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Eu3 + as a luminescent probe for the local structure of trivalent dopant ions in barium zirconate-based proton conductors
Solid State Ionics 247–248 (2013) 94–97
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Eu3 + as a luminescent probe for the local structure of trivalent dopant ions in barium zirconate-based proton conductors P. Haro-González a, M. Karlsson b, S.M. Gaita c, C.S. Knee d, M. Bettinelli a,⁎ a
Luminescent Materials Laboratory, Department of Biotechnology, University of Verona and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Institute of Nuclear Science & Technology, University of Nairobi, P.O. Box 30197 00100, Nairobi, Kenya d Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden b c
a r t i c l e
i n f o
Article history: Received 14 November 2012 Received in revised form 17 June 2013 Accepted 21 June 2013 Keywords: Luminescence Eu3 + BaZrO3 Proton conducting oxides Hydration Local structure Proton-defect association
a b s t r a c t The luminescence spectra and decay kinetics of dry and hydrated samples of BaZr0.9Y0.099Eu0.001O2.95 have been measured at room temperature. The spectra of the dry sample evidence two 5D0 → 7F0 bands clearly indicating that Eu3+ (replacing Y3+ in the Zr4+ position of the average cubic perovskite structure) occupies two different local structures, whilst only one Eu3+ site is observed for the hydrated material. From the spectral data, it is possible to identify the nature of these two sites and to propose point group symmetries for the average local geometry around Eu3+. The decay time of the 5D0 level becomes shorter upon hydration, due to the interaction with high-frequency O–H stretching vibrations, indicating an attractive interaction between protons and dopant atoms. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrated, acceptor doped, ABO3 type perovskites, such as the barium zirconate-based compounds BaZr1 − xMxO3 − x/2 (M = Y, In, Sc and Ga, etc., x N 0), are well known to be proton conductors at elevated temperatures [1,2]. The introduction of trivalent dopant atoms on the tetravalent Zr sites creates an oxygen-deficient structure and in a humid atmosphere H2O molecules may dissociate into OH− ions, which fill the oxygen vacancies, and protons (H+), which bind to lattice oxygens. Hence, it is easy to understand that the local structure around the proton and the trivalent M ion is a key factor for the proton conduction process. Local structural investigations of such materials are therefore of great importance for the development of a detailed understanding of the, yet not fully understood, proton conduction phenomenon in hydrated perovskites. For this reason, we have used luminescence spectroscopy for the investigation of the local structure at Y3+ in 10% Y-doped BaZrO3 when 1% of the Y is replaced with Eu. The Eu3+ ion is a well-known spectroscopic probe [3,4] used in luminescence spectroscopy of inorganic materials and it has been used before for structural investigations of proton conducting perovskites (e.g. in the case of orthorhombic SrCe1−xYxO3 and SrZr1−xYxO3) [5,6]. These earlier studies [5,6], however, were, in contrast to our investigations, performed
⁎ Corresponding author. Tel.: +39 045 8027902. E-mail address: [email protected] (M. Bettinelli). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.06.008
on orthorhombic rather than cubic-structured materials. In our investigations we additionally present the decay times of the excited emitting state, for both dry and hydrated samples, which gives information about the interaction of the Eu3+ ion with O–H oscillators present in the hydrated material. The obtained information can be useful for understanding what factors influence the proton transport properties in proton conducting oxides. 2. Experimental Dry and hydrated samples of BaZr0.9Y0.099Eu0.001O2.95 were prepared following a procedure previously described for this class of materials [7], with Eu(NO3)3 · 6H2O acting as the source of Eu. A final reaction temperature of 1400 °C was used to obtain the as-prepared cubic perovskite (space group Pm–3m) that was judged to be phase pure based on powder X-ray diffraction (Cu-Kα1 radiation) data. A portion of the sample was then hydrated by cooling from 800 °C to 200 °C in steps of 100 °C with 1 h dwells at each temperature under a flow of Ar gas saturated with water vapour at room temperature (pH2O ≈ 0.025 atm.). The dry sample was prepared by annealing the as-prepared sample under vacuum (2 × 10−6 mbar) at 900 °C for 10 h. Room temperature laser excited luminescence spectra were measured upon excitation at 473 nm by using a continuous wave diode laser. The Eu3+ transitions from the 5D0 level were detected using an ANDOR-NEWTON CCD spectrograph with 0.5 nm resolution and
P. Haro-González et al. / Solid State Ionics 247–248 (2013) 94–97
analyzed with the specific software of the spectrograph. Emission decay curves were obtained at room temperature upon pulsed laser excitation with high-energy pulses of about 5 ns of duration at 532 nm, using a Nd-YAG laser. The detection chain was formed by a TRIAX-180 monochromator with 1 nm resolution and the output of the photomultiplier tube was registered by a digital oscilloscope TEKTRONIX-2430A.
