- Email: [email protected]
Electron spin resonance study of the conversion of Mn4+ to Mn2+ in the Pb1−xEuxTi1−yMnyO3 ceramic system
PERGAMON
Solid State Communications 118 (2001) 371±376
www.elsevier.com/locate/ssc
Electron spin resonance study of the conversion of Mn 41 to Mn 21 in the Pb12xEuxTi12yMnyO3 ceramic system D. RamõÂrez-Rosales a,*, R. Zamorano-Ulloa a, O. PeÂrez-MartõÂnez b a
Depto. de FõÂsica, Escuela Superior de FõÂsica y MatemaÂticas del IPN, Edif. 9, U.P., Zacatenco, Col. San Pedro Zacatenco, MeÂxico, D.F., C.P. 07738, Mexico b Facultad de FõÂsica, Instituto de Materiales y Reactivos, Universidad de la Habana, San LaÂzaro y L. Vedado, La Habana 10400, Cuba Received 17 January 2001; accepted 8 February 2001 by R.C. Dynes; received in ®nal form by the Publisher 7 March 2001
Abstract Electron spin resonance (ESR) studies of the Pb0.88Eu0.08 Ti12yMnyO3 (y 0.0, 0.1, 0.2, 0.3); PbTi0.98Mn0.02O3 and PbTiO3 ferroelectric ceramics are reported. The conversion of Mn 41 to Mn 21 in the Pb0.88Eu0.08Ti12yMnyO3 ceramics compositions have been clearly demonstrated. The presence of the Eu cation is necessary for such conversion to take place and it does not take place in the PbTi0.98Mn0.02O3 ceramic. Analysis of its ESR spectra indicates that Mn 41 substitution occurs at the Ti 41 at two different ESR-distinguishable locations. Both factors, namely the Mn 41 ! Mn 21 conversion and the Eu 31 presence are responsible for the large microstrain and the deterioration of crystallinity observed in this ceramics, which seemed to play a crucial role for reading large piezoelectric anisotropy in the system. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 76.30.2v; 77.84.Dy; 77.65.2j; 76.30.Da Keywords: E. Electron spin resonance
1. Introduction Dense ceramics have been obtained by the partial substitution of alkaline metals [1] or rare earth elements (RE) [2] in the modi®ed lead titanate structure. When this is done, an anisotropy in the piezoelectric effect is manifested in these ceramics, because kt the electromechanical coupling factor for thickness dilatational vibration, increases with poling ®eld, while kp, the coupling factor for planar extensional vibration, seems to disappear. These materials therefore become very attractive as high frequency transducers [3]. As demonstrated recently [4], a large percentage of 908 domain rotation during the poling process is necessary, but it is not a suf®cient condition for the ultrahigh electromechanical anisotropy (kp ! 0) manifestation in the ceramics based on the Pb0.88Eu0.08Ti12yMnyO3 composition. The inherent anisotropy of the piezoelectric effect depends primarily on the degree of microstress or structural defects, including pseudo-random distribution of oxygen defects, which are present in these materials before the poling process. * Corresponding author. E-mail address: [email protected] (D. RamõÂrez-Rosales).
In these ceramics, the introduction of Eu to lead titanate produces one lead vacancy for every two Eu 31 ions. These defects locally break the translational periodicity of the lattice resulting in a substantial decrease in the ®rst-order discontinuity signature of the phase transition, i.e. a breakdown of the conventional ®rst-order ferroelectric phase transition [5]. It should be noted that these ceramics also require a modi®cation on the B site to produce ultrahigh electromechanical anisotropy. Indeed, PeÂrez-MartIÂnez et al. [4] have recently observed a marked detriment in the crystallinity due to the partial Ti by Mn substitution by means of DTA and DXR analysis. In that work, the authors presumed that such loss of crystallinity is due to oxygen vacancies induced by an assumed Mn 41 ! Mn 21 reduction during the sintering process of the samples [4]. Although extensive studies have been conducted on the (Pb,RE)(Ti,Mn)O3 system [1,2,6,7], the Mn reduction has not been considered a factor responsible for the large electromechanical anisotropy manifestation in this ceramic system. Nevertheless, the effects of reduction of manganese tetravalent ions on doped lead zirconate titanate (PZT) ceramics have been investigated in detail by Izaki et al. [8]. They conclude that Mn 41 is stable at low temperatures
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00072-2
372
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 1. (a) X-band ceramic powder ESR spectrum of PbTiO3 obtained at 77 K and microwave frequency 9.0549 GHz. (b) Central region of the powder ESR spectrum of PbTiO3.
and is reduced to Mn 21 during sintering, given that Mn 21 is the stable ionic state at high temperatures. Electron spin resonance is the most powerful spectroscopic method available to determine unambiguously the valence state of the Mn ion be it 4 1 or 2 1 and also for obtaining local structural information and symmetry of paramagnetic-incorporated metal ions [9±12]. Indeed, a large number of ESR studies has reported detailed information on these parameters when intentional or non-intentional incorporation of transition metal ions like Fe, Cr, Mn and Co has been carried out in single crystals of SrTiO3, BaTiO3 and of course PbTiO3 [10,11,15±22]. In this study, we have used the ESR technique to obtain information about the oxidation state of the Mn ion, its local environment and its crystalline electric ®eld in the Pb0.88Eu0.08Ti12yMnyO3, PbTi0.98Mn0.02O3 and PbTiO3 ceramics.
