Influence of Sr/Hf ratio and annealing treatment on structural and scintillating properties of sol–gel Ce3+-doped strontium hafnate powders
Introduction
Among the new solid-state scintillators of the last decades, SrHfO3 material appears as promising host lattice when activating with rare earths due to its high density and relative high light output [1], [2]. Indeed, the characteristic of such orthorhombic phase crystallized phosphor material is to emit light in the visible range under X-rays excitation [3]. This material has potential large impact in high-energy nuclear medical applications such as computer tomography and positron emission tomography, where Ce3+ emission is very efficient.
In this study, cerium (1 mol%)-doped strontium hafnate powders are prepared using the sol–gel process. The sol–gel process is known to be well adapted to provide high quality oxide materials and to allow the incorporation of doping agents with a good homogeneity at the molecular scale [4]. Nevertheless, the preparation by sol–gel process of crystallized bimetallic compounds (ABO3) in pure phase is sometimes difficult due to the difference of reactivity of the two metallic precursors [5]. Indeed, previous study conducted on sol–gel SrHfO3 powders indicates that the equimolar [Sr/Hf] ratio in the starting solution provides both HfO2 and SrHfO3 phases in the corresponding powder heat-treated above 900 °C [6].
This paper presents the influence of both the [Sr/Hf] molar ratios in the starting solutions and the annealing temperatures of the powder on either the structural characteristics or the luminescence properties under X-rays excitation of cerium (1 mol%)-doped strontium hafnate sol–gel material.
The structural properties of the powders are determined using X-rays diffraction, Raman spectroscopy and Fourier transformed infrared spectroscopy. Emission spectra under X-rays excitation are conducted using an home made equipment. Correlations between structural and emission properties are discussed.
Section snippets
Experimental
The starting solution is prepared using mixtures of hafnium tetra-ethoxide (Hf(OC2H5)4), (99.9%—Gelest) and strontium ethoxide, (Sr(OC2H5)2). Strontium alkoxide is synthesized by reacting strontium metal with ethanol under reflux and isolated as a white powder. Various Sr/Hf molar ratios (ranged from 0.66 to 1.63) are used in the precursors solutions. Cerium nitrate pentahydrate (Ce(NO3)3, 5H2O), (99.9%—Aldrich) is directly added as dopant under stirring in order to obtain a 1% mol
Results and discussions
The cerium (1 mol%)-doped strontium hafnate powder starts to crystallize from amorphous phase at 600 °C whatever the [Sr/Hf] molar ratio as observed by Raman spectroscopy and X-rays diffraction. Micro Raman and X-rays experiments conducted on powders heat-treated above 600 °C indicated that the crystallized phases depend on annealing temperature of the powders and on the [Sr/Hf] molar ratio of the precursor solution. As an example, the XRD patterns of the powders [Sr/Hf] = 1 heat-treated from 600 °C
Conclusions
We have synthesized the Ce3+-doped (1 mol%) strontium hafnate sol–gel powders at different annealing temperatures. The Sr/Hf molar ratios of the starting solutions vary from 0.66 to 1.63. We have studied the structural phases of the sol–gel powder using X-rays diffraction, micro Raman spectroscopy and FTIR. An excess of strontium in the starting solution is necessary to provide the pure SrHfO3 phase powder, but induces formation of strontium carbonate. Scintillation performances under X-rays
Acknowledgement
The authors are grateful to CONACyT-SFERE Program 2001 for M.V.-I. scholarship. The authors acknowledge Dr. A. Pillonet-Minardi for helpful discussions.
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