5
7
D0
F2
Hydrated
Intensity (a.u.)
7
3. Results and discussion In almost all oxide hosts, the stable oxidation state of the europium ion is +3, unless the material is prepared under reducing conditions, which was not the case in the present investigation. Eu2+ gives rise to strong, broad, emission and excitation bands in the ultraviolet and visible regions [8,9]. We found no evidence for such spectral features and we conclude therefore that only Eu3+ is present in the samples under investigation. Moreover, it has been convincingly shown that Y3+ enters the Zr4+ site of the perovskite structure in BaZrO3 [10]. The ionic radii of Eu3+, Y3+, and Zr4+ in six-fold coordination (i.e. the B site in ABO3 type perovskites) are 0.947, 0.900 and 0.720 Å, respectively, whilst the ones for Eu3+ and Ba2+ in twelve-fold coordination (the A site has a coordination number (CN) of 12, the value for Eu3+ was obtained from extrapolation of the radii for lower CNs) are 1.29 and 1.61 Å, respectively [11]. These values strongly indicate that Eu3+ will substitute for Y3+ ions, and therefore suggest that Eu3+ is a suitable probe for the local structure around the Y3+ ions. The luminescence spectra of the Eu3+ ion in oxide hosts are generally dominated by emission from the non-degenerate 5D0 level (Fig. 1) [3]. The room temperature luminescence spectra of the two samples (dry and hydrated) of BaZr0.9Y0.099Eu0.001O2.95 are shown in Fig. 2, after normalization of the intensities of the 5D0 → 7F2 band peaking around 615–620 nm. It must be noted that the spectra have been measured upon excitation at 473 nm in the inhomogeneous profile of the 7F0 → 5D2 excitation band, i.e. exciting all the Eu3+ sites present in the structure. For this reason, the spectra show a significant degree of inhomogeneous broadening, due to the presence of a range of different local structural configurations around the Eu3+ as induced by weak structural perturbations caused by the presence of dopant atoms (Y and Eu) and oxygen vacancies [12]. Cooling the sample to low temperature does not give rise to significantly sharper spectral features, as inhomogeneous broadening is related to structural disorder and is not dependent on temperature. It is evident from Fig. 2 that the spectra of the two materials are not identical. Firstly, the relative intensities of the hypersensitive
95
F4
7 7
F0
F1 7
F3
Dry 550
600
650
700
750
Wavelength (nm) Fig. 2. Room temperature luminescence spectra of BaZr0.9Y0.099Eu0.001O2.95 samples measured upon excitation at 473 nm. The spectra have been vertically offset for the sake of clarity.
5
D0 → 7F2 transition and the magnetic dipole allowed 5D0 → 7F1 transition significantly change upon hydration. It is well known that the hypersensitive transitions (characterized by |ΔJ| = 2, where J is the total angular momentum quantum number) exhibit an intensity that depends strongly on the nature of the local structural environment around the Eu3+ ions, whilst the intensity of magnetic dipole transitions is not influenced [13]. The asymmetry ratio R, defined as R = I(5D0 → 7F2)/I(5D0 → 7F1) [14], where I represents the integrated intensity of the f–f transition, allows the evaluation of the degree of asymmetry in the local environment around the Eu3+ dopant. Our spectroscopic data show that R decreases as the material is hydrated, suggesting a more symmetrical local structure around the dopant atoms in the hydrated material. It should be noted that the values reported in Table. 1 are in both cases compatible with a significant asymmetry of the Eu3+ site(s), but clearly indicate that on average the geometry of the site(s) accommodating Eu3+ is more distorted in the case of the dry material. The spectral region of the 0–0 transition (5D0 → 7F0, located between 570 and 580 nm, see Fig. 1) is shown in detail in Fig. 3. This transition connects two non-degenerate levels having J = 0 that cannot be split by crystal-field effects. For this reason, the number of bands related to this transition reflects the number of sites occupied by Eu3+ ions in the material under investigation [13]. That is, each Eu3+ in a given site can be expected to be manifested by one separate spectral feature. Inspection of Fig. 3 suggests therefore the presence of two Eu3+ main sites in the dry sample whereas only one in the hydrated. This is in agreement with the luminescence spectroscopy study on Ba0.995La0.005Zr0.995Eu0.005O3 (Eu3+ used as a structural probe), a similar but more complex material, as reported by Alarcon et al. [15]. In their study, however, the results were interpreted as possible occupation by Eu3+ on both the Ba and Zr sites in the structure, which cannot be expected in our study due to ionic size considerations [11] and to experimental [16] and computational [17] evidence. On the basis of these arguments, we propose therefore to assign the band around 577 nm, which is located in the usual wavelength range for the
Table 1 Asymmetry ratios, and experimental and radiative 5D0 lifetimes for the materials under investigation.
Fig. 1. Energy-level diagram for the Eu3+ ion.
BaZr0.9Y0.099Eu0.001O2.95
R
τexp/μs
τR/μs
dry hydrated
7.8 5.5
470 420
431 538
96
P. Haro-González et al. / Solid State Ionics 247–248 (2013) 94–97
From the observed spectrum and the refractive index of the material it is possible to obtain a reasonable estimate of the radiative lifetime of the 5D0 level τR, using the equation [23]: 1 3 I tot ¼ AMD;0 n τR IMD
Fig. 3. Luminescence spectra of BaZr0.9Y0.099Eu0.001O2.95 samples in the 5D0 → 7F0 region, measured upon excitation at 473 nm.
5
D0 → 7F0 transition [18] and is present in both materials, to Eu3+ ions located in Zr4+ sites with no nearby oxygen vacancies. On the other hand, we note that the band at about 572 nm is located at very high energy, compared to its position in many hosts [18,19]. We therefore assign this band to sites perturbed by oxygen vacancies close-by, as found by Sommerdijk and Bril for AMgF3 (A = K, Rb, Cs) [20]. The explanation of this unusual behavior lies in the increased interaction due to the nearby charge compensation of the 5D0 level with charge transfer states, therefore inducing a shift to higher energies and concomitantly a corresponding shift of the 5D0 → 7F0 transition to a lower wavelength [20]. It should also be pointed out that many hosts in which the 5D0 → 7 F0 transition is located at low wavelength are characterized by charge compensation [21]. Upon hydration, oxygen vacancies are filled by OH− groups and this family of sites emitting at 572 nm disappears creating more sites emitting at 577 nm. We note that Yugami et al. [6] also observed in the proton conducting perovskite Y3+-doped SrCeO3 the disappearance upon hydration of a low wavelength band (around 575 nm), and the appearance of another band at higher wavelength (close to 580 nm). They interpreted this observation as a red shift of the low wavelength line. The differences in the spectroscopic behavior between our results and the ones obtained by Yugami and coworkers may be related to the different crystal structures (BaZrO3 is cubic, whilst SrCeO3 is orthorhombic) and the level of Y doping in the investigated materials (10% in the present study compared to a maximum of 4.9% in [6]). As discussed above, for both the dry and the hydrated sample, the 5 D0 → 7F2 hypersensitive band is dominating and much stronger than magnetic dipole allowed 5D0 → 7F1. The observation of the 5D0 → 7F0 band and the relative strength of the 5D0 → 7F2 transition indicates that the average local symmetry belongs to a point group containing Y1m spherical harmonics, as discussed at length by Peacock [22]. The highest symmetrical point groups of this type are C2v and C4v, which can be obtained by distortion of the original octahedral symmetry around Zr4+ in the cubic BaZrO3 perovskite structure. The decay times of the luminescence from 5D0 were measured at room temperature upon pulsed excitation at 532 nm. These results extend the ones previously obtained by Yugami et al. [5,6] who reported on persistent hole burning and site selective spectroscopy for SrZr1−xYxO3 and SrCe1−xYxO3, respectively [5,6]. The decay curves (not shown) are almost perfectly exponential; a suitable fit yields for the two samples rather similar decay times, slightly faster in the case of the hydrated sample, but with the same exponential behavior. The corresponding 5D0 lifetimes (τexp) are reported in Table. 1.
