SCINT 2017 - 14th Int. Conference on Scintillating Materials and their Applications

Thermoluminescence evidence of grain boundary structural disorder in LuAG:Ce optical ceramics

21 Sept 2017, 10:00

1h

Congress Centre "Le Majestic"

Poster presentation P5_characterization Poster Session 3

Speaker

Dr F. Moretti (Department of Materials Science, University of Milano-Bicocca, Italy)

Description

Scintillating optical ceramics (OC) based on garnet structures have been the subject of intense R&D. The interest on this kind of scintillating optical materials spawned from the excellent results obtained on Nd:YAG laser OC, whose production route allowed better control on dopant concentration and spatial distribution in the final laser active element, substantially outperforming the single crystal counterparts.${}^1$ However, the results obtained on scintillating garnet OC by using the same synthesis strategies employed for laser OC have not been as successful, and only recently OC with comparable or better performances with respect to those of single crystals have been reported, based on a divalent ion codoping strategy.${}^2$ Indeed, sintering aids are used to improve the powder sinterability leading to a reduced amount of light scattering centres and thus to a better transparency. However, sintering aids can also give rise to point defects which strongly affect carrier migration and transfer to luminescence centres. The recent improvements are substantially related to a better understanding of the sintering aid role on defect formation${}^3$ and the scintillation mechanisms, and the use of different sintering aids with respect to those used for the production of laser OCs.
In this contribution we will present results obtained on two different LuAG:Ce OC (with and without MgO as sintering aid) and on a Czochralski grown single crystal as a reference. Optical absorption (OA), Raman scattering, radio- (RL) and thermo-luminescence (TSL) results will be presented and discussed in view of the detected differences among the optical ceramics and the single crystal, with the aim to evaluate defect localization in the OC. OA measurements evidenced the typical Ce3+ transitions accompanied, in the case of Mg codoped ceramic, by the presence of Ce4+ charge transfer transition. RL spectra show only minor variations in the Ce3+ emission shape and intensity, related to the different cerium contents in the three samples. Raman spectra of all the samples are practically coincident, clearly demonstrating that the three samples are, from optical and vibrational points of view, indistinguishable.
TSL results, on the other hand, evidenced clear differences among the OC and the single crystal. The glow curves of the ceramics are in fact much broader. Trap energy evaluation indeed evidenced the presence of broad distributions that are usually encountered when structural (or compositional) disorder is present. This leads us to conclude, also taking into consideration the other results, that the detected broad TSL peaks are mostly related to defects at grain boundaries. In such regions, structural disorder occurs, giving rise to inhomogeneous broadening of defect levels.
Therefore, TSL appears as an extremely effective technique to evidence the presence of structural disorder in OC which would, otherwise, be practically undetectable by other more commonly used bulk characterization methods.
This work has been supported by the H2020 project AIDA-2020, GA no. 654168.

[1] A. Ikesue et al. Nature Photonics 2(2008)721
[2] S. Liu et al. Advanced Opt. Mater. 4(2016)731
[3] Y. Shen et al. J. Am. Ceram. Soc. 95(2012)2130