Influência da composição da matriz nas propriedades espectroscópicas e luminescentes dos vidros aluminosilicato de cálcio dopados com Er e Cr

Abstract

The effects of matrix composition on the spectroscopic properties of calcium aluminosilicate glasses (CAS) doped with Erbium (Er) and Chromium (Cr) were studied in this work. Different matrix composition with silica ranging from 7 to 55 wt% were considered by keeping the amount of Er2O3 constant (0.5 wt%). In order to prevent the occurrence of OH-, sample synthesis was carried out under vacuum atmosphere condition. Samples containing these silica proportions are referred as low silica calcium aluminosilicate and calcium aluminosilicate, respectively. For the samples doped with Er, the lifetime of Er3+ (4I11/2) showed a significant decrease by increasing amount of silica, which was related to an increase of phonon energy. By comparing the experimental lifetime with those obtained theoretically by the Judd-Ofelt model, we observed a difference of three orders of magnitude, i.e., the values measured were about thousand times smaller than those predicted by the theoretical model. To calculate the probability of non-radiative transitions, we have established a system of equations taking into account the major interactions of calcium aluminosilicate glasses doped with Er3+. We found that the decay process from the 4I11/2 level of Er3+ is ruled by multiphonon relaxation process. With regard to study the effects of Cr, matrices with 7 and 34 wt% of silica were doped with 0.1 and 0.5 wt% of Cr2O3. Different valence states, namely Cr3+, Cr4+ and Cr6+ were observed in the samples. The presence of Cr6+ were observed in both compositions evidenced by the intense absorption band in the ultraviolet region, which is associated with charge transfer band. In CAS34 samples, we noted the formation of the (+3) valence state in octahedral symmetry with weak interaction with the crystal field. This valence state is the origin of the intense broadband emission centered at 860 nm. In the case of LSCAS samples, the valence state observed was Cr4+ in tetrahedral symmetry, resulting in an emission band at 1400 nm. As a final point, we verified that the formation of (+3) and (+4) valence states depends on the number of non-bridging oxygen (NBOs) in the host material. We think that silica concentration is the central issue accountable for the formation of different valences.