where n is the refractive index of the host, AMD,0 is the spontaneous emission probability of the 5D0 → 7F1 transition in vacuo (14.65 s−1, [23]) and Itot/IMD is the ratio of the total area of the Eu3+ emission spectrum to the area of the 5D0 → 7F1 band. This is due to the fact that the absolute spontaneous emission probability of the magnetic dipole 5 D0 → 7F1 transition depends only on the refractive index of the host and therefore can be easily calculated. Assuming for the materials under investigation the value n = 2.20 reported for the refractive index of undoped BaZrO3 [24,25], the radiative lifetimes reported in Table. 1 are obtained. In the case of the dry sample, the observed decay time is close to the radiative one, and therefore shows no quenching, either due to accidental hydration or to energy transfer to impurities. On the other hand, for the hydrated sample the decay time is about 25% less than the radiative one. This indicates significant quenching of the Eu3+ emission due to multiphonon relaxation induced by high-frequency O–H oscillators [26], bridging through molecular vibrations the energy gap separating the emitting 5D0 level from the lower lying non-emitting levels 7FJ (J = 6,…0). It follows that several, if not all, of the protons in the hydrated material are located in the vicinity of the Eu3+ ions. That is, there seems to be an associative interaction between the protons and the Y (Eu) atoms. This is in agreement with results obtained from both computational [27] and experimental [28] studies. Further luminescence spectroscopy studies along these lines, for example as a function of dopant concentration, may therefore give more detailed information for such a behavior and hence be very rewarding.
4. Conclusions In this work, we have shown that the Eu3+ ion is a useful spectroscopic probe of the environment of the trivalent M cations in proton conducting perovskite type oxides of the form BaZr1 − xMxO3 − x/2. In particular, the present study extends previous investigations on other proton conducting oxides [5,6], through the measurement of luminescence decay curves and the analysis of the relative intensities of the 5 D0 → 7FJ bands. On the basis of luminescence spectra and decay curves, we find that for the dry material there are two main sites present, one close to oxygen vacancies and one farther away. For the hydrated material, the site close to oxygen vacancies has disappeared. For both materials, the Eu3+ sites appear to be distorted from the original octahedral symmetry. Furthermore, hydration is shown to significantly shorten the decay time of the 5D0 level of Eu3+, indicating that high-frequency O–H vibrations provide an efficient non-radiative relaxation pathway, suggesting an associative interaction between the protons and the dopant (Eu, Y) atoms. Finally we note that the use of the Eu3+ luminescent probe can be successfully extended to other classes of proton conductors containing trivalent cations that can be substituted by this lanthanide ion without significant distortion of the local structure.
Acknowledgements Erica Viviani (Univ. Verona) is gratefully acknowledged for expert technical assistance. S. M. Gaita acknowledges the International Science Programme, hosted by Uppsala University, Sweden, for funding. M. Karlsson thanks the Swedish Research Council for research funding (Grant no. 2010-3519).
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K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. M. Karlsson, I. Ahmed, A. Matic, S.G. Eriksson, Solid State Ionics 181 (2010) 126. G. Jia, P.A. Tanner, C.-K. Duan, J. Dexpert-Ghys, J. Phys. Chem. C 114 (2010) 2769. D. Tu, Y. Liu, H. Zhu, R. Li, L. Liu, X. Chen, Angew. Chem. Int. Ed. 52 (2013) 1128. H. Yugami, S. Matsuo, M. Ishigame, Solid State Ionics 77 (1995) 195. H. Yugami, Y. Chiba, M. Ishigame, Solid State Ionics 77 (1995) 201. A. D'Epifanio, E. Fabbri, E. Di Bartolomeo, S. Licoccia, E. Traversa, Fuel Cells 8 (2008) 69. S.H.M. Poort, W. Janssen, G. Blasse, J. Alloys Compd. 260 (1997) 93. A.M. Srivastava, H.A. Comanzo, S. Camardello, S.B. Chaney, M. Aycibin, U. Happek, J. Lumin. 129 (2009) 919. D. Han, K. Kishida, K. Shinoda, H. Inuia, T. Uda, J. Mater. Chem. A 1 (2013) 3027. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Clarendon Press, Oxford, 1999. 235. J.-C.G. Bünzli, G.R. Choppin, Lanthanide probes in life, chemical and earth sciences: Theory and practice, Elsevier, New York, 1989.
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[14] E.W.J.L. Oomen, A.M.A. Van Dongen, J. Non-Cryst. Solids 111 (1989) 205. [15] J. Alarcon, D. van der Voort, G. Blasse, Mater. Res. Bull. 27 (1992) 467. [16] K.D. Kreue, St. Adams, W. Münch, A. Fuchs, U. Klock, J. Maier, Solid State Ionics 145 (2001) 295. [17] P.G. Sundell, M.E. Björketun, G. Wahnström, Phys. Rev. B 73 (2006) 104112. [18] P. Caro, O. Beaury, E. Antic, J. Phys. 37 (1976) 671. [19] G. Sperka, M. Bettinelli, Inorg. Chim. Acta 149 (1988) 147. [20] J.L. Sommerdijk, A. Bril, J. Lumin. 12/13 (1976) 669. [21] L.S. Gaigerova, O.F. Dudnik, V.F. Zolin, V.A. Kudryashova, in: F. Williams (Ed.), Luminescence of crystals, molecules and solutions, Plenum, New York, 1973, p. 514. [22] R.D. Peacock, Struct. Bond. 22 (1975) 83. [23] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002) 1542. [24] M.J. Weber, Handbook of Optical Materials, CRC Press, Boca Raton, FL, 2002. [25] Q. Sun, J. Shi, J. Phys. Chem. C 114 (2010) 3230. [26] Y. Haas, G. Stein, J. Phys. Chem. 76 (1972) 1093. [27] M.E. Björketun, P.G. Sundell, G. Wahnström, Phys. Rev. B 76 (2007) 054307. [28] Y. Yamazaki, F. Blanc, Y. Okuyama, L. Buannic, J.C. Lucio-Vega, C.P. Grey, S.M. Haile, Nat. Mater. 12 (2013) 647.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Eu3 + as a luminescent probe for the local structure of trivalent dopant ions in barium zirconate-based proton conductors P. Haro-González a, M. Karlsson b, S.M. Gaita c, C.S. Knee d, M. Bettinelli a,⁎ a
Luminescent Materials Laboratory, Department of Biotechnology, University of Verona and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Institute of Nuclear Science & Technology, University of Nairobi, P.O. Box 30197 00100, Nairobi, Kenya d Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden b c
a r t i c l e
i n f o
Article history: Received 14 November 2012 Received in revised form 17 June 2013 Accepted 21 June 2013 Keywords: Luminescence Eu3 + BaZrO3 Proton conducting oxides Hydration Local structure Proton-defect association
a b s t r a c t The luminescence spectra and decay kinetics of dry and hydrated samples of BaZr0.9Y0.099Eu0.001O2.95 have been measured at room temperature. The spectra of the dry sample evidence two 5D0 → 7F0 bands clearly indicating that Eu3+ (replacing Y3+ in the Zr4+ position of the average cubic perovskite structure) occupies two different local structures, whilst only one Eu3+ site is observed for the hydrated material. From the spectral data, it is possible to identify the nature of these two sites and to propose point group symmetries for the average local geometry around Eu3+. The decay time of the 5D0 level becomes shorter upon hydration, due to the interaction with high-frequency O–H stretching vibrations, indicating an attractive interaction between protons and dopant atoms. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrated, acceptor doped, ABO3 type perovskites, such as the barium zirconate-based compounds BaZr1 − xMxO3 − x/2 (M = Y, In, Sc and Ga, etc., x N 0), are well known to be proton conductors at elevated temperatures [1,2]. The introduction of trivalent dopant atoms on the tetravalent Zr sites creates an oxygen-deficient structure and in a humid atmosphere H2O molecules may dissociate into OH− ions, which fill the oxygen vacancies, and protons (H+), which bind to lattice oxygens. Hence, it is easy to understand that the local structure around the proton and the trivalent M ion is a key factor for the proton conduction process. Local structural investigations of such materials are therefore of great importance for the development of a detailed understanding of the, yet not fully understood, proton conduction phenomenon in hydrated perovskites. For this reason, we have used luminescence spectroscopy for the investigation of the local structure at Y3+ in 10% Y-doped BaZrO3 when 1% of the Y is replaced with Eu. The Eu3+ ion is a well-known spectroscopic probe [3,4] used in luminescence spectroscopy of inorganic materials and it has been used before for structural investigations of proton conducting perovskites (e.g. in the case of orthorhombic SrCe1−xYxO3 and SrZr1−xYxO3) [5,6]. These earlier studies [5,6], however, were, in contrast to our investigations, performed
⁎ Corresponding author. Tel.: +39 045 8027902. E-mail address: [email protected] (M. Bettinelli). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.06.008
