in alkali germanates.

Multicomposition EPSR: Toward Transferable Potentials To Model Chalcogenide Glass Structures

“…Through the combination of MC-EPSR and minimal constraints we have produced a set of interatomic potentials that are consistent with experimental techniques and are transferable to new glass compositions. In agreement with previous glass structure studies [27][28] we find that including a lone-pair is required to constrain the model to the expected interatomic arrangement and coordination numbers.…”

Lone-pair distribution and plumbite network formation in high lead silicate glass, 80PbO·20SiO2

“…High lead oxide glasses from PbO-M 2 O 3 systems such as the lead borates, 79 83 includes claims of a composition of 90 mol% PbO, however, the laboratory x-ray diffraction data are of insufficient quality to rule out Al 2 O 3 contents (arising from crucible contamination) larger than those stated, and the absence of a Pb-O peak in the radial distribution function was not explained. Takaishi et al 14 claimed to have studied an 89 mol% PbO lead silicate glass, however, their radial distribution function is consistent with a much lower Pb concentration, estimated at 72 mol% PbO from their reported glass density of 7.50 g/cm 3 . Combined with the fact that the authors 14 do not take into account apodization effects, their quoted coordination numbers are subject to large uncertainties.…”

“…However, the role of the stereochemically active electron lone-pairs on the Pb 2+ ions, particularly in relation to the free volume, was not considered. A different structural model for lead silicate glasses has been proposed by Takaishi et al, 14 in which the Pb cations are present predominantly as PbO 3 trigonal pyramids, which are interconnected by edges to form Pb 2 O 4 moieties, and have opposed orientations arising from steric repulsion between their respective electron lone-pairs. Whether or not a three-dimensional space-filling model, with the correct atomic number density, can be constructed based on such motifs was not addressed.…”

“…A clear picture of the role of lone-pairs (LPs) of electrons in a glass is important for an understanding of non-linear optical (NLO) properties, 1,2 and their location in the network is of interest from a fundamental structural point of view. 3 However, the location of LPs in a structure cannot be measured directly, but has to be derived indirectly from other structural information. Therefore it is necessary to build up a comprehensive model of the structure of a material in order to infer the location of the LPs, and this is what we have done for a lead silicate glass with an extremely high lead content.…”

Phonon Sideband Analysis and Near-Infrared Emission in Heavy Metal Oxide Glasses

“…They are also interesting from the structural point of view. The presence of minima or maxima in physicochemical properties of germanate-based glasses under addition of various network-modifiers usually well correlate with the coordination change GeO 4 tetrahedra ⟷ GeO 6 octahedra, the formation of Ge-O-Ge bridging bonds and/or the creation of nonbridging oxygen atoms NBO’s [ 12 , 13 ]. The structural mechanism responsible for the germanate anomaly well-known in the literature was proposed for glasses based on PbO-GeO 2 [ 14 , 15 ].…”