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Title: The structure of vitreous binary oxides : silicate, germanate and plumbite networks
Author: Alderman, Oliver L. G.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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Structural aspects of three related binary oxide glass systems have been studied in detail using a combination of neutron and x-ray diffraction, empirical modelling techniques, and information from bulk property and spectroscopic measurements. The local Pb2+ environment in PbO SiO2 glasses changes only subtly with composition, having, on average 3.33 short (2:70 Å) and 1.3 long (2.70 < rPbO < 3:27 Å) Pb- O bonds at 35 mol% PbO, and 3.57 short and 1.3(2) long Pb-O bonds at 80 mol% PbO. Therefore, over the entire series, lead behaves as a glass network forming cation, with highly asymmetric ligand distribution and stereochemically active electron lone-pair (LP), with gradual transition toward more axially symmetric environments (cf. crystalline PbO) as the PbO content increases. Structural modelling of the highest lead content oxide glass (80 PbO < 20 SiO2) to date reveals organisation of LPs to form voids, analogous to interlayer spaces in crystalline PbO polymorphs, and channels found within other crystalline lead oxide compounds such as Pb11Si3O17. The plumbite glass network is characterised by a high degree of intermediate-range ordering, evidenced by a very narrow first sharp diffraction peak (FSDP), as well as significant edge-sharing of Pb polyhedra and high oxygen-cation coordination numbers, e.g. [OPb4], [OPb3Si]. PbOGeO2 glass formation was found as high as 75 mol% PbO using rapid twinroller quenching. Such high lead glasses are analogues of the silicates, but with a 10% enhancement in correlation length derived from the FSDP. In the low PbO region a broad maximum in Ge-O coordination number of nGeO = 4:14(3) at 26:5(5) mol% PbO was observed. This is much smaller, and at higher metal oxide content than in Na, K and Cs germanate glasses and contradicts published results [N. Umesaki et al., Physica B 213, (1995), 490]. The weakness of the effect is attributed to Pb2+ playing a predominantly network forming role, although Ge-O and Pb-O coordination numbers showed positive correlation, such that the presence of [GeO5] or [GeO6] is indicative of Pb having some ‘modifying’ character. It was argued, based on empirical models and the known crystal chemistry, that Pb may occupy sites of intermediate character, with mixed s-p LP character, rather than invoking a bimodal distribution of network forming and modifying Pb sites. Homogeneous calcium germanate glasses were obtained in the region 21 to 41 mol% CaO by rapid twin-roller quenching, with glass-in-glass phase separation occuring in the low CaO region, and crystallisation around the CaGeO3 composition. A very broad maximum of nGeO = 4:30(3) at 28:0(5) mol% CaO was observed, correlating approximately with maxima in atomic number densities and glass transition temperatures. Nonbridging oxygen (NBO) are present in all glasses, again in contrast to alkali germanates. The position of the maximum is attributed to stabilisation of vertex sharing [GeOm>4] and/or [GeO6] units by divalent Ca2+. The presence of NBOs is related to the Ca-O coordination number of 7, with higher nGeO and less NBO predicted in Sr and Ba germanate glasses, where M-O coordination is expected to be larger. In both Ca and Pb germanate systems, the equilibrium crystals have much larger nGeO than the glasses, and this implies a decrease of nGeO with liquid, supercooled liquid and glass fictive temperatures. This tentative conclusion is supported by measurements on two 40 PbO 60 GeO2 glasses with different thermal histories.
Supervisor: Not available Sponsor: Science and Technology Facilities Council (Great Britain) (STFC) (CMPC09105) ; Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry