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ISSN 1063 7834, Physics of the Solid State, 2012, Vol. 54, No. 5, pp. 975­979. © Pleiades Publishing, Ltd., 2012. Original Russian Text © I.A. Sluchinskaya, A.I. Lebedev, A. Erko, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 5, pp. 917­920.

PROCEEDINGS OF THE XIX ALL RUSSIAN CONFERENCE ON PHYSICS OF FERROELECTRICS (VKS XIX)
(Moscow, Russia, June 19­23, 2011)

XAFS Studies of the Local Structure and Charge State of the Pr Impurity in SrTiO3
I. A. Sluchinskayaa, *, A. I. Lebedeva, and A. Erkob
a

Moscow State University, Moscow, 119991 Russia * e mail: irinasluch@nm.ru

b

Berliner Elektronenspeicherring Gesellschaft fÝr Synchrotronstrahlung (BESSY), Albert Einstein Str. 15, Berlin, 12489 Germany

Abstract--Solid solutions of (Sr1­ xPrx)TiO3 have been studied using X ray methods. It has been shown that, with an increase in the praseodymium concentration, the temperature of the structural phase transition to the phase with space group I4/mcm increases and, at x 0.15, the structure at 300 K is tetragonal. X ray absorp tion fine structure (XAFS) spectroscopy studies have revealed that Pr ions are predominantly in the charge state 3+ and occupy the Sr sites. No indications of the off centering of Pr atoms at the Sr sites have been revealed. The local environment of Pr atoms is characterized by a strong relaxation of the oxygen atoms, the value of which corresponds to the difference between the ionic radii of Pr3+ and Sr2+. It has been found that, in the second shell, there occurs a significant repulsion of the Pr3+ and Ti4+ ions, which is responsible for the weak dependence of the lattice parameter in the solid solution on the praseodymium concentration. DOI: 10.1134/S1063783412050381

1. INTRODUCTION Strontium titanate SrTiO3 is an incipient ferroelec tric with a cubic structure at 300 K, in which quantum fluctuations stabilize the paraelectric phase down to very low temperatures. At temperatures close to 105 K, it undergoes a structural phase transition to the tetrag onal phase with space group I4/mcm due to rotations of the octahedra. The ferroelectric state in SrTiO3 can be induced either by applying external electric or strain fields or by doping this compound with impuri ties. In this case, the temperature of the phase transi tion, as a rule, remains relatively low [1]. In this respect, the results recently obtained by DurÀn et al. [2] for SrTiO3 samples doped with the praseodymium impurity appeared to be interesting and unexpected. It was found that these samples exhibit a maximum of the dielectric constant at approximately 240°C and hysteresis loops at room temperature [2], which were explained by the ferro electric phase transition occurring in the crystals. A specific feature of this phase transition was a weak influence of the Pr concentration on the temperature corresponding to the maximum of the dielectric con stant, which differed from the influence exerted by other impurities on the ferroelectric phase transition in SrTiO3 [1]. An analysis of the X ray photoelectron spectra enabled the authors of [2] to draw the conclu sion that the Pr atoms in SrTiO3 are in two charge

states (3+ and 4+). Synchrotron X ray diffraction studies [3] revealed a tetragonal lattice distortion in (Sr0.85Pr0.15)TiO3 whose structure was determined as P4mm. Further investigations [4­6] carried out using X ray powder diffraction, neutron diffraction, and Raman spectroscopy demonstrated that, with an increase in the Pr concentration, the temperature of the structural phase transition to the I4/mcm phase rapidly increases and, at the praseodymium concen tration x = 0.05, is close to 300 K. The temperature dependence of the frequency of soft TO phonons in (Sr0.975Pr0.025)TiO3 crystals did not reveal specific fea tures at the temperature of the dielectric anomaly [4]; therefore, the conclusion was drawn that no ferroelec tric phase transition occurs in the SrTiO3 matrix. The observation of the dispersion of the dielectric constant [4] showed that the system demonstrates signs of a high temperature relaxor ferroelectric and that the temperature of the transition to the ferroelectric state is not related to the