2. Experimental A set of samples of the (PT±Eu) (Ti±Mn)O3 system was prepared by a conventional ceramic route using PbO (98%), TiO2 (99.9%), Eu2O3 (99.9%) and MnO2 (99%) reagents. The general formula is Pb123x/2EuxTi12yMnyO3 and they
are denoted by PT, when x y 0; PT±2Mn, when x 0 and y 0.02, and keeping the Eu concentration at x 0.08: PTEu, PTEu±1Mn; PTEu±2Mn and PTEu±3Mn for y 0; y 0.01; y 0.02 and y 0.03, respectively. The stoichometric mixture of the powders was calcined at 9008C for 2 h. The calcined powders were conformed as disks, by cold-pressing and sintering in air at 12208C for 2 h in a well-covered platinum crucible and were cooled down to room temperature. ESR measurements of the ceramic powder were carried out at the X-band on a JEOL JES-RES 3X spectrometer, operating at a 100 kHz ®eld modulation and equipped with an X-band low-temperature accessory. The ESR X-band spectra were recorded at variable temperatures ranging from 300 K down to 77 K. The magnetic ®eld sweeps from zero to 8000 G. The g values were calculated from the accurate measurements of the magnetic ®eld with resolution of ^0.1 G and microwave frequency parameters and using an Mn 21 marker (weak pitch). Theoretical spectra for S 5/2 and S 3/2 systems were calculated by means of a set of numerical treatment programs specially developed for this purpose [23]. In these methods we included the matrix diagonalization and numerical exact eigenvalues, eigenfunction solutions, transition probabilities and transition ®elds, along the lines of the calculations made by Dowsing and Gibson [24], Griscom and Griscom [12] and more recently Pilbrow [9] and Mabbs and Collison [25]. The calculations have already been successfully applied to other Mn 21, S 1/2 rhombic systems [26]. 3. Results and discussion ESR spectra were recorded at variable temperatures ranging from 300 K down to 77 K. The spectra do not show any changes within this temperature range, except for a better resolution of the spectral features at lower temperatures. Hence, for all species identi®ed in this work, none of the hyper®ne and crystal ®eld parameters is dependent on temperatures between 300 K and 77 K. For this reason the 77 K ESR spectra are shown and discussed in this work. 3.1. Unmodi®ed PbTiO3 sample Fig. 1(a) shows well-de®ned signals labeled J, B, K and L. Hyper®ne splitting of 77 G is measured for the sextet B (g 3.777), which is assigned undoubtedly to Mn 41 with S 3/2 [10,11,25]. Fig. 1(b) shows the set of lines J in the central region. This set is composed of a weak signal at g 2.055 which is identi®ed as being due to Cu 21 ion impurity traces present in the sample [9,13] and other signals at g 1.984 and g 1.952 that are identi®ed as the probable parallel components of the K and B signals, respectively. The K signal at g 5.931 is common for isolated paramagnetic centers, S 5/2, in octahedral symmetry with
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 2. X-band ceramic powder ESR spectrum of manganesemodi®ed PbTiO3 obtained at 77 K and microwave frequency 9.0448 GHz.
tetragonal (axial) distortion. For this PT sample, it can be assigned either to high spin Mn 21 and/or high spin Fe 31 [9± 14] ions present as impurity traces proceeding from the starting reagents. Owing to its extremely narrow linewidth of 11.7 G and the lack of hyper®ne structure, we favor the assignment of this K signal to Fe 31, S 5/2 ions substituting in Ti 41 sites where the oxygen octahedra have lost an oxygen atom to compensate charge and so producing the strong tetragonal distortion. This type of paramagnetic center is common in these perovskite systems and are well understood [21,27,28]. Following Serway et al. [20] and Lewis et al. [21], we designate this paramagnetic center as Fe 31 ±VO to emphasize the role the oxygen vacancy is playing. The L signal, on the left-hand side of the spectrum in Fig. 1(a), appears to be a zero magnetic ®eld effect. This fact indicates that a crystalline ®eld of approximately 0.314 cm 21 is present and at least one of the paramagnetic centers is experiencing it. It is worthwhile noting that even though Mn and Fe are present at the level of impurity traces, they originate two well-differentiated signals B and K that indicate two different local environments at the B site. In the region of higher than 5500 G, a £ 20 ampli®ed view (inset) shows clearly two signals at g 1.147 and g 1.099 that we will call the incipient F p and F signals. These are discussed in the next section. 3.2. Mn-modi®ed PbTiO3 (PT±2Mn) sample Several signi®cant changes can be observed by the partial substitution of Ti 41 by Mn 41 (y 0.02) in the lead titanate sample. In Fig. 2, the L and J signals are no longer present, while B and K contributions remain and four new signals B p, M, F and F p are observed in the ESR spectrum. The six peaks of Mn 41 (B signal) are being superposed by the new signal B p, which shows itself as a strong and wide singlet (G B p < 300 G) at g ù 3.567. In view of this g value, B p is also assigned to Mn 41, but showing a dipolar interac-
373
tion. With the aim of displaying more explicitly the B p signal parameters, we performed a simulation of B p and the best parameters obtained were g 3.567 and ABp 59:7 G. The different ESR parameters for signals B and B p leads to consideration of the possibility of two structural Mn 41 locations occupying slightly different B sites. These sites are de®ned by their ESR distinguishability. The strong dipole±dipole interaction that shows B p is a salient characteristic that differentiates it from the B signal that even with its growth in intensity, it retains its hyper®ne resolution. We think that this difference makes the two sites B and B p geographically distinct. Probably making the Mn 41 centers that elicit the B p signal being more magnetically concentrated, perhaps closer to and/or at the grain boundaries and domain walls, while the Mn 41 centers eliciting the B signal are more diluted magnetically, indicating that they are more likely ªbulk centersº. The presence of spectral features such as F p and F at high ®elds in the region of 5000±6000 G is quite common in systems with S $ 1, such as transition metal ions where the crystalline ®eld starts to become an important fraction or even a multiple of the energy of the microwave quantum hn 0.31 cm 21 [24±26]. Though perturbation theory is not applicable in theses cases for the adequate assignment of these high ®eld transitions, it is required for a complete solution of the Hamiltonian j k ~ H ~ 1 D S2z 2 1=3S S 1 1 (1)H^ gbS? ~ ~I 1 E S2x 2 S2y 1 AS?
1
where the ®rst term is the Zeeman interaction and the second and the third terms correspond to the crystal ®eld splitting and the nuclear hyper®ne interaction, respectively. D and E are the usual axial and rhombic crystal ®eld parameters [15,20,23±26]. It is customary to de®ne the parameter l E=D, which can take values from zero to 1/3, where the l 0 value represents the totally axial case of the tensor D and the l 1/3 value represents the maximum rhombic distortion, E D=3, of the crystal ®eld tensor. Values of l outside this range reproduce the cases already included in the 0 , l , 1=3 range [24,26]. Several authors has presented solutions to this Hamiltonian for S 5/2 and S 3/2 [24±26,29]. It should be noted that transition ®elds and transition probabilities should also be explicitly calculated. These solutions are ~ x^, H ~ y^ and normally presented as D vs Hres plots for H ~ z^ orientations and only transitions with nonzero probH ability are plotted. In the present work, we have calculated the theoretical ®eld and probability transitions as was indicated in the Experimental section for spin systems S 3/2 and S 5/2. By careful comparison of the spectrum shown in Fig. 2 with the graphical solutions, the best agreements were achieved by considering that: (1) B, M and F are the transitions 3/2 $ 1/2, 1/2 $ 2 1/2 and 21/2 $ 2 3/2,
374
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 3. X-band ceramic powder ESR spectrum of europiummodi®ed PbTiO3 obtained at 77 K and microwave frequency 9.0543 GHz.
