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Preparation and X-ray luminescence of Ba4±xCe3±xF17±x solid solutions
D. S. Yasyrkina, S. V. Kuznetsov, A. A. Alexandrov, S. Kh. Batygov, V. V. Voronov, P. P. Fedorov Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
darya.yasyrkina@gmail.com
DOI 10.17586/2220-8054-2021-12-4-505-511
Single-phase BaF2:Ce solid solutions containing 30 - 40 mol. % cerium with the simultaneous entry of potassium into the crystal lattice in an amount of 0.7 - 0.8 mol. % were prepared by coprecipitation from aqueous nitrate solutions with potassium fluoride as the fluoridating agent. The cerium X-ray luminescence intensity increases in response to increasing cerium concentration contrary to the concentration quenching effect. Keywords: barium fluoride, cerium, X-ray luminescence. Received: 21 July 2021
1. Introduction
Barium fluoride is a well-known heavy scintillator with two X-ray luminescence bands. The 220-nm band exhibits a very short decay time (0.8 ns), and a broad exciton band peaking around 300 nm has a decay time of 600 - 800 ns [1,2]. In cerium-doped barium fluoride, exciton luminescence transforms to cerium ion luminescence induced by d-f electron transitions in Ce3+ ion [3-8]. The highest luminescence light yield occurs when the CeF3 concentration is approximately 0.1 mol. %. At higher cerium concentrations, the luminescence intensity declines dramatically due to the concentration quenching phenomenon [9]. The decline was observed both for single crystals and for BaF2:Ce3+ optical ceramics [10-15]. The concentration decay effect was assigned to defect cluster formation in fluorite-type lattices upon formation of Ba1-xCexF2+x solid solutions [9]. The composition corresponding to the highest luminescence light yield is the one with the highest concentration of free Ce3+-F- dipoles in the BaF2 lattice [16]. In addition, barium fluoride dissolves up to 52 mol. % CeF3, which radically changes the physical properties of its crystals, in particular, appreciably increases their density [17,18].
A natural strategy to gain X-ray luminescence in high-cerium Ba1-xCexF2+x solid solutions is to destroy defect clusters to release Ce3+-F- free dipoles. This can be fulfilled by doping the solid solution with univalent cations, e.g., potassium. Potassium fluoride is frequently used to prepare precursors of fluoride optical ceramics [19-23]. It was due to the use of potassium fluoride in the synthesis of precursors that X-ray luminescence was induced in BaCeF5 ceramics [24].
The goals of this work were to prepare Ba1-xCexF2+x solid solutions by coprecipitation from aqueous nitrate solutions with KF as the precipitating agent and to study X-ray luminescence of Ce3+ ions.
2. Experimental section
The initial reagents were used Ce(NO3)3 • 6H2O (99.99% pure, Lanhit, Moscow, Russia), Ba(NO3)2 (specialty grade, Vekton, St. Petersburg, Russia), KF2H2O (reagent grade, the Fluoride Salts Plant, Russia), and bidistilled water. The potassium fluoride was stored in a desiccator. The reagents were used as received.
Samples were prepared by coprecipitation from aqueous solutions [25] by reaction (1):
(1 -x)Ba(NO3)2 +xCe(NO3)3-6H2O+(2+x)KF• 2H2O = Bai-xCe^+x I +(2+x)KNO3 + (10 + 2x)H2O. (1)
Barium nitrate and cerium nitrate solutions (0.08 mol/L each) were mixed, and the mixed solution was added drop by drop to a 0.16 M potassium fluoride solution under vigorous stirring on a magnetic stirrer. The potassium fluoride was used in a 7% excess over the stoichiometry. Once dropwise addition was over, the resulting suspension was stirred for 2 hours. After a precipitate settled, the mother solution was decanted, and the precipitate was washed with bidistilled water. Nitrate leaching was monitored by the diphenylamine test. The as-washed precipitate was air-dried at 45 °C. The cerium fluoride content in samples ranged from 30 to 55 mol. %. X-ray phase analysis of samples were carried out on Bruker D8 ADVANCE diffractometer using CuKa radiation. The unit cell parameters and coherent scattering domain (CSD) sizes were calculated in the TOPAS software (Rwp < 10). The size of the particles and the morphology of the samples were studied by means of Carl Zeiss NVision 40 scanning electron microscope (Germany) with an Oxford Instruments XMAX microprobe analyzer (UK) (80 mm2) for energy dispersive X-ray spectroscopy. The particle size was averaged over 25 particles in the ImageJ software. The X-ray luminescence (XRL) spectra of the prepared samples were measured on a laboratory installation built of an X-ray source (tungsten anode) and an FSD-10
mini-spectrometer (Optofiber LLC, Moscow, Russia). A test sample was placed onto the holder horizontally under the beam of the X-ray source operating at a voltage of 40 kV and a current of 35 mA. The XRL signal was collected by a waveguide and transferred to the FSD-10 spectrometer. The luminescence was recorded in the 200 - 1000 nm range of the spectrum.
