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aänetic Resonance in Solids
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Volume 19, Issue 2 Paper No 17209, 1-9 pages 2017
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Yoshio Kitaoka (Osaka University,
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Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jörg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University, Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentine Zhikharev (KNRTU,
Kazan)
* In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
Short cite this: Magn. Reson. Solids 19, 17209 (2017)
Electron paramagnetic resonance and X-ray diffraction study of PbF2 fine powders mechanochemically doped with Er3+ ions
I.A. Irisova1'*, I.N. Gracheva1, Y.V. Lysogorskiy1, A.A. Shinkarev2, A.A. Rodionov1,
D.A. Tayurskii1, R.V. Yusupov1 1Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia 2Kazan National Research Technological University, K. Marx str. 68, 420015 Kazan, Russia
* E-mail: IAIrisova@kpfu.ru
(Received December 9, 2017; revised December 26, 2017; accepted December 26, 2017; published December 30, 2017)
Investigation of the mechanochemical doping of PbF2 powders with Er3+ ions with electron paramagnetic resonance and X-ray diffraction is presented. In the analysis of the results a possibility of the structural transformation between the cubic ,0-PbF2 and orthorhombic a-PbF2 phases in the course of synthesis was taken into account. It is shown that regardless of the initial state of PbF2 it reveals high efficiency of the mechanochemical doping with Er3+ ions. Obtained particles are found in (a/^)-PbF2 structurally inhomogeneous state with the majority of the Er3+ ions located in the equilibrium a-PbF2 fraction. Preferrable location of the Er3+ ions in the a-PbF2 phase is related to the fact that the formation of the cation vacancies necessary for a mechanically activated diffusion of erbium ions into the particles and nucleation of the a-PbF2 phase proceed in parallel and is mediated by dislocations created in the course of synthesis. Annealing of the sample leads to a conversion of its entire volume into the metastable ,0-PbF2 phase with all the Er3+ centers possessing the cubic symmetry.
PACS: 73.20.Hb, 76.30.-v, 76.30.Kg
Keywords: fluorite, rare-earth ions, mechanically activated doping, electron paramagnetic resonance
1. Introduction
Nowadays, ^-PbF2 single-crystalline optical fibers doped with rare-earth (RE) ions compete with the traditional ones produced from doped glasses [1]. Optical properties of the crystalline ^-PbF2 fibers are more attractive than those of the glass fibers. However, the technology of the crystalline fiber production is significantly more complex and expensive. An alternative approach is based on the synthesis of the RE-doped ^-PbF2 glass-ceramics, as it is much simpler while the optical properties are comparable [2,3].
Mechanical activation or, briefly, "mechanoactivation" can serve as an alternative and effective means for incorporation of the RE ions into the crystal structure as well as one of the steps of the doped ceramics synthesis. Mechanoactivation in general is a complex, multistage process of changing the state of a solid under a delivery of the mechanical energy [4]. Moreover, mechanochemical synthesis has an important advantage with respect to the traditional method of the doped crystal growth by means of directional crystallization, namely, the low (room) temperature, which allows to overcome the restrictions associated with different melting points of the components, vapor pressure, thermal decomposition, and other factors [5]. Therefore, it is important to study processes that occur in the course of the mechanochemical doping of PbF2 with Er3+ ions.
Our previous studies [6-8] of the mechanochemical doping of MF2 (M = Ca, Sr, Ba) fine powders with Er3+ ions performed with electron paramagnetic resonance (EPR) spectroscopy
have shown that the cubic symmetry Er3+ ion impurity centers are formed. Investigations of the EPR spectra intensity dependences on the particle size have shown that doping proceeds differently for various MF2 hosts. In the case of CaF2, impurity centers are located in a thin surface layer of the particles. In SrF2, the impurity is distributed over the particle volume. In BaF2, there is a layer of a finite thickness for which the probability of the mechanochemical doping is small, and RE impurities are located in the cores of the large enough particles. These observations were explained assuming that the result of the mechanosynthesis of the fluorite-structure particles doped with Er3+ ions is governed by two processes: mechanically-activated diffusion of RE ions into the particles, and segregation of the impurity ions to the grain boundaries. In this case, characteristic depth values for various MF2 differ considerably from each other. Also, it has been shown that MF2 powders mechanochemically doped with Er3+ ions are in a long-lived metastable state characterized by a high concentration of vacancies and dominating cubic symmetry Er3+ ion centers [9]. Annealing of the samples brings the powders to the ground state with most of the vacancies healed, and trigonal symmetry Er3+ centers are formed in SrF2 and BaF2 due to the local charge compensation by the interstitial fluorine ion.