on orthorhombic rather than cubic-structured materials. In our investigations we additionally present the decay times of the excited emitting state, for both dry and hydrated samples, which gives information about the interaction of the Eu3+ ion with O–H oscillators present in the hydrated material. The obtained information can be useful for understanding what factors influence the proton transport properties in proton conducting oxides. 2. Experimental Dry and hydrated samples of BaZr0.9Y0.099Eu0.001O2.95 were prepared following a procedure previously described for this class of materials [7], with Eu(NO3)3 · 6H2O acting as the source of Eu. A final reaction temperature of 1400 °C was used to obtain the as-prepared cubic perovskite (space group Pm–3m) that was judged to be phase pure based on powder X-ray diffraction (Cu-Kα1 radiation) data. A portion of the sample was then hydrated by cooling from 800 °C to 200 °C in steps of 100 °C with 1 h dwells at each temperature under a flow of Ar gas saturated with water vapour at room temperature (pH2O ≈ 0.025 atm.). The dry sample was prepared by annealing the as-prepared sample under vacuum (2 × 10−6 mbar) at 900 °C for 10 h. Room temperature laser excited luminescence spectra were measured upon excitation at 473 nm by using a continuous wave diode laser. The Eu3+ transitions from the 5D0 level were detected using an ANDOR-NEWTON CCD spectrograph with 0.5 nm resolution and
P. Haro-González et al. / Solid State Ionics 247–248 (2013) 94–97
analyzed with the specific software of the spectrograph. Emission decay curves were obtained at room temperature upon pulsed laser excitation with high-energy pulses of about 5 ns of duration at 532 nm, using a Nd-YAG laser. The detection chain was formed by a TRIAX-180 monochromator with 1 nm resolution and the output of the photomultiplier tube was registered by a digital oscilloscope TEKTRONIX-2430A.
5
7
D0
F2
Hydrated
Intensity (a.u.)
7
3. Results and discussion In almost all oxide hosts, the stable oxidation state of the europium ion is +3, unless the material is prepared under reducing conditions, which was not the case in the present investigation. Eu2+ gives rise to strong, broad, emission and excitation bands in the ultraviolet and visible regions [8,9]. We found no evidence for such spectral features and we conclude therefore that only Eu3+ is present in the samples under investigation. Moreover, it has been convincingly shown that Y3+ enters the Zr4+ site of the perovskite structure in BaZrO3 [10]. The ionic radii of Eu3+, Y3+, and Zr4+ in six-fold coordination (i.e. the B site in ABO3 type perovskites) are 0.947, 0.900 and 0.720 Å, respectively, whilst the ones for Eu3+ and Ba2+ in twelve-fold coordination (the A site has a coordination number (CN) of 12, the value for Eu3+ was obtained from extrapolation of the radii for lower CNs) are 1.29 and 1.61 Å, respectively [11]. These values strongly indicate that Eu3+ will substitute for Y3+ ions, and therefore suggest that Eu3+ is a suitable probe for the local structure around the Y3+ ions. The luminescence spectra of the Eu3+ ion in oxide hosts are generally dominated by emission from the non-degenerate 5D0 level (Fig. 1) [3]. The room temperature luminescence spectra of the two samples (dry and hydrated) of BaZr0.9Y0.099Eu0.001O2.95 are shown in Fig. 2, after normalization of the intensities of the 5D0 → 7F2 band peaking around 615–620 nm. It must be noted that the spectra have been measured upon excitation at 473 nm in the inhomogeneous profile of the 7F0 → 5D2 excitation band, i.e. exciting all the Eu3+ sites present in the structure. For this reason, the spectra show a significant degree of inhomogeneous broadening, due to the presence of a range of different local structural configurations around the Eu3+ as induced by weak structural perturbations caused by the presence of dopant atoms (Y and Eu) and oxygen vacancies [12]. Cooling the sample to low temperature does not give rise to significantly sharper spectral features, as inhomogeneous broadening is related to structural disorder and is not dependent on temperature. It is evident from Fig. 2 that the spectra of the two materials are not identical. Firstly, the relative intensities of the hypersensitive
95
F4
7 7
F0
F1 7
F3
Dry 550
600
650
700
750
Wavelength (nm) Fig. 2. Room temperature luminescence spectra of BaZr0.9Y0.099Eu0.001O2.95 samples measured upon excitation at 473 nm. The spectra have been vertically offset for the sake of clarity.