3. Results and discussion
Prior studies [26-29] imply that the coprecipitation of barium and cerium fluorides from aqueous solutions by various precipitating agents yields two-phase samples (pure barium fluoride and a fluorite phase containing 30 -45 mol. % CeF3). Single-phase samples can be prepared at 32-58 mol. % CeF3 with hydrofluoric acid or ammonium fluoride as the precipitating agent [28].
X-ray diffraction patterns for samples with as-batch cerium concentrations of 30.0, 35.0, 40.0, 42.5, 45.0, 47.5, 50.0, and 55.0 mol. % shows on Fig. 1. X-ray diffraction experiments showed that our syntheses of solid solutions yielded single-phase powders with the fluorite structure containing 30 - 40 mol. % cerium. When the cerium concentration increased to 42.5 mol. %, a second (CeF3-based) phase appeared, and its X-ray reflection intensity increased as the cerium concentration increased further (Fig. 1).
_A
30 mol.% Ce
A A A A . 35 mol.% Ce
A A . 40 mol.% Cc
Ai A . 42.5 mol.% Ce
45 mol.% Ce
aH a . 47.5 mol.% Ce
AH a 50 mol.% Ce
......_ _________All. A____ 55 mol.% Ce
20 30 40 50
2 theta, degree
60
70
Fig. 1. X-ray diffraction patterns of BaF2-CeF3 samples with various cerium concentrations (the peaks marked with arrow refer to the CeF3 tysonite phase)
Table 1 displays the calculated unit cell parameters and CSDs of prepared samples. An inspection of the tabulated values shows that, as the cerium concentration increases, the unit cell parameter of the fluorite phase decreases systematically due to the cerium radius being smaller than the barium radius [31].
The unit cell parameter versus rare-earth-ion concentration relationship for Ba1-xCexF2+x fluorite solid solutions borrowed from [30] was used to estimate the cerium concentration as
a = ao + kx, (2)
where a0 = 6.2000 A is the BaF2 unit cell parameter and k = -0.36 (Table 2). The estimated compositions were richer in cerium compared to the as-batch compositions for single-phase samples and were depleted in cerium for biphase samples.
Figure 2 shows the SEM images of single-phase samples. The mean particle sizes in these samples were 15 nm, in match with the CSD calculations (Table 1).
The EDX results reveals that potassium does enter the solid solution lattice when the as-batch cerium concentration is 35.0 or 40.0 mol. % (Table 2). Despite the additional entrance of potassium, there is a satisfactory match between the estimates by Eq. (2) andEDX measurements. Between the as-analyzed and as-batch concentrations, there is a reasonable match.
X-ray luminescence spectra for single-phase Ba1-xCexF2+x samples presented on Fig. 3.
An inspection of the X-ray luminescence spectra elucidated an increase in intensity of the 347-nm cerium luminescence in response to increasing cerium concentration.
