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Cerium doped mixed oxide garnets are promising luminescent materials to use as LED phosphors or scintillators. Their properties can be adjusted via composition engineering for use in a specific application [1]. The cubic crystal structure allows preparation of highly transparent scintillation ceramics, which is a more cost efficient process than crystal growth.
Every change in the composition affects multiple parameters, meaning that the final composition is usually a compromise between several significant factors. One of these factors is the temperature stability of the scintillator. For many applications it is crucial to use a scintillator that shows stable signal in the range of operating temperatures.
The main goal of this study is the characterization of Ce3+ doped garnet temperature dependent properties and the analysis of mechanisms responsible for said properties. Luminescent properties of the following samples have been studied: Y3GaxAl5-xO12:Ce3+ (x=1, 2, 3, 4) and Gd3GaxAl5-xO12:Ce3+ (x=1, 2, 3). The temperature dependence of the Ce3+ luminescence intensity has been measured using continuous X-ray excitation (40 kV, 10 mA). Measurements of the Ce3+ luminescence lifetime have been performed at different temperatures using pulsed UV (445 nm) excitation.
The samples show strong Ce3+ 5d1 → 4f band emission in the 500-600 nm range. The maximum of the Ce3+ emission experiences a blue shift with increasing Ga content. For all samples the Ce3+ luminescence decreases upon heating above certain temperature. This process is usually called temperature quenching (TQ). According to modern concepts, TQ in Ce doped garnets is a result of thermal ionization of electrons from the Ce3+ excited 5d1 state [2,3]. Replacement of Y with Gd or admixture of Ga into the compositions causes TQ to start at lower temperature meaning that the energy gap between the Ce3+ 5d1 excited state and the bottom of the conduction band decreases. Values of this energy gap have been determined for every composition by fitting the measured temperature dependence of the decay time with the Mott formula. Comparison of obtained values with literature is given.
While Y3GaxAl5-xO12:Ce3+ samples have almost constant luminescence intensity at low temperature, Gd3GaxAl5-xO12:Ce3+ show a significant decrease in intensity upon cooling from room temperature to liquid nitrogen temperature (negative TQ). An increase of the Ga content shifts the negative TQ region to even lower temperatures. Thermally stimulated luminescence has been measured for every composition. In the temperature region where the negative TQ takes place in Gd3GaxAl5-xO12:Ce3+ samples, large thermoluminescence peaks have been observed which led to the conclusion that this behavior is most probably the result of significant localization of charge carriers on traps.
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References:
- K. Kamada, T. Endo, K. Tsutumi, Cryst. Growth Des. 11 (10), 4484–4490 (2011)
- G. Blasse, W. Schipper, J.J. Hamelink, Inorganica Chimica Acta, 189, 77-80 (1991)
- J. Ueda, P. Dorenbos, A.J.J. Bos et al., J. Phys. Chem. C., 119, 25003−25008 (2015)