5
D0 → 7F2 transition and the magnetic dipole allowed 5D0 → 7F1 transition significantly change upon hydration. It is well known that the hypersensitive transitions (characterized by |ΔJ| = 2, where J is the total angular momentum quantum number) exhibit an intensity that depends strongly on the nature of the local structural environment around the Eu3+ ions, whilst the intensity of magnetic dipole transitions is not influenced [13]. The asymmetry ratio R, defined as R = I(5D0 → 7F2)/I(5D0 → 7F1) [14], where I represents the integrated intensity of the f–f transition, allows the evaluation of the degree of asymmetry in the local environment around the Eu3+ dopant. Our spectroscopic data show that R decreases as the material is hydrated, suggesting a more symmetrical local structure around the dopant atoms in the hydrated material. It should be noted that the values reported in Table. 1 are in both cases compatible with a significant asymmetry of the Eu3+ site(s), but clearly indicate that on average the geometry of the site(s) accommodating Eu3+ is more distorted in the case of the dry material. The spectral region of the 0–0 transition (5D0 → 7F0, located between 570 and 580 nm, see Fig. 1) is shown in detail in Fig. 3. This transition connects two non-degenerate levels having J = 0 that cannot be split by crystal-field effects. For this reason, the number of bands related to this transition reflects the number of sites occupied by Eu3+ ions in the material under investigation [13]. That is, each Eu3+ in a given site can be expected to be manifested by one separate spectral feature. Inspection of Fig. 3 suggests therefore the presence of two Eu3+ main sites in the dry sample whereas only one in the hydrated. This is in agreement with the luminescence spectroscopy study on Ba0.995La0.005Zr0.995Eu0.005O3 (Eu3+ used as a structural probe), a similar but more complex material, as reported by Alarcon et al. [15]. In their study, however, the results were interpreted as possible occupation by Eu3+ on both the Ba and Zr sites in the structure, which cannot be expected in our study due to ionic size considerations [11] and to experimental [16] and computational [17] evidence. On the basis of these arguments, we propose therefore to assign the band around 577 nm, which is located in the usual wavelength range for the
Table 1 Asymmetry ratios, and experimental and radiative 5D0 lifetimes for the materials under investigation.
Fig. 1. Energy-level diagram for the Eu3+ ion.
BaZr0.9Y0.099Eu0.001O2.95
R
τexp/μs
τR/μs
dry hydrated
7.8 5.5
470 420
431 538
96
P. Haro-González et al. / Solid State Ionics 247–248 (2013) 94–97
From the observed spectrum and the refractive index of the material it is possible to obtain a reasonable estimate of the radiative lifetime of the 5D0 level τR, using the equation [23]: 1 3 I tot ¼ AMD;0 n τR IMD
Fig. 3. Luminescence spectra of BaZr0.9Y0.099Eu0.001O2.95 samples in the 5D0 → 7F0 region, measured upon excitation at 473 nm.