Fig. 2. SEM images of Ba1-xCexF2+x samples containing (a) 30.0 mol. %, (b) 35.0 mol. %, and (c) 40.0 mol. % nominal composition
D. S. Yasyrkina, S. V Kuznetsov, A. A. Alexandrov, et al. Table 1. Unit cell parameters and CSDs of prepared samples
Nominal composition Phase Unit cell parameter, A CSD, nm
Bao.7oCeo.3oF2.3o Fluorite 6.071(1) 18(1)
Ba0.65Ce0.35F2.35 Fluorite 6.059(1) 19(1)
Bao.6oCeo.4oF2.4o Fluorite 6.050(1) 17(1)
Bao.575 Ceo.425F2.425 Fluorite 6.054(1) 16(1)
presence a tysonite phase
Bao.55Ceo.45F2.45 Fluorite 6.048(1) 16(1)
presence a tysonite phase
Bao.525 Ceo.475F2.475 Fluorite 6.046(1) 16(1)
presence a tysonite phase
Bao.5oCeo.5oF2.5o Fluorite 6.039(1) 15(1)
presence a tysonite phase
Bao.45 Ceo.55F2.55 Fluorite 6.035(1) 15(1)
presence a tysonite phase
Table 2. Estimated compositions of prepared samples
Nominal composition As calculated from the unit cell parameter As probed by EDX
Bao.7oCeo.3oF2.3o Bao.642 Ceo.358F2.358 Bao.645Ceo.355F2.355
Bao.65Ceo.35F2.35 Bao.608 Ce0.392F2.392 Bao.594Ceo.398Ko.oo8F2.39o
Bao.6oCeo.4oF2.4o Bao.583 Ce0.417F2.417 Bao.563Ceo.43oKo.oo7F2.423
Bao.575 Ceo.425F2.425 Bao.594 Ce0.406F2.406
Bao.55Ceo.45F2.45 Bao.578 Ce0.422F2.422
Bao.525 Ceo.475F2.475 Bao.572 Ceo.428F2.428
Bao.5oCeo.5oF2.5o Ba0.553 Ce0.447F2.447
Bao.45Ceo.55F2.55 Ba0.542 Ce0.458F2.458
The unit cell parameter of the fluorite solid solution changes due to the interplay of two factors: the substitution of barium by a smaller cerium ion [31] and the entrance of an additional (interstitial) fluoride ion for providing charge compensation as
Ba2+ ^ Ce3+ + F-nt (3)
Individual dipoles are observed only for low cerium concentrations in the solid solutions. The formation of nnn dipoles of trigonal symmetry is characteristic of low concentrations of cerium in the BaF2 matrix [16,32].
The association of dipoles in the barium fluoride matrix occurs with an increase in the cerium concentration, which leads to the formation of defect clusters. There is, as yet, no common structural description in terms of defect clusters for concentrated Ba1-xCexF2+x solid solutions in spite of numerous structural studies for chemically similar Ba1-xLaxF2+x [33-40] solid solutions. An essential factor for a reliable interpretation of the results is the concentration range over which fluorite phases are formed in low-temperature syntheses of solid solutions in BaF2-RF3 systems. High-temperature syntheses in these systems yield ordered fluorite-related phases of Ba4R3F17 stoichiom-etry [18,42-45]. These phases were found in all BaF2-RF3 systems for R = Ce-Lu, Y when their phase equilibria were studied [18,42,46]. Kieser and Greis [46] prepared an ordered Ba4Ce3F17 phase by heat-treating a disordered solid solution of the appropriate concentration at 400 °C. The fluorite-related Ba4R3F17 phases are distinguished by a trigonal distortion of the fluorite-type lattice caused by spatial ordering of R6F37 clusters [44]. Accordingly, the synthesis of single-phase fluorite solid solutions containing 30 - 40 mol. % cerium may be regarded as the synthesis of a Ba4±xCe3±xF17±x phase, whose ordering is not manifested at the nanoscale level [47-51]. It is quite natural to suggest that R6F37 clusters exist in the structure of this phase, substituting for parts of the Ba6F32 lattice in accordance
50000-
40000-
30000-
Ce = 40.0 mol.% Ce = 35.0 mol.% Ce = 30.0 mol.%
3
CO
20000 -
10000-
200
400 600 800
Wavelength, nm
1000
Fig. 3. X-ray luminescence spectra of single-phase Ba1-xCexF2+x solid solution samples
with the isomorphic substitution scheme advanced by Bevan et al. [52]. The shift of the X-ray luminescence peak from 330 to 347 nm may serve as an argument for this suggestion. The observation of X-ray luminescence in the prepared samples both by us and by Chen and Wu [24] may be regarded as the verification of the expected increase in free dipole concentration due to additional entrance of potassium in the barium fluoride host.
The concerted entrance of univalent and trivalent cations to form solid solutions is typical of the fluorite lattice [53]. Potassium was systematically observed to enter the host fluorite lattice in nanofluorides synthesized by coprecipitation of solid solutions [19]. In the NaF-BaF2-GdF3 ternary system, univalent sodium enters the lattice of the Ba4Gd3F17 phase thereby extending its existence area [54].
4. Conclusions
A range of single-phase Ba1-xCexF2+x (x = 0.30 - 0.40) solid solutions, with an average particle size of 15 nm, prepared by coprecipitation from aqueous solutions, has been determined. It has been suggested that these solid solutions actually have a more complex composition, namely, Ba4±xCe3±xF17±x. The X-ray luminescence intensity in the solid solution samples increased in response to cerium concentration, increasing from 30 to 40 mol. % without concentration quenching. The absence of concentration decay was most likely due to potassium (0.7 - 0.8 mol. %) entering the crystal lattice, and thus, precluding cerium-based cluster formation.
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