B and P Glass Formers as Stabilizers of Luminescent Ce(III) in Silica-Based Glasses
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B and P Glass Formers as Stabilizers of Luminescent Ce(III) in Silica-Based Glasses C.Canevali1, M. Mattoni1, F. Morazzoni1, R.Scotti1, R. Krsmanovic2, S. Polizzi2, M.Bettinelli3, A. Speghini 3 1 INSTM, Dept. Scienza dei Materiali, Univ.Milano-Bicocca, via Cozzi, 53, 20125 Milano, Italy 2 Dept. Chimica Fisica, Univ. Ca’ Foscari, via Torino, 155/b, 30172 Venezia-Mestre, Italy 3 Dept. Scientifico e Tecnologico, Univ.Verona, strada le Grazie, 15, 37134 Verona, Italy
ABSTRACT Ce3+-doped boro, phospho and borophosphosilicate monolithic glasses (BSG, PSG and BPSG) were prepared by sol-gel method. The inclusion of the glass formers P and B in silica lowered the tendency of rare earth (RE) to segregate as CeO2 with respect to pure SiO2 matrix (SG). In PSG and BPSG, Ce3+ co-ordination to P=O groups was responsible for the inhibition of both CeO2 segregation and oxidation of isolated Ce3+ ions to Ce4+. The better ion dispersion in the host matrix and the stabilization of Ce3+oxidation state improved PSG and BPSG luminescence properties with respect to BSG and SG.
INTRODUCTION Ce3+-doped crystalline compounds received great interest owing to the rare earth (RE) luminescence properties in UV or visible region [1, 2] (5d–4f transition). The luminescence yield is unfavorably affected by Ce3+ aggregation phenomena which occur when the ion distance becomes comparable to the critical radius, the distance at which the radiative decay probability equals the energy transfer rate, leading to non-radiative processes [3, 4]. Besides, Ce3+ can be oxidized to not luminescent Ce4+. In order to obtain the highest concentration of homogeneously dispersed luminescent activators, RE doped glasses seemed to be a very suitable alternative to crystalline materials [5, 6]. Their main drawback is the low efficiency in the energy transfer from the host glass matrix to the emission centers; this effect is caused by point defects and OH groups, which induce non-radiative recombination channels. High purity and controlled composition luminescent glasses with RE ions as activators were obtained by the sol-gel technique [7, 8] at lower temperatures than by traditional melting methods. Recently we prepared Ce-doped SiO2 glasses by sol-gel method [9-11] with promising luminescence yield and scintillation decay time. These properties showed to be depending both on the glass densification temperature, whose increase induced the progressive desorption of OH groups responsible for non-radiative recombination channels and on the RE concentration. In particular luminescence quenching occurred at Ce concentration higher than Ce:Si molar ratio 1•10-3, along with the segregation of CeO2 nanocrystalline particles in the host matrix [12] . In order to make the glass matrix more suitable to embed rare-earth ions, avoiding RE clustering and segregation of oxidized phase, silica was modified by the inclusion of glass formers, P and B. These were expected to induce local distortions in the silica network,
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resulting in a less compact glass texture
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