respectively, of an S 3/2 system in a crystal ®eld with D 0.1005 cm 21 and E 0.0335 cm 21 (l ù 1/3); (2) B p, M and F p are the transitions 3/2 $ 1/2, 1/2 $ 2 1/2 and 21/ 2 $ 2 3/2, respectively, of an S 3/2 system in the crystal ®eld with DBp 0:075 m21 and EBp 0:00098 cm21 (l ù 0.01), anywhere from 0.0 to 0.0166 cm 21 since for these low values of D, the solutions of equation (1) do not comply with the values of l from 0 to 0.22. The spin levels are labeled by their high-®eld wavefunction [24,29]. The set of the transition B p, B, M, F and F p could not be reproduced with any set of Hamiltonian parameters g, D, E, A when considering the system to be a single S 5/2 or S 3/2 paramagnetic species. This last result obtained from calculated ®eld and probability transitions derived from the exact solutions of the spin Hamiltonian of equation (1), is fully compatible with the spectral analysis of B and B p made in the previous paragraph. When Mn 41 is added to the PbTiO3 ferroelectric system, it is found in two slightly different locations, substituting Ti at the B site. These two different B-site locations are distinguishable by their slightly different ESR parameters and in addition, the Mn 41 cations in the B p locations experience considerable dipole±dipole interactions. We propose that these two B sites correspond to two different geographical locations: one more surface and/or domain wall concentrated while the other is more bulk-like magnetically diluted. 3.3. Eu-modi®ed PbTiO3 (PTEu) ceramic When instead of Mn 41, Eu 31 is incorporated into the ceramic, and the ESR spectrum registered, strong changes occur by the partial substitution of Pb 21 by Eu 31 (PTEu sample). Fig. 3 shows the complete disappearance of the B, J and incipient F p and F signals registered in unmodi®ed lead titanate. The signals B p, M, F and F p present in the PT± 2Mn ceramic are not observed in the PTEu sample. In other
words, no signs of any of the Mn 41 signals have remained with the inclusion of Eu 31 to lead titanate. Only the K signal remains in the ESR spectrum. In addition, a wide signal K p with line-width G 290 G emerges in the H 1110 G region, g < 5.859, with the K signal superposed on it. The dashed line is drawn as an aid to the eye. The signal K p is at a position common to signals originating from high-spin Fe 31 and Mn 21 isolated paramagnetic species. It seems as if the partial substitution of Eu 31 into Pb 21 sites, in order to compensate charge, in addition to creating Pb vacancies, facilitates oxygen de®ciency by also promoting the change Mn 41 ! Mn 21 as judged by the total disappearance of the B and incipient F p and F signals and the appearance of the K p signal. Manganese paramagnetic centers in this g region are well known in single crystals of PbTiO3 and indeed have been assigned to Mn 21 ±VO centers where the oxygen vacancy associated with the Mn 21 cation is included to account for the large tetragonal distortion from the octahedral symmetry that these ESR parameters are re¯ecting [10,11,15,20]. The partial Eu 31 substitution in the Pb positions has also brought about dipole±dipole magnetic interactions, namely Eu 31 ±Fe 31 and Eu 31 ± (Mn 21 ±VO) ions, which are manifested in the broadening of the K signal and the lack of hyper®ne splitting in the K p signal. In addition, the K signal has changed to clear axial symmetry, showing a weak but very clear peak to the right of the K peak. This means that the local environment experienced by Fe 31 ±VO has changed, in particular the symmetry is now lower. It seems safe to say that the presence of the magnetic moment mEu and the extra charge of Eu 31, neighboring the Fe 31 ±VO centers are responsible for this local environment change. In the central region, the main changes are the appearance of a wellresolved Cu(II) spectrum with gk 2.375 and g' 2.057 and other lines that are taken as the parallel components of K and K p. Note that a new L p zero ®eld absorption has also appeared at the left side of the spectrum in Fig. 3. This zero-®eld effect is indicating that at least one of the K or K p paramagnetic centers is experiencing a crystal ®eld of the order of magnitude of the energy of the microwave quantum hn < 0.31 cm 21. 3.4. Eu and Mn-modi®ed PbTiO3 (PTEu±yMn) ceramics When the lead titanate is modi®ed by the partial substitution of Eu 31 by Pb 21 and Mn 41 by Ti 41,the ESR spectra as shown in Fig. 4 are obtained. In these samples the Eu concentration was maintained at x 0.08 and the manganese concentration changed being: (a) 1 mol%, (b) 2 mol% and (c) 3 mol%, respectively. An appreciable increase in the pro®le and a strong broadening of both the K and K p signals are observed, being more pronounced for K p. The broadening is associated with an important increment of manganese responsible for the dipole±dipole interactions. Surprisingly, there are no signs of ESR lines due to Mn 41 (B and B p signals in the region of g # 4.0, signal M at g ù 1.983,
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
375
duced with any set of spin Hamiltonian parameters if the paramagnetic center was considered as S 3/2 (Mn 41). In this series of Eu and Mn-modi®ed PbTiO3 ferroelectric ceramics, the whole spectrum changes and the most important result observed is that Mn contributes very signi®cantly to the K p, T and G signals, which correspond to the ®ne transitions mentioned above belonging to an S 5/2 system in a crystal ®eld with axial component D 0.1012 cm 21 and rhombic component E 0.0051 cm 21. The complete disappearance of the Mn 41 signals in these PET±yMn samples indicate that Mn 41 to Mn 21 reduction took place during the sintering process. When this occurs, oxygen vacancies must be produced in order to keep the electrical neutrality of the compositions. In addition, severe changes in the local environment of the Mn and its crystalline electric ®eld take place.
4. Conclusions
Fig. 4. Sequence of ESR spectra taken at 77 K: (a) PET±1Mn; (b) PET±2Mn; and (c) PET±3Mn.