5
D0 → 7F0 transition [18] and is present in both materials, to Eu3+ ions located in Zr4+ sites with no nearby oxygen vacancies. On the other hand, we note that the band at about 572 nm is located at very high energy, compared to its position in many hosts [18,19]. We therefore assign this band to sites perturbed by oxygen vacancies close-by, as found by Sommerdijk and Bril for AMgF3 (A = K, Rb, Cs) [20]. The explanation of this unusual behavior lies in the increased interaction due to the nearby charge compensation of the 5D0 level with charge transfer states, therefore inducing a shift to higher energies and concomitantly a corresponding shift of the 5D0 → 7F0 transition to a lower wavelength [20]. It should also be pointed out that many hosts in which the 5D0 → 7 F0 transition is located at low wavelength are characterized by charge compensation [21]. Upon hydration, oxygen vacancies are filled by OH− groups and this family of sites emitting at 572 nm disappears creating more sites emitting at 577 nm. We note that Yugami et al. [6] also observed in the proton conducting perovskite Y3+-doped SrCeO3 the disappearance upon hydration of a low wavelength band (around 575 nm), and the appearance of another band at higher wavelength (close to 580 nm). They interpreted this observation as a red shift of the low wavelength line. The differences in the spectroscopic behavior between our results and the ones obtained by Yugami and coworkers may be related to the different crystal structures (BaZrO3 is cubic, whilst SrCeO3 is orthorhombic) and the level of Y doping in the investigated materials (10% in the present study compared to a maximum of 4.9% in [6]). As discussed above, for both the dry and the hydrated sample, the 5 D0 → 7F2 hypersensitive band is dominating and much stronger than magnetic dipole allowed 5D0 → 7F1. The observation of the 5D0 → 7F0 band and the relative strength of the 5D0 → 7F2 transition indicates that the average local symmetry belongs to a point group containing Y1m spherical harmonics, as discussed at length by Peacock [22]. The highest symmetrical point groups of this type are C2v and C4v, which can be obtained by distortion of the original octahedral symmetry around Zr4+ in the cubic BaZrO3 perovskite structure. The decay times of the luminescence from 5D0 were measured at room temperature upon pulsed excitation at 532 nm. These results extend the ones previously obtained by Yugami et al. [5,6] who reported on persistent hole burning and site selective spectroscopy for SrZr1−xYxO3 and SrCe1−xYxO3, respectively [5,6]. The decay curves (not shown) are almost perfectly exponential; a suitable fit yields for the two samples rather similar decay times, slightly faster in the case of the hydrated sample, but with the same exponential behavior. The corresponding 5D0 lifetimes (τexp) are reported in Table. 1.
where n is the refractive index of the host, AMD,0 is the spontaneous emission probability of the 5D0 → 7F1 transition in vacuo (14.65 s−1, [23]) and Itot/IMD is the ratio of the total area of the Eu3+ emission spectrum to the area of the 5D0 → 7F1 band. This is due to the fact that the absolute spontaneous emission probability of the magnetic dipole 5 D0 → 7F1 transition depends only on the refractive index of the host and therefore can be easily calculated. Assuming for the materials under investigation the value n = 2.20 reported for the refractive index of undoped BaZrO3 [24,25], the radiative lifetimes reported in Table. 1 are obtained. In the case of the dry sample, the observed decay time is close to the radiative one, and therefore shows no quenching, either due to accidental hydration or to energy transfer to impurities. On the other hand, for the hydrated sample the decay time is about 25% less than the radiative one. This indicates significant quenching of the Eu3+ emission due to multiphonon relaxation induced by high-frequency O–H oscillators [26], bridging through molecular vibrations the energy gap separating the emitting 5D0 level from the lower lying non-emitting levels 7FJ (J = 6,…0). It follows that several, if not all, of the protons in the hydrated material are located in the vicinity of the Eu3+ ions. That is, there seems to be an associative interaction between the protons and the Y (Eu) atoms. This is in agreement with results obtained from both computational [27] and experimental [28] studies. Further luminescence spectroscopy studies along these lines, for example as a function of dopant concentration, may therefore give more detailed information for such a behavior and hence be very rewarding.
4. Conclusions In this work, we have shown that the Eu3+ ion is a useful spectroscopic probe of the environment of the trivalent M cations in proton conducting perovskite type oxides of the form BaZr1 − xMxO3 − x/2. In particular, the present study extends previous investigations on other proton conducting oxides [5,6], through the measurement of luminescence decay curves and the analysis of the relative intensities of the 5 D0 → 7FJ bands. On the basis of luminescence spectra and decay curves, we find that for the dry material there are two main sites present, one close to oxygen vacancies and one farther away. For the hydrated material, the site close to oxygen vacancies has disappeared. For both materials, the Eu3+ sites appear to be distorted from the original octahedral symmetry. Furthermore, hydration is shown to significantly shorten the decay time of the 5D0 level of Eu3+, indicating that high-frequency O–H vibrations provide an efficient non-radiative relaxation pathway, suggesting an associative interaction between the protons and the dopant (Eu, Y) atoms. Finally we note that the use of the Eu3+ luminescent probe can be successfully extended to other classes of proton conductors containing trivalent cations that can be substituted by this lanthanide ion without significant distortion of the local structure.
Acknowledgements Erica Viviani (Univ. Verona) is gratefully acknowledged for expert technical assistance. S. M. Gaita acknowledges the International Science Programme, hosted by Uppsala University, Sweden, for funding. M. Karlsson thanks the Swedish Research Council for research funding (Grant no. 2010-3519).
P. Haro-González et al. / Solid State Ionics 247–248 (2013) 94–97
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