In [6-9], we studied MF2 fluorites that exist only in the cubic phase. It was interesting to study the mechanochemical doping with RE ions of a crystalline host with higher complexity. It is known that under normal conditions PbF2 can exist in the two structural phases [10]: thermodynamically equilibrium orthorhombic a-PbF2 (space group Pnma) and metastable cubic ft-PbF2 phase. The latter has the fluorite structure with the space group Fm3m.
It follows from the above that the studies of the mechanosynthesis of the PbF2 powders doped with rare-earth ions are promising and relevant, due to both the possibility of a simplified doped ceramics synthesis as well as an assessment of the approach applicability to different MF2 hosts.
2. Sample Preparation and Experiment Techniques
PbF2 powders mechanochemically doped with Er3+ ions were obtained by grinding of the (97 wt.% PbF2 + 3 wt.% ErF3) mixtures of high purity crystalline salts in an agate mortar in extra-pure isopropyl alcohol. The choice of isopropyl alcohol as a buffer medium is due to its low chemical activity in the series of alcohols. Grinding of the powders in the isopropanol allowed to avoid their prolonged contact with air and, accordingly, excluded the interactions with oxygen and water vapor, which could significantly complicate the interpretation of the results. To control better the concentration of the RE impurity, before grinding of the mixture the mortar was cleaned with a corundum-based abrasive followed by a two-time self-lining with the PbF2 compound.
Since PbF2 can exist at room temperature in two modifications (a, ft)-PbF2 [10], it was necessary to control a sample phase composition, both in the initial mixture and in the prepared fine powders. From the X-ray diffraction (XRD) analysis, the major part of the PbF2 powder initially was in the orthorhombic a-PbF2 phase. Transformation of the orthorhombic to the cubic phase (ft-PbF2) can be achieved by the annealing at a temperature of 650°C. Subsequent cooling down to the room temperature at atmospheric pressure does not lead to a reverse transition. In this study, the annealing of the a-PbF2 powder for 12 hours has led to the transformation of more than 90% of its volume to the ft-PbF2 cubic phase. ft-PbF2 is in fact a metastable state, and, according to [10], under a mechanical stress a-PbF2 nucleates in the ft-PbF2 at the structural defects like slip lines and bands. As far as the ErF3 is concerned, according to the powder-XRD analysis its whole volume was in the orthorhombic phase (space group Pnma [11]).
Series of the fine powder samples with different mean particle size (d) were prepared from the (97 wt.% PbF2 + 3 wt.% ErF3) mixture with the predominant ft-PbF2 phase by grinding for 12 h in an agate mortar. Separation of the fractions with different particle sizes was performed by means of the successive sedimentation in isopropanol. Prior to sedimentation the powders were dispersed in a small amount of the solvent with ultrasound. Studied series that differed only in the grain size consisted of fine powders obtained after 48, 17, 6 and 2 hour sedimentation.
Samples were characterized with the Philips XL30 scanning electron microscopy (SEM). In order to determine the mean particle sizes (d), the distributions of grain sizes were fit to the log-normal distribution function [8]. The results of this analysis are presented in Table 1.
EPR spectra of the samples were measured with the continuous wave X-band 9.5 GHz) Bruker ESP300 spectrometer. Experiments were performed at a temperature of 15 K. The temperature was controlled using an Oxford Instruments ESR-9 liquid helium flow cryostat.
Powder X-ray diffraction patterns were obtained with the Bruker D8 Advance diffractometer using the Cu-Ka radiation (A = 1.5418A) in the Bragg-Brentano geometry.