signal F p at g ù 1.197 and the signal F at g ù 1.064) at any concentration, or at any gain of the spectrometer. A large new and signi®cantly wide signal T at g 2.056, appears now in the central region for all the PTEu±yMn samples. If it is considered that the knee shown by the T signal splits, then g1 2.035 and g2 2.081 are measured. It should be stressed that this signal T is quite different from the M signal that appears when Ti is partially substituted by Mn in the PbTi0.98Mn0.02O3 composition (see the PT±2Mn spectrum) in the absence of Eu 31. In addition, a new G signal at H 5495 G, g 1.177 appears, while the F p and F signals are not present in the spectrum. As can be seen, the presence of a high ®eld transition such as G is again indicative of the presence of several ®ne transition lines of S $ 1 systems experiencing crystal ®elds comparable to hn < 0.31 cm 21. The spectra in Fig. 4 were compared in detail with the theoretical ®eld and probability transitions calculated from the exact solutions of the spin Hamiltonian in equation (1). The best agreement was achieved with the parameters D 0.1012 cm 21 and E 0.0051 cm 21 (l E=D 0:05) for a S 5/2 species, where the ESR signals K p, T and G correspond to the transitions (25/2 $ 2 3/2; 23/2 $ 1/2), 1/2 $ 2 1/2 and 21/2 $ 2 3/2, respectively. The notation of the spin states is as before. There are two important observations: (a) K p signal is at 6 # g # 4.3 and (b) T signal shows a knee at g 2.056. Both facts are only accounted for if a rhombic component E of the crystalline ®eld is included for the S 5/2 system. On the other hand, the facts above could not be repro-
The oxidation state of the Mn ion in the Pb123x/2EuxTi12yMnyO3 system was investigated using electron spin resonance. The impressionable resolutions of the ESR technique unmask the Cu, Fe and Mn impurities in the ªpureº PbTiO3 ceramic. These impurities proceed from the starting reagents and are also present in all the samples studied. Different effects on the ESR spectra were observed in the ceramics based on the modi®ed lead titanate with Mn 41 according to the presence of rare earth (Eu) element: (1) when there is no Eu 31 in the structure (impure PbTiO3 and PbTi0.98Mn0.02O3), all the Mn stays as Mn 41 substituting Ti 41 and experiencing two ESR distinguishable locations inside the oxygen octahedra, the two sites B and B p. The main difference is that the B p site represents a much more magnetically concentrated location than the one represented by B signal; (2) when Eu 31 is present (Pb0.88Eu0.08TiO3), Mn 41 to Mn 21 conversion takes place, even when the isolated Mn 41 centers proceed from impurities in the starting reagents; (3) when Eu and Mn-modi®ed PbTiO3 ceramic Pb0.88Eu0.08 Ti12yMnyO3 signals K p, T and G are assigned to the converted Mn 21 cations revealing yet more local structural environments as the different ESR parameters indicate. Two environments accommodate these Mn 21 cations. The Mn 41 ions at the sites of Ti 41 are able to dipole± dipole interact with other Mn 41 ions at Ti 41 sites and with Fe 31 ±VO centers. When Eu is incorporated, the Eu 31 ions at the sites of Pb 21 do not elicit a signal by themselves but are able to dipole±dipole interact with Fe 31 ±VO and elicit the L p zero ®eld absorption and the K p signal. The reduction of Mn 41 to Mn 21 for all Pb0.88Eu0.08 Ti12yMnyO3 ceramic compositions has been demonstrated using spin paramagnetic resonance. This reduction occurs during the sintering process and, as a consequence, oxygen vacancies have to be created to compensate for the charge imbalance. The presence of the Eu 31 ions plays a crucial role in the
376
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
above-mentioned charge state conversion. The results support the hypothesis that the anisotropy of the piezoelectric effect depends primarily on the degree of structural defects or microstress induced by ionic vacancies that are present in these ceramics before the poling process. Acknowledgements The authors acknowledge the Third World Academic of Science and the ICTP, Trieste, Italy, for ®nancial support of the Latin-American Network of Ferroelectric Materials (NET-43). References [1] Y. Yamashita, K. Yokoyama, H. Honda, H. Okuma, Jpn. J. Appl. Phys. 20 (Suppl. 20-4) (1981) 183. [2] H. Takeuchi, S. Jyomura, E. Yamamoto, Y. Ito, J. Acoust. Soc. Amer. 72 (4) (1982) 1114. [3] I. Ueda, Jpn. J. Appl. Phys. 11 (4) (1972) 450. [4] O. PeÂrez, J.M. Saniger, A. PelaÂiz, F. CalderoÂn, J. Mater. Res. 41 (7) (1999) 3083. [5] G.A. Rossetti, L.E. Cross, J.P. Cline, J. Mater. Sci. 30 (1995) 24. [6] P. Duran, J.F. Fdez, F. Capel, C. Moure, J. Mater. Sci. 23 (1988) 4463. [7] P. Duran, J.F. Fdez, F. Capel, C. Moure, J. Mater. Sci. 24 (1989) 447. [8] T. Izaki, H. Haneda, A. Watanabe, Y. Uchida, J. Tanaka, S. Shirasaki, Jpn. J. Appl. Phys. 31 (1992) 3045. [9] J.R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon Press, Oxford, 1990.
[10] D.J. Keeble, E.H. Poindexter, G.J. Gerardi, Appl. Spectroscopy 51 (1997) 117. [11] D.J. Keeble, Z. Li, E.H. Poindexter, Phys. Condens. Matter 7 (1995) 6327. [12] D.L. Griscom, R.E. Griscom, J. Chem. Phys. 47 (1967) 2711. [13] R.S. Drago, Physical Methods in Chemistry, Saunders, Eastbourne, 1977. [14] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. [15] K.A. MuÈller, W. Berlinger, K.W. Blazey, J. Albers, Sol. St. Commun. 61 (1987) 21. [16] T. Izaki, H. Haneda, A. Watanabe, Y. Uchida, J. Tanaka, The Sumitomo Search 53 (1993) 29. [17] D.J.A. Gainon, Phys. Rev. 134 (5A) (1964) A1300. [18] H. Ikushima, S. Hayakawa, J. Phys. Soc. Jpn 27 (2) (1969) 414. [19] T. Isaki, A. Watanabe, Fourth Euro Ceramics 5 (1995) 33. [20] R.A. Serway, W. Berlinger, K.A. MuÈller, R.W. Collins, Phys. Rev. B 16 (11) (1977) 4761. [21] O. Lewis, G. Wessel, Phys. Rev. B 13 (7) (1976) 2742. [22] E. Siegel, K.A. MuÈller, Phys. Rev. B 20 (9) (1979) 3587. [23] Basurto-Uribe, E., SimulacioÂn de espectros EPR de compuestos lantaÂnidos, Master Degree Thesis, ESFM.IPN, MeÂxico, 1996. [24] R.D. Dowsing, J.F. Gibson, J. Chem. Phys. 50 (1969) 294. [25] F.E. Mabbs, D. Collison, Electron Paramagnetic Resonance of d Transition Metal Compounds, Elsevier, Amsterdam, 1992. [26] O. JimeÂnez-Sandoval, D. RamõÂrez-Rosales, M.J. Rosales-Hoz, M.E. Sosa-Torres, R. Zamorano-Ulloa, J. Chem. Soc. Dalton Trans. (1998) 1551. [27] E.S. Kirkpatrick, K.A. MuÈller, R.S. Rubins, Phys. Rev. A86 (1964) 135. [28] Th. Von Waldkirch, K.A. MuÈller, W. Berlinger, Phys. Rev. B5 (1972) 4324. [29] R.D. Dowsing, J.F. Gibson, D.M.L. Goodgame, M. Goodgame, P.J. Hayward, J. Chem. Soc. A (1969) 1242.
