Structure and Properties of Orthoborate Glasses in the Eu2O3–(Sr,Eu)O–B2O3 Quaternary
2015
Winterstein-Beckmann, Anja | Moncke, Doris | Palles, Dimitrios | Kamitsos, Efstratios I. | Wondraczek, Lothar
The structure and properties of melt-quenched glasses and partially crystallized samples from the borate series (1–2x)Eu₂O₃–x((Eu,Sr)O–B₂O₃) were investigated in the supermodified regime of x < 0.5, using Raman, infrared (IR), electron spin resonance (ESR), and UV–vis absorption and fluorescence spectroscopic techniques. ESR and optical spectroscopy showed that, despite the strongly reducing synthesis conditions, the Eu²⁺/Eu³⁺ equilibrium remained shifted to the side of trivalent Eu³⁺. Stable and transparent overmodified borate glasses were produced for compositions with x ≥ 0.36. Higher europium oxide concentrations resulted in precipitation of crystalline Eu₂Sr₃(BO₃)₄ and EuBO₃ phases, as traced by X-ray diffraction. Raman and IR spectroscopy showed that the metaborate configuration which is present at x = 0.46 transforms gradually, with increasing Eu₂O₃ levels, into orthoborate [BO₃]³– triangular units. At higher europium oxide content (x ≤ 0.36), the presence of Eu³⁺ supports the formation of orthoborate [BØ₂O₂]³– tetrahedral species. These units organize into [B₃O₉]⁹– rings, which exist in equilibrium with [BO₃]³– triangles. As a consequence, distinct variations can be observed also in the macroscopic properties such as density, glass transition temperature, refractive index, optical basicity, and oxygen polarizability. This observation confirms previous findings on manganese–strontium borates with high modification levels.
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