Speaker
Dr
Jan Bárta
(Czech Technical University (CZ); Acad. of Sciences of the Czech Rep. (CZ))
Description
Multicomponent oxide scintillators with the garnet structure (space group Ia-3d) have attracted significant attention in recent decade due to the possibility of alleviating structural defects and changing the luminescence properties by tuning the composition. Very often, the admixture of Ga into Ce-doped aluminate garnets is used to lower the conduction band minimum and thus to bury the anti-site defect traps in the conduction band [1], improving scintillation intensity. Moreover, the emission wavelength is slightly blue-shifted. At the same time, the thermal ionization of the Ce3+ 5d state becomes more probable, necessitating further modifications of the composition.
In a recent DFT simulation of A3B5O12 garnet structures (A = Ln, Y; B = Al, Ga, In, As, Sb), the substitution of the trivalent ion B was shown to have significant influence over the conduction band minimum and valence band maximum relative to LuAG (A = Lu; B = Al) [2]. Among these, the hypothesized Sb- and As-containing garnets exhibited a markedly raised valence band maximum, which would improve the transport of holes toward Ce3+ while preserving the Ce3+ 5d state position relative to conduction band. As both As2O3 and Sb2O3 are very volatile, the formation of such compounds precludes high calcination temperatures, let alone preparation using melting.
In this study, we utilized a photo-induced formation of solid precursors to GGAG (A = Gd; B = Ga+Al) and Lu3(Al,Sb)5O12 (LuSbAG) garnets in aqueous solutions of their salts and ammonium formate, followed by calcination in various atmospheres to form nanocrystalline garnets [3]. The resulting powder materials were examined by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), differential thermal analysis (DTA) and luminescence measurements.
The solid precursors to GGAG:Ce were prepared with almost 100% yield and contained identical concentrations of all elements as the initial solutions. During their calcination, a sharp exothermic DTA peak was detected between 800 and 900 °C (its temperature increased with decreasing Ga concentration), probably related to the crystallization of garnet phase. However, samples with high Ga concentration contained the garnet phase along with Gd3O2GaO4 even when calcined at 1200 °C – this impurity disappeared only after further heating. Radioluminescence spectra featured an intense Ce emission band, a weak broad band at ~370 nm (caused by anti-site defects) with lower intensity in samples with high Ga concentration, and several emission lines of Eu impurity.
The LuSbAG precursor was prepared by a two-step process due to fast Sb(III) hydrolysis into Sb(III) oxide – the precursor containing Lu and Al was formed in the solution, into which antimony acetate was then added. The produced solid phase with homogeneous distribution of elements was then calcined in different atmospheres. XRF and XRD confirmed that calcination in air somewhat prevents the loss of Sb from the material during heating at the cost of oxidation into Sb(V) oxides. Calcination in reducing or inert atmosphere led to significant losses of Sb, but a pure garnet phase with acceptable Sb content was formed after calcination at 700 °C for 3 hours in Ar. However, the additional heating required for high luminescence intensity causes further losses of Sb.
This research has been supported by the Czech Science Foundation grant GA 17-06479S and EC H2020 project ASCIMAT no. 690599.
[1] M. Fasoli, A. Vedda, M. Nikl, C. Jiang, B. P. Uberuaga, D. A. Andersson, K. J. McClellan, C. R. Stanek; Phys. Rev. B 84 (2011) 081102.
[2] S. K. Yadav, B. P. Uberuaga, M. Nikl, C. Jiang, C. R. Stanek; Phys. Rev. Applied 4 (2015) 054012.
[3] J. Bárta, V. Čuba, M. Pospíšil, V. Jarý, M. Nikl; J. Mater. Chem. 22 (2012) 16590-16597.
Author
Dr
Jan Bárta
(Czech Technical University (CZ); Acad. of Sciences of the Czech Rep. (CZ))
Co-authors
Dr
Vítězslav Jarý
(Acad. of Sciences of the Czech Rep. (CZ))
Alena Beitlerova
(Acad. of Sciences of the Czech Rep. (CZ))
Prof.
Václav Čuba
(Czech Technical University (CZ))
Prof.
Martin Nikl
(Acad. of Sciences of the Czech Rep. (CZ))