Solid State Communications 118 (2001) 371±376
www.elsevier.com/locate/ssc
Electron spin resonance study of the conversion of Mn 41 to Mn 21 in the Pb12xEuxTi12yMnyO3 ceramic system D. RamõÂrez-Rosales a,*, R. Zamorano-Ulloa a, O. PeÂrez-MartõÂnez b a
Depto. de FõÂsica, Escuela Superior de FõÂsica y MatemaÂticas del IPN, Edif. 9, U.P., Zacatenco, Col. San Pedro Zacatenco, MeÂxico, D.F., C.P. 07738, Mexico b Facultad de FõÂsica, Instituto de Materiales y Reactivos, Universidad de la Habana, San LaÂzaro y L. Vedado, La Habana 10400, Cuba Received 17 January 2001; accepted 8 February 2001 by R.C. Dynes; received in ®nal form by the Publisher 7 March 2001
Abstract Electron spin resonance (ESR) studies of the Pb0.88Eu0.08 Ti12yMnyO3 (y 0.0, 0.1, 0.2, 0.3); PbTi0.98Mn0.02O3 and PbTiO3 ferroelectric ceramics are reported. The conversion of Mn 41 to Mn 21 in the Pb0.88Eu0.08Ti12yMnyO3 ceramics compositions have been clearly demonstrated. The presence of the Eu cation is necessary for such conversion to take place and it does not take place in the PbTi0.98Mn0.02O3 ceramic. Analysis of its ESR spectra indicates that Mn 41 substitution occurs at the Ti 41 at two different ESR-distinguishable locations. Both factors, namely the Mn 41 ! Mn 21 conversion and the Eu 31 presence are responsible for the large microstrain and the deterioration of crystallinity observed in this ceramics, which seemed to play a crucial role for reading large piezoelectric anisotropy in the system. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 76.30.2v; 77.84.Dy; 77.65.2j; 76.30.Da Keywords: E. Electron spin resonance
1. Introduction Dense ceramics have been obtained by the partial substitution of alkaline metals [1] or rare earth elements (RE) [2] in the modi®ed lead titanate structure. When this is done, an anisotropy in the piezoelectric effect is manifested in these ceramics, because kt the electromechanical coupling factor for thickness dilatational vibration, increases with poling ®eld, while kp, the coupling factor for planar extensional vibration, seems to disappear. These materials therefore become very attractive as high frequency transducers [3]. As demonstrated recently [4], a large percentage of 908 domain rotation during the poling process is necessary, but it is not a suf®cient condition for the ultrahigh electromechanical anisotropy (kp ! 0) manifestation in the ceramics based on the Pb0.88Eu0.08Ti12yMnyO3 composition. The inherent anisotropy of the piezoelectric effect depends primarily on the degree of microstress or structural defects, including pseudo-random distribution of oxygen defects, which are present in these materials before the poling process. * Corresponding author. E-mail address: [email protected] (D. RamõÂrez-Rosales).
In these ceramics, the introduction of Eu to lead titanate produces one lead vacancy for every two Eu 31 ions. These defects locally break the translational periodicity of the lattice resulting in a substantial decrease in the ®rst-order discontinuity signature of the phase transition, i.e. a breakdown of the conventional ®rst-order ferroelectric phase transition [5]. It should be noted that these ceramics also require a modi®cation on the B site to produce ultrahigh electromechanical anisotropy. Indeed, PeÂrez-MartIÂnez et al. [4] have recently observed a marked detriment in the crystallinity due to the partial Ti by Mn substitution by means of DTA and DXR analysis. In that work, the authors presumed that such loss of crystallinity is due to oxygen vacancies induced by an assumed Mn 41 ! Mn 21 reduction during the sintering process of the samples [4]. Although extensive studies have been conducted on the (Pb,RE)(Ti,Mn)O3 system [1,2,6,7], the Mn reduction has not been considered a factor responsible for the large electromechanical anisotropy manifestation in this ceramic system. Nevertheless, the effects of reduction of manganese tetravalent ions on doped lead zirconate titanate (PZT) ceramics have been investigated in detail by Izaki et al. [8]. They conclude that Mn 41 is stable at low temperatures
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00072-2
372
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 1. (a) X-band ceramic powder ESR spectrum of PbTiO3 obtained at 77 K and microwave frequency 9.0549 GHz. (b) Central region of the powder ESR spectrum of PbTiO3.
and is reduced to Mn 21 during sintering, given that Mn 21 is the stable ionic state at high temperatures. Electron spin resonance is the most powerful spectroscopic method available to determine unambiguously the valence state of the Mn ion be it 4 1 or 2 1 and also for obtaining local structural information and symmetry of paramagnetic-incorporated metal ions [9±12]. Indeed, a large number of ESR studies has reported detailed information on these parameters when intentional or non-intentional incorporation of transition metal ions like Fe, Cr, Mn and Co has been carried out in single crystals of SrTiO3, BaTiO3 and of course PbTiO3 [10,11,15±22]. In this study, we have used the ESR technique to obtain information about the oxidation state of the Mn ion, its local environment and its crystalline electric ®eld in the Pb0.88Eu0.08Ti12yMnyO3, PbTi0.98Mn0.02O3 and PbTiO3 ceramics.
