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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2014, 5 (6), P. 752-756
X-RAY LUMINESCENCE OF BaF2:Ce3+ POWDERS
S.Kh. Batygov, M.N. Mayakova, S. V. Kuznetsov, P.P. Fedorov A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russia
PACS 61.46
We studied the mechanism for the formation of cerium-activated barium fluoride scintillation ceramics and especially X-ray luminescence of its powdered precursors, prepared by co-precipitation of barium and cerium fluorides from aqueous solutions. We have found that the Ce3+ luminescence, which is typical for cerium (III)-containing ceramics and single crystals, was not observed for such polycrystalline precursors, and the intensity of barium fluoride's own luminescence decreases with increasing amounts of the cerium dopant in the specimens. We have interpreted our results as two-phase precipitation of barium hydrofluoride (BaF2 •HF) and cerium fluoride, respectively. Cerium (III) became incorporated in fluorite-type barium fluoride lattice only later, in the course of ceramics synthesis by the hot-pressing technique.
Keywords: Fluoride powder, nanoparticles, scintillators, X-ray luminescence. Received: 30 October 2014
1. Introduction
The study of fluoride nanoceramics is one of the most promising areas of modern photonics [1-3], and barium fluoride is one of the most interesting and actively studied fast scintillators [3-8]. BaF2 ceramics and single crystals exhibit two types of emission: a broad excitonic line with maximum near 300 nm (slow luminescence, t = 600 - 800 ns), and a short-wavelength line at 220 nm, corresponding to the superfast luminescence core-valence transitions (t = 0.8 ns). Addition of CeF3 to the BaF2 matrix leads to the transfer of exciton energy to cerium ion and powerful Ce3+-luminescence at 300 - 350 nm [7,9-12]. Because BaF2:Ce3+ ceramics have an enhanced scintillation yield compared to the same composition single crystals [11,12], we have developed a technology for the further improvement of its optical quality that included precipitation of barium hydrofluoride BaF2HF from aqueous solutions followed by its thermal decomposition with HF evolution. The latter has been found to be an active flu-orinating reagent that eliminated oxygen impurities from the batch materials [13,14]. Thus, the topic of the present paper concerns the influence of cerium doping on the synthesis of barium fluoride scintillation ceramics and its precursors.
2. Experimental
99.99 wt. % pure Ba(NO3)2, Ce(NO3)3, HF and doubly-distilled water were used as starting materials. All precipitation experiments were carried in polypropylene reactors at room temperature under air. All samples were prepared by co-precipitation from aqueous solutions [13-15]. Premixed corresponding amounts of starting 0.17 M metal nitrate solutions (Ce/(Ce+Ba) (molar ratios varied from 0.01 to 1%) were treated in a dropwise manner with concentrated aqueous HF under vigorous stirring. The HF content in the formed matrix solution was maintained at about 47 vol. %. Formed precipitates were additionally treated with conc. HF, decanted, thoroughly washed with doubly-distilled water and dried under air at 40 °C.
Further thermal treatment included heating of the synthesized precipitates at 10 °C/min to 550 °C followed by 1-hour annealing at the same (550 °C) temperature under air.
The phase composition of the synthesized samples was determined by X-ray diffraction analysis (Bruker D8 diffractometer, CuKa radiation, TOPAS software for lattice parameter calculations and profile treatment of the X-ray diffraction pattern lines). All sample X-ray-initiated luminescence spectra were recorded at room temperature (KSVU spectrometer, Cu anode, 10 kV , 10 mA ).
3. Results and discussion
Typical X-ray diffraction patterns of the synthesized samples are presented in Fig. 1.
Fig. 1. Typical X-ray diffraction patterns of barium fluoride samples: mono-clinic BaF2HF [13,14] precipitated by HF from aqueous solution at room temperature (a); and the same sample after its washing with water, cubic BaF2 (b)
All barium difluoride samples had a0 = 6.189 - 6.200 A lattice parameters that were in a good agreement with the known literature data for pure BaF2 [15].
These precipitates did not exhibit any luminescence without additional treatment, but after annealing at 550 °C, intrinsic BaF2 produced luminescence at 320 nm if excited by X-ray radiation (Fig. 2; for comparison, please see there a similar spectrum of monocrystalline BaF2, too).
Lines in both spectra have about the same maximum wavelengths (its location for the single crystals is associated with self-trapped exciton irradiation), and luminescence intensity for the powdered BaF2 batch is comparable to that of single crystals.
The luminescence spectra for cerium-doped BaF2 batch powders are presented in Fig. 3. The addition of cerium to BaF2 samples decreases the intensity of its own line at ca. 320 nm, but - in contrast with cerium-doped single crystals and ceramics - such spectra also lack of Ce3+-related luminescence lines (i.e., lines attributed to Ce3+ ions in BaF2 matrix) (Fig. 4).
0.1% Ce-doped ceramics and single crystals have BaF2-related luminescence almost completely suppressed, and the only lines visible in the luminescence spectra are those related
i-■-1-1-1-1-r -1-1-1-1-T"
200 250 300 350 400 450 500
wavelength, nm
Fig. 2. X-Ray luminescence spectra of intrinsic BaF2: single crystal (1) and powder (2) (recorded under the same ambient conditions)
—i-1-j-1-1-1-1-1-1-1-1-1-r~
200 250 300 350 400 450 500
wavelength, nm
Fig. 3. Luminescence spectra of intrinsic (1) and cerium-doped BaF2 batch powders: 0.01% Ce (2), 0.1% Ce (3), and 1% Ce (4)
to the 5d — 4f transitions in Ce3+ ions [8-12]. However, our cerium-doped BaF2 batch samples did not exhibit such Ce3+-related lines, and suppression of the BaF2-related luminescence line was much less than expected. Also, increasing the cerium concentration in the doped samples caused a shift of the line toward the higher wavelength part of the spectrum (Fig. 3). The negligible influence of cerium dopant on BaF2 luminescence and lack of Ce3+-related lines in
Fig. 4. X-ray luminescence spectra of polycrystalline (1) and ceramic (2) BaF2 -0.1% Ce
the recorded spectra may indicate that cerium did not get incorporated in BaF2 matrix during precipitation (co-precipitation) from aqueous solutions and formed an additional solid phase.
4. Conclusions
1. X-Ray radiation excitation produces luminescence at ca. 320 nm in co-precipitated barium-cerium fluoride powders, which is typical for intrinsic BaF2 and is usually associated with the auto-localized exciton in intrinsic BaF2 single crystals and BaF2 ceramics (intensity of such lines for monocrystalline and polycrystalline (ceramic) BaF2 is about the same under similar conditions).
2. Doping of BaF2 with cerium during their co-precipitation from aqueous solutions leads to a decrease in the intensity of the BaF2-related luminescence line, and there is no cerium-related luminescence line observed (in contrast to the same composition BaF2:Ce ceramics or single crystals).
3. The latter indicates that co-precipitation of BaF2 and CeF3 from aqueous solutions results in the formation of two separate fluoride phases, and cerium only becomes incorporated in fluorite-type BaF2 crystalline lattice only during hot-pressing ceramics synthesis.
Acknowledgements
This work was supported by the Scholarship of the President of Russian Federation (SP-6467.2013.2) and RFBR grant No. 13-02-12162-ofi-m. Authors thank V.V. Voronov and R. P. Ermakov for their help with X-ray diffraction experiments.
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