3. Results and Discussion
In Fig. 1 EPR spectra are shown of the two PbF2:Er3+ samples obtained by means of mechanosyn-thesis from lead fluoride with the dominating a-PbF2 (volume fraction 94%) and ft-PbF2 (90%) phases (see Table 1). Both spectra in Fig. 1 contain a component at the g-factor of 6.78, characteristic for the cubic symmetry Er3+ impurity ion centers [2]. This is additionally approved by the observation for a sample produced from the ft-PbF2 of the hyperfine structure due to the 167Er-isotope (nuclear spin I = 7/2, natural abundance 22.9%). However, for the sample synthesized from the ft-PbF2 the intensity of the line at g ~ 6.78 is almost an order of magnitude higher than for the sample produced from the a-PbF2. In the EPR spectra of both samples, a new spectral component at g ~ 12.7 appears which has an asymmetric shape characteristic for the powder spectra of anisotropic centers. The intensity of the new component, which was not observed in the MF2 before is similar for the samples produced from the a- and ft-PbF2.
Since high-purity chemical components were used for the preparation of the samples, and no signals were observed in the EPR spectrum of the mixture before grinding neither at g = 6.78 nor at g = 12.7, it may indicate that both signals originate from the impurity Er3+ centers. One may expect that in the course of grinding phase transformation between the a- and ft-PbF2 phases occurred with the transition from the metastable ft - into the equilibrium a-phase more probable. Obviously, impurity Er3+ ion in the orthorhombic a-PbF2 phase cannot have symmetry higher than the symmetry of the nearest surrounding, and therefore, should possess anisotropic g-tensor.
Table 1. Mean particle size and phase composition for a series of PbF2 fine powder samples mechanochemically doped with Er3+ ions.
Fraction Size (^m) ft-PbF2 (%) a-PbF2(%)
Initial PbF2 ~ 100 6 ± 2 94 ± 2
Annealed PbF2 ~ 100 90 ± 2 10 ± 2
48 h sedimentation 0.15 ± 0.01 15 ± 2 85 ± 2
17 h sedimentation 0.23 ± 0.02 20 ± 2 80 ± 2
6 h sedimentation 0.66 ± 0.09 31 ± 2 69 ± 2
2 h sedimentation 1.49 ± 0.12 36 ± 2 64 ± 2
B (mT)
Figure 1. EPR spectra of PbF2 fine powders mechanochemically doped with Er3+ ions and produced by 3-hour long mechanosynthesis from (97 wt.% PbF2 + 3 wt.% ErF3) mixtures with dominating a-PbF2 (red line) and ^-PbF2 (black line) phases; v = 9.4864 GHz, T = 15 K.
B (mT)
Figure 2. EPR spectra of PbF2 fine powder mechanochemically doped with Er3+ ions (initial a-PbF2) before (a) and after (b) the annealing for 12 hours at 650oC; v = 9.4892 GHz, T = 15 K.
To check whether the above assumptions are true, the PbF2:Er3+ sample produced from the a-PbF2 was annealed for 12 hours at 650°C. This annealing is a typical procedure used to transform PbF2 from the a- to the ft-phase. The results of the annealing are shown in Fig. 2. Clearly, EPR spectrum of the sample has changed drastically: while the asymmetric component at g ~ 12.7 has vanished, the spectrum of the cubic Er3+ center has increased in intensity more than 20 times, and hyperfine structure due to 167Er-isotope has become obvious.
Thus, impurity Er3+ centers that resided in the orthorhombic phase due to the annealing and structural transition have acquired the cubic symmetry. Consequently, the intensity of the Er3+ cubic center spectrum increased at the expense of the anisotropic component. The intensity increase of the cubic Er3+-center spectrum may seem too large. However, as the measured signal is in fact a derivative of the absorption spectrum, its amplitude for the powder samples in the case of a substantial g-factor anisotropy is significant mainly at the edges of the absorption pattern characteristic for powders. As an appropriate measure for the amount of paramagnetic centers is the integral intensity of the absorption spectrum, there is no inconsistency in our observations.