2. Experimental A set of samples of the (PT±Eu) (Ti±Mn)O3 system was prepared by a conventional ceramic route using PbO (98%), TiO2 (99.9%), Eu2O3 (99.9%) and MnO2 (99%) reagents. The general formula is Pb123x/2EuxTi12yMnyO3 and they
are denoted by PT, when x y 0; PT±2Mn, when x 0 and y 0.02, and keeping the Eu concentration at x 0.08: PTEu, PTEu±1Mn; PTEu±2Mn and PTEu±3Mn for y 0; y 0.01; y 0.02 and y 0.03, respectively. The stoichometric mixture of the powders was calcined at 9008C for 2 h. The calcined powders were conformed as disks, by cold-pressing and sintering in air at 12208C for 2 h in a well-covered platinum crucible and were cooled down to room temperature. ESR measurements of the ceramic powder were carried out at the X-band on a JEOL JES-RES 3X spectrometer, operating at a 100 kHz ®eld modulation and equipped with an X-band low-temperature accessory. The ESR X-band spectra were recorded at variable temperatures ranging from 300 K down to 77 K. The magnetic ®eld sweeps from zero to 8000 G. The g values were calculated from the accurate measurements of the magnetic ®eld with resolution of ^0.1 G and microwave frequency parameters and using an Mn 21 marker (weak pitch). Theoretical spectra for S 5/2 and S 3/2 systems were calculated by means of a set of numerical treatment programs specially developed for this purpose [23]. In these methods we included the matrix diagonalization and numerical exact eigenvalues, eigenfunction solutions, transition probabilities and transition ®elds, along the lines of the calculations made by Dowsing and Gibson [24], Griscom and Griscom [12] and more recently Pilbrow [9] and Mabbs and Collison [25]. The calculations have already been successfully applied to other Mn 21, S 1/2 rhombic systems [26]. 3. Results and discussion ESR spectra were recorded at variable temperatures ranging from 300 K down to 77 K. The spectra do not show any changes within this temperature range, except for a better resolution of the spectral features at lower temperatures. Hence, for all species identi®ed in this work, none of the hyper®ne and crystal ®eld parameters is dependent on temperatures between 300 K and 77 K. For this reason the 77 K ESR spectra are shown and discussed in this work. 3.1. Unmodi®ed PbTiO3 sample Fig. 1(a) shows well-de®ned signals labeled J, B, K and L. Hyper®ne splitting of 77 G is measured for the sextet B (g 3.777), which is assigned undoubtedly to Mn 41 with S 3/2 [10,11,25]. Fig. 1(b) shows the set of lines J in the central region. This set is composed of a weak signal at g 2.055 which is identi®ed as being due to Cu 21 ion impurity traces present in the sample [9,13] and other signals at g 1.984 and g 1.952 that are identi®ed as the probable parallel components of the K and B signals, respectively. The K signal at g 5.931 is common for isolated paramagnetic centers, S 5/2, in octahedral symmetry with
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 2. X-band ceramic powder ESR spectrum of manganesemodi®ed PbTiO3 obtained at 77 K and microwave frequency 9.0448 GHz.
tetragonal (axial) distortion. For this PT sample, it can be assigned either to high spin Mn 21 and/or high spin Fe 31 [9± 14] ions present as impurity traces proceeding from the starting reagents. Owing to its extremely narrow linewidth of 11.7 G and the lack of hyper®ne structure, we favor the assignment of this K signal to Fe 31, S 5/2 ions substituting in Ti 41 sites where the oxygen octahedra have lost an oxygen atom to compensate charge and so producing the strong tetragonal distortion. This type of paramagnetic center is common in these perovskite systems and are well understood [21,27,28]. Following Serway et al. [20] and Lewis et al. [21], we designate this paramagnetic center as Fe 31 ±VO to emphasize the role the oxygen vacancy is playing. The L signal, on the left-hand side of the spectrum in Fig. 1(a), appears to be a zero magnetic ®eld effect. This fact indicates that a crystalline ®eld of approximately 0.314 cm 21 is present and at least one of the paramagnetic centers is experiencing it. It is worthwhile noting that even though Mn and Fe are present at the level of impurity traces, they originate two well-differentiated signals B and K that indicate two different local environments at the B site. In the region of higher than 5500 G, a £ 20 ampli®ed view (inset) shows clearly two signals at g 1.147 and g 1.099 that we will call the incipient F p and F signals. These are discussed in the next section. 3.2. Mn-modi®ed PbTiO3 (PT±2Mn) sample Several signi®cant changes can be observed by the partial substitution of Ti 41 by Mn 41 (y 0.02) in the lead titanate sample. In Fig. 2, the L and J signals are no longer present, while B and K contributions remain and four new signals B p, M, F and F p are observed in the ESR spectrum. The six peaks of Mn 41 (B signal) are being superposed by the new signal B p, which shows itself as a strong and wide singlet (G B p < 300 G) at g ù 3.567. In view of this g value, B p is also assigned to Mn 41, but showing a dipolar interac-
373
tion. With the aim of displaying more explicitly the B p signal parameters, we performed a simulation of B p and the best parameters obtained were g 3.567 and ABp 59:7 G. The different ESR parameters for signals B and B p leads to consideration of the possibility of two structural Mn 41 locations occupying slightly different B sites. These sites are de®ned by their ESR distinguishability. The strong dipole±dipole interaction that shows B p is a salient characteristic that differentiates it from the B signal that even with its growth in intensity, it retains its hyper®ne resolution. We think that this difference makes the two sites B and B p geographically distinct. Probably making the Mn 41 centers that elicit the B p signal being more magnetically concentrated, perhaps closer to and/or at the grain boundaries and domain walls, while the Mn 41 centers eliciting the B signal are more diluted magnetically, indicating that they are more likely ªbulk centersº. The presence of spectral features such as F p and F at high ®elds in the region of 5000±6000 G is quite common in systems with S $ 1, such as transition metal ions where the crystalline ®eld starts to become an important fraction or even a multiple of the energy of the microwave quantum hn 0.31 cm 21 [24±26]. Though perturbation theory is not applicable in theses cases for the adequate assignment of these high ®eld transitions, it is required for a complete solution of the Hamiltonian j k ~ H ~ 1 D S2z 2 1=3S S 1 1 (1)H^ gbS? ~ ~I 1 E S2x 2 S2y 1 AS?
1
where the ®rst term is the Zeeman interaction and the second and the third terms correspond to the crystal ®eld splitting and the nuclear hyper®ne interaction, respectively. D and E are the usual axial and rhombic crystal ®eld parameters [15,20,23±26]. It is customary to de®ne the parameter l E=D, which can take values from zero to 1/3, where the l 0 value represents the totally axial case of the tensor D and the l 1/3 value represents the maximum rhombic distortion, E D=3, of the crystal ®eld tensor. Values of l outside this range reproduce the cases already included in the 0 , l , 1=3 range [24,26]. Several authors has presented solutions to this Hamiltonian for S 5/2 and S 3/2 [24±26,29]. It should be noted that transition ®elds and transition probabilities should also be explicitly calculated. These solutions are ~ x^, H ~ y^ and normally presented as D vs Hres plots for H ~ z^ orientations and only transitions with nonzero probH ability are plotted. In the present work, we have calculated the theoretical ®eld and probability transitions as was indicated in the Experimental section for spin systems S 3/2 and S 5/2. By careful comparison of the spectrum shown in Fig. 2 with the graphical solutions, the best agreements were achieved by considering that: (1) B, M and F are the transitions 3/2 $ 1/2, 1/2 $ 2 1/2 and 21/2 $ 2 3/2,
374
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
Fig. 3. X-band ceramic powder ESR spectrum of europiummodi®ed PbTiO3 obtained at 77 K and microwave frequency 9.0543 GHz.
