Optical glasses are used in a variety of applications in the fields of optics and photonics. Special glass compositions in the borosilicate and phosphate systems combine low thermal expansion coefficient and high resistance to thermal shocks with low refractive index and high transmission from the ultraviolet to the infrared range. Due to their properties such systems are considered as prototypes for the development of high performance optical glasses. For the systematic variation of their properties and/or the improvement of the glass-forming ability prototype glasses are often doped with polyvalent metal ions or conditional glass forming compounds (e.g. metal fluorides).
Doped borosilicate and phosphate glasses are being developed and investigated in collaboration with the Otto-Schott Institut (Friedrich-Schiller University, Jena, Germany), aiming at the establishment of correlations between structure/chemistry and macroscopic properties of glass. For example the performance of glass in photonics, and in particular its role as laser host material, depends strongly on the nature of the local anionic environments coordinating the guest metal ions. The effectiveness of the metal ion-hosting sites is controlled by the electron donating ability of the glass matrix, and this property is being quantified by the optical basicity, Ë, introduced in the pioneering works of Duffy and Ingram.
Phosphate-based oxyfluoride glasses were developed in the system xSr(PO3)2-(100-x)(MgF2, CaF2, SrF2, AlF3), denoted by PFx where x is the Sr(PO3)2 content in mol%. Studies of optical basicity for glasses in the PFx system revealed pronounced differences between calculated, Ëth, and experimental optical basicity probed by Pb2+ ions, ËPb, for intermediate Sr(PO3)2 contents (x). A structural study of these glasses with Raman spectroscopy showed that for x=0 and x=80-100 the key anionic species available for coordination with Pb2+ ions are fluoride and oxide ions in metaphosphate units (Q2, phosphate tetrahedra with 2 bridging oxygen atoms), respectively. For intermediate x values the Raman spectra reveal the presence of highly charged pyrophosphate (Q1) and orthophosphate (Q0) anionic sites coordinating Pb2+ ions. Such sites exhibit local basicity considerably higher than the average glass basicity.
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L.L. Velli, C.P.E. Varsamis, E.I. Kamitsos, D. Moencke and D. Ehrt, ''Optical basicity and refractivity in mixed oxyfluoride glasses'', Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B 49, 182 (2008).
Optical Basicity of Oxyfluoride Glasses
Effect of Sr(PO3)2 content on the calculated, Ëth, and measured optical basicity, ËPb, of FPx glasses, xSr(PO3)2-(100-x)(MgF2, CaF2, SrF2, AlF3).
Raman Spectra of Oxyfluoride Glasses
The evolution of the Raman spectra of glasses in the FPx system, xSr(PO3)2-(100-x)(MgF2, CaF2, SrF2, AlF3) with x denoting the Sr(PO3)2 content in mol%, manifest a progressive change in the nature of phosphate Qn tetrahedral units (n is the number of bridging oxygen atoms).