The shape of the observed anisotropic Er3+-center powder spectrum allows us to conclude that, first, the largest of its g-tensor components corresponding to the low-field edge of the
absorption spectrum, is ~ 12.7. This value is quite ordinary for anisotropic Er3+ impurity centers; similar values of the g-factor were found for Er3+ ions in, e.g., CaO, Y2O3, YCl3-6H2O, LaF3 [12], SrY2O4 [13]. Second, low-symmetry Er3+ centers dominate in the sample produced from the orthorhombic a-PbF2: less than 5% of the impurity Er3+ centers possess the cubic symmetry.
To learn more about the structural transformations in PbF2 powders that occur in the course of the mechanosynthesis we studied it with X-ray diffraction. Sample series was prepared from ft-PbF2 by means of successive sedimentation from the (97 wt.% PbF2 + 3 wt.% ErF3) mixture ground for 12 hours. In Fig. 3 powder-XRD patterns of the series are presented. Clearly, two principal components dominate, from a- and ft-PbF2. Phase compositions obtained from the data analysis, are indicated in Figs. 3 and 4 and in Table 1.
In the initial mixture, about 90% of the PbF2-powder volume was in the cubic ft-phase. After grinding, the volume fraction of the cubic phase has decreased significantly. Thus, grinding
20 30 40 50 60
29° Cu Kr
Figure 3. X-ray diffraction patterns of PbF2:Er3+ powder samples produced by mechanosynthesis from (97 wt.% PbF2 + 3 wt.% ErF3) mixture with dominating ß-PbF2 phase. Mean particle size (from top to bottom): 1.49 ^m, 0.66 ^m, 0.23 ^m and 0.15 ^m.
1/<d> (|im"1)
Figure 4. Dependence of the a- and ft-phase content in the PbF2 fine powders mechanochemically doped with Er3+ ions on grain size and its linear fit.
of PbF2 in a mortar indeed leads to the transition of a part of the volume from the metastable cubic to the equilibrium orthorhombic phase. In our case, grinding for 12 hours has led to the transformation of most of the PbF2 volume into a rhombic a-phase. Moreover, the content of the latter systematically increases with a decrease of the grain size reaching 85% for the finest fraction (Fig. 4).
Formation of the a-phase during the mechanochemical doping of ft-PbF2, in our opinion, proceeds in the following way. According to [10], on stress application the a-PbF2 phase nucleates in the ft-PbF2 matrix at structural defects - slip lines and bands - created when deformation exceeds the elastic limit. Then the volume fraction of the a-PbF2 increases under stress application/variation due to the growth of the a-phase droplets in the ft-phase host. Therefore, an amount of the orthorhombic a-PbF2 phase in the samples increases with the duration of the mechanosynthesis. Then one can expect that in the smaller particles the volume of the initially dominating cubic phase will be less that in the larger ones. This indeed is observed in our data. After 12-hour grinding in all the fractions of the sample most of the volume (64-85%) is in the a-PbF2 phase.
In this situation, it is not surprising to observe in the EPR spectra of the ft-PbF2-based ground mixture the signal of the low-symmetry Er3+ ion centers. This signal originates from the impurity Er3+ ions in the a-PbF2 phase. High intensity of the cubic Er3+-center component in the EPR spectrum of the sample produced from the initial ft-PbF2 phase in Fig. 1, in our opinion, is due to the shorter duration of grinding (3 hours) compared with the series described above.
Knowledge of the anisotropic spectrum origin and the analysis of the spectra shown in Figs. 1 and 2 bring us to the following conclusions. First, we note that while the intensities of the anisotropic peaks at g ~ 12.7 for the samples obtained from a- and ft-PbF2 are practically identical (Fig. 1), the intensity of the cubic Er3+-center spectrum for the sample obtained from the ft-PbF2 is approximately an order of magnitude greater. Therefore, it becomes clear that mechanically activated doping of the cubic ft-PbF2 phase is notably more efficient than of the rhombic a-PbF2. Second, comparing the intensities of the Er3+ cubic center in the sample obtained from the a-PbF2 before and after the annealing (Fig. 2), and the intensities of the anisotropic spectrum in the samples produced from the a- and ft-phases (Fig. 1), one can conclude that in the sample prepared from ft-PbF2 the number of Er3+ anisotropic centers is greater than that of the cubic ones.