respectively, of an S 3/2 system in a crystal ®eld with D 0.1005 cm 21 and E 0.0335 cm 21 (l ù 1/3); (2) B p, M and F p are the transitions 3/2 $ 1/2, 1/2 $ 2 1/2 and 21/ 2 $ 2 3/2, respectively, of an S 3/2 system in the crystal ®eld with DBp 0:075 m21 and EBp 0:00098 cm21 (l ù 0.01), anywhere from 0.0 to 0.0166 cm 21 since for these low values of D, the solutions of equation (1) do not comply with the values of l from 0 to 0.22. The spin levels are labeled by their high-®eld wavefunction [24,29]. The set of the transition B p, B, M, F and F p could not be reproduced with any set of Hamiltonian parameters g, D, E, A when considering the system to be a single S 5/2 or S 3/2 paramagnetic species. This last result obtained from calculated ®eld and probability transitions derived from the exact solutions of the spin Hamiltonian of equation (1), is fully compatible with the spectral analysis of B and B p made in the previous paragraph. When Mn 41 is added to the PbTiO3 ferroelectric system, it is found in two slightly different locations, substituting Ti at the B site. These two different B-site locations are distinguishable by their slightly different ESR parameters and in addition, the Mn 41 cations in the B p locations experience considerable dipole±dipole interactions. We propose that these two B sites correspond to two different geographical locations: one more surface and/or domain wall concentrated while the other is more bulk-like magnetically diluted. 3.3. Eu-modi®ed PbTiO3 (PTEu) ceramic When instead of Mn 41, Eu 31 is incorporated into the ceramic, and the ESR spectrum registered, strong changes occur by the partial substitution of Pb 21 by Eu 31 (PTEu sample). Fig. 3 shows the complete disappearance of the B, J and incipient F p and F signals registered in unmodi®ed lead titanate. The signals B p, M, F and F p present in the PT± 2Mn ceramic are not observed in the PTEu sample. In other
words, no signs of any of the Mn 41 signals have remained with the inclusion of Eu 31 to lead titanate. Only the K signal remains in the ESR spectrum. In addition, a wide signal K p with line-width G 290 G emerges in the H 1110 G region, g < 5.859, with the K signal superposed on it. The dashed line is drawn as an aid to the eye. The signal K p is at a position common to signals originating from high-spin Fe 31 and Mn 21 isolated paramagnetic species. It seems as if the partial substitution of Eu 31 into Pb 21 sites, in order to compensate charge, in addition to creating Pb vacancies, facilitates oxygen de®ciency by also promoting the change Mn 41 ! Mn 21 as judged by the total disappearance of the B and incipient F p and F signals and the appearance of the K p signal. Manganese paramagnetic centers in this g region are well known in single crystals of PbTiO3 and indeed have been assigned to Mn 21 ±VO centers where the oxygen vacancy associated with the Mn 21 cation is included to account for the large tetragonal distortion from the octahedral symmetry that these ESR parameters are re¯ecting [10,11,15,20]. The partial Eu 31 substitution in the Pb positions has also brought about dipole±dipole magnetic interactions, namely Eu 31 ±Fe 31 and Eu 31 ± (Mn 21 ±VO) ions, which are manifested in the broadening of the K signal and the lack of hyper®ne splitting in the K p signal. In addition, the K signal has changed to clear axial symmetry, showing a weak but very clear peak to the right of the K peak. This means that the local environment experienced by Fe 31 ±VO has changed, in particular the symmetry is now lower. It seems safe to say that the presence of the magnetic moment mEu and the extra charge of Eu 31, neighboring the Fe 31 ±VO centers are responsible for this local environment change. In the central region, the main changes are the appearance of a wellresolved Cu(II) spectrum with gk 2.375 and g' 2.057 and other lines that are taken as the parallel components of K and K p. Note that a new L p zero ®eld absorption has also appeared at the left side of the spectrum in Fig. 3. This zero-®eld effect is indicating that at least one of the K or K p paramagnetic centers is experiencing a crystal ®eld of the order of magnitude of the energy of the microwave quantum hn < 0.31 cm 21. 3.4. Eu and Mn-modi®ed PbTiO3 (PTEu±yMn) ceramics When the lead titanate is modi®ed by the partial substitution of Eu 31 by Pb 21 and Mn 41 by Ti 41,the ESR spectra as shown in Fig. 4 are obtained. In these samples the Eu concentration was maintained at x 0.08 and the manganese concentration changed being: (a) 1 mol%, (b) 2 mol% and (c) 3 mol%, respectively. An appreciable increase in the pro®le and a strong broadening of both the K and K p signals are observed, being more pronounced for K p. The broadening is associated with an important increment of manganese responsible for the dipole±dipole interactions. Surprisingly, there are no signs of ESR lines due to Mn 41 (B and B p signals in the region of g # 4.0, signal M at g ù 1.983,
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
375
duced with any set of spin Hamiltonian parameters if the paramagnetic center was considered as S 3/2 (Mn 41). In this series of Eu and Mn-modi®ed PbTiO3 ferroelectric ceramics, the whole spectrum changes and the most important result observed is that Mn contributes very signi®cantly to the K p, T and G signals, which correspond to the ®ne transitions mentioned above belonging to an S 5/2 system in a crystal ®eld with axial component D 0.1012 cm 21 and rhombic component E 0.0051 cm 21. The complete disappearance of the Mn 41 signals in these PET±yMn samples indicate that Mn 41 to Mn 21 reduction took place during the sintering process. When this occurs, oxygen vacancies must be produced in order to keep the electrical neutrality of the compositions. In addition, severe changes in the local environment of the Mn and its crystalline electric ®eld take place.
4. Conclusions
Fig. 4. Sequence of ESR spectra taken at 77 K: (a) PET±1Mn; (b) PET±2Mn; and (c) PET±3Mn.
signal F p at g ù 1.197 and the signal F at g ù 1.064) at any concentration, or at any gain of the spectrometer. A large new and signi®cantly wide signal T at g 2.056, appears now in the central region for all the PTEu±yMn samples. If it is considered that the knee shown by the T signal splits, then g1 2.035 and g2 2.081 are measured. It should be stressed that this signal T is quite different from the M signal that appears when Ti is partially substituted by Mn in the PbTi0.98Mn0.02O3 composition (see the PT±2Mn spectrum) in the absence of Eu 31. In addition, a new G signal at H 5495 G, g 1.177 appears, while the F p and F signals are not present in the spectrum. As can be seen, the presence of a high ®eld transition such as G is again indicative of the presence of several ®ne transition lines of S $ 1 systems experiencing crystal ®elds comparable to hn < 0.31 cm 21. The spectra in Fig. 4 were compared in detail with the theoretical ®eld and probability transitions calculated from the exact solutions of the spin Hamiltonian in equation (1). The best agreement was achieved with the parameters D 0.1012 cm 21 and E 0.0051 cm 21 (l E=D 0:05) for a S 5/2 species, where the ESR signals K p, T and G correspond to the transitions (25/2 $ 2 3/2; 23/2 $ 1/2), 1/2 $ 2 1/2 and 21/2 $ 2 3/2, respectively. The notation of the spin states is as before. There are two important observations: (a) K p signal is at 6 # g # 4.3 and (b) T signal shows a knee at g 2.056. Both facts are only accounted for if a rhombic component E of the crystalline ®eld is included for the S 5/2 system. On the other hand, the facts above could not be repro-
The oxidation state of the Mn ion in the Pb123x/2EuxTi12yMnyO3 system was investigated using electron spin resonance. The impressionable resolutions of the ESR technique unmask the Cu, Fe and Mn impurities in the ªpureº PbTiO3 ceramic. These impurities proceed from the starting reagents and are also present in all the samples studied. Different effects on the ESR spectra were observed in the ceramics based on the modi®ed lead titanate with Mn 41 according to the presence of rare earth (Eu) element: (1) when there is no Eu 31 in the structure (impure PbTiO3 and PbTi0.98Mn0.02O3), all the Mn stays as Mn 41 substituting Ti 41 and experiencing two ESR distinguishable locations inside the oxygen octahedra, the two sites B and B p. The main difference is that the B p site represents a much more magnetically concentrated location than the one represented by B signal; (2) when Eu 31 is present (Pb0.88Eu0.08TiO3), Mn 41 to Mn 21 conversion takes place, even when the isolated Mn 41 centers proceed from impurities in the starting reagents; (3) when Eu and Mn-modi®ed PbTiO3 ceramic Pb0.88Eu0.08 Ti12yMnyO3 signals K p, T and G are assigned to the converted Mn 21 cations revealing yet more local structural environments as the different ESR parameters indicate. Two environments accommodate these Mn 21 cations. The Mn 41 ions at the sites of Ti 41 are able to dipole± dipole interact with other Mn 41 ions at Ti 41 sites and with Fe 31 ±VO centers. When Eu is incorporated, the Eu 31 ions at the sites of Pb 21 do not elicit a signal by themselves but are able to dipole±dipole interact with Fe 31 ±VO and elicit the L p zero ®eld absorption and the K p signal. The reduction of Mn 41 to Mn 21 for all Pb0.88Eu0.08 Ti12yMnyO3 ceramic compositions has been demonstrated using spin paramagnetic resonance. This reduction occurs during the sintering process and, as a consequence, oxygen vacancies have to be created to compensate for the charge imbalance. The presence of the Eu 31 ions plays a crucial role in the
376
D. RamõÂrez-Rosales et al. / Solid State Communications 118 (2001) 371±376
above-mentioned charge state conversion. The results support the hypothesis that the anisotropy of the piezoelectric effect depends primarily on the degree of structural defects or microstress induced by ionic vacancies that are present in these ceramics before the poling process. Acknowledgements The authors acknowledge the Third World Academic of Science and the ICTP, Trieste, Italy, for ®nancial support of the Latin-American Network of Ferroelectric Materials (NET-43). References [1] Y. Yamashita, K. Yokoyama, H. Honda, H. Okuma, Jpn. J. Appl. Phys. 20 (Suppl. 20-4) (1981) 183. [2] H. Takeuchi, S. Jyomura, E. Yamamoto, Y. Ito, J. Acoust. Soc. Amer. 72 (4) (1982) 1114. [3] I. Ueda, Jpn. J. Appl. Phys. 11 (4) (1972) 450. [4] O. PeÂrez, J.M. Saniger, A. PelaÂiz, F. CalderoÂn, J. Mater. Res. 41 (7) (1999) 3083. [5] G.A. Rossetti, L.E. Cross, J.P. Cline, J. Mater. Sci. 30 (1995) 24. [6] P. Duran, J.F. Fdez, F. Capel, C. Moure, J. Mater. Sci. 23 (1988) 4463. [7] P. Duran, J.F. Fdez, F. Capel, C. Moure, J. Mater. Sci. 24 (1989) 447. [8] T. Izaki, H. Haneda, A. Watanabe, Y. Uchida, J. Tanaka, S. Shirasaki, Jpn. J. Appl. Phys. 31 (1992) 3045. [9] J.R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon Press, Oxford, 1990.
[10] D.J. Keeble, E.H. Poindexter, G.J. Gerardi, Appl. Spectroscopy 51 (1997) 117. [11] D.J. Keeble, Z. Li, E.H. Poindexter, Phys. Condens. Matter 7 (1995) 6327. [12] D.L. Griscom, R.E. Griscom, J. Chem. Phys. 47 (1967) 2711. [13] R.S. Drago, Physical Methods in Chemistry, Saunders, Eastbourne, 1977. [14] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. [15] K.A. MuÈller, W. Berlinger, K.W. Blazey, J. Albers, Sol. St. Commun. 61 (1987) 21. [16] T. Izaki, H. Haneda, A. Watanabe, Y. Uchida, J. Tanaka, The Sumitomo Search 53 (1993) 29. [17] D.J.A. Gainon, Phys. Rev. 134 (5A) (1964) A1300. [18] H. Ikushima, S. Hayakawa, J. Phys. Soc. Jpn 27 (2) (1969) 414. [19] T. Isaki, A. Watanabe, Fourth Euro Ceramics 5 (1995) 33. [20] R.A. Serway, W. Berlinger, K.A. MuÈller, R.W. Collins, Phys. Rev. B 16 (11) (1977) 4761. [21] O. Lewis, G. Wessel, Phys. Rev. B 13 (7) (1976) 2742. [22] E. Siegel, K.A. MuÈller, Phys. Rev. B 20 (9) (1979) 3587. [23] Basurto-Uribe, E., SimulacioÂn de espectros EPR de compuestos lantaÂnidos, Master Degree Thesis, ESFM.IPN, MeÂxico, 1996. [24] R.D. Dowsing, J.F. Gibson, J. Chem. Phys. 50 (1969) 294. [25] F.E. Mabbs, D. Collison, Electron Paramagnetic Resonance of d Transition Metal Compounds, Elsevier, Amsterdam, 1992. [26] O. JimeÂnez-Sandoval, D. RamõÂrez-Rosales, M.J. Rosales-Hoz, M.E. Sosa-Torres, R. Zamorano-Ulloa, J. Chem. Soc. Dalton Trans. (1998) 1551. [27] E.S. Kirkpatrick, K.A. MuÈller, R.S. Rubins, Phys. Rev. A86 (1964) 135. [28] Th. Von Waldkirch, K.A. MuÈller, W. Berlinger, Phys. Rev. B5 (1972) 4324. [29] R.D. Dowsing, J.F. Gibson, D.M.L. Goodgame, M. Goodgame, P.J. Hayward, J. Chem. Soc. A (1969) 1242.