B (mT) 1/<cf> (^m"1)
Figure 5. EPR spectra of the mechanosynthesized PbF2:Er3+ fine powder samples with different grain sizes (a); spectra intensities are normalized to the sample mass, v = 9.484 GHz, T = 15 K. Dependences on the particle size of the Er3+ impurity concentration in the a- and ft-phases of PbF2 and their linear fits (b).
Fig. 5(a) shows the EPR spectra of the PbF2:Er3+ grain size series. At first sight, there is no significant difference in the intensities of the principal spectral components. However, since both a- and ft-phases are present in the samples, correct interpretation of the results demands the intensity of each component to be normalized to the volume fraction of the respective phase. The results are shown in Fig. 5(b). Here, the peak-to-peak intensity of the cubic Er3+ center spectrum served as a measure of its concentration, and the peak amplitude at g ~ 12.7 was used for Er3+ ions in the a-PbF2 phase. The latter is correct within an assumption that the shape of the absorption spectrum of the Er3+ ions in the a-PbF2 does not change from sample to sample.
The used approach obviously provides with an information on the relative changes in concentrations of two types of the Er3+ centers but not on the absolute amount of centers. It can be seen from Fig. 5(b) that the concentration of Er3+ ions in a-PbF2 does not depend on the particle size. It means that the rhombic phase is evenly doped with Er3+ ions. Concentration of the Er3+ ions in the ft-phase of PbF2 increases with the decrease of the particle size. On plastic deformation of ft-PbF2 the a-phase is found both in the near-surface layer and in the volume of ft-PbF2 particles [10]. Rhombic a-PbF2 nucleates at structural defects like slip lines and bands. Formation of these defects is associated with the creation of dislocations. Motion and annihilation of dislocations in turn generates cationic vacancies that serve as a necessary step in mechanoactivated diffusion of RE-ions into cubic fluorites CaF2 and SrF2. Thus, the source of the defects at which a-phase nucleates in ft-PbF2 and of those promoting RE-ions into fluorites is the same. Therefore, it is not a big surprise that on grinding of (ft-PbF2 + ErF3) mixture impurity Er3+-ions are located mainly in the rapidly growing a-phase. Concentration of the Er3+ ions in a-PbF2 does not change with the particle size of the powder, since, probably, the growth of a-PbF2 occurs homogeneously throughout the sample.
Increase in the impurity Er3+ ion concentration in the cubic phase of PbF2 with the decrease of the particle size can be explained in the following simple way. Since the volume fraction of the cubic phase is reduced in the particles of a smaller size, it is likely that the droplets of the residual cubic phase in these particles are smaller than in the larger ones. Then, assuming that a diffusion depth of the Er3+ ions from the saturated rhombic phase to the cubic one is small compared to the size of the cubic phase inclusions, it would be an expected situation that the concentration of the erbium ions in the cubic phase of large particles is smaller than in the small ones. Then, mechanochemical doping of the ft-PbF2 phase results in formation of a structurally
Figure 6. Schematic models of the inhomogeneous structure of larger (left) and smaller (right) PbF2 fine particles mechanochemically doped with Er3+ ions.
phase-separated, inhomogeneously doped with Er3+ impurity powders schematically shown in Fig. 6 for larger and smaller particles.
4. Summary
Thus, a complementary study of mechanochemical doping of PbF2 powders with Er3+ ions with electron paramagnetic resonance and X-ray diffraction has been performed. It has been shown that whatever is the initial crystal structure of PbF2 (a/ft-phase) the doping proceeds efficiently. Grains of PbF2:Er3+ fine powders obtained by mechanosynthesis are in the structurally-mixed state with most of the volume in the orthorhombic a-phase. Mechanoactivated diffusion and transformation to the a-PbF2 proceed in parallel and is mediated by dislocations created in the course of the synthesis. Full volume of the powders can be brought to the cubic ft-PbF2 phase by the appropriate annealing.
Acknowledgments
The reported study was funded by RFBR according to the research project No.16-32-00152 mol_a (I.A.I., I.N.G. and Y.V.L.) and by the subsidy allocated to Kazan Federal University for the state assignment in the sphere of scientific activities, Nos. 3.8138.2017/8.9 (D.A.T.) and 3.7704.2017/4.6 (R.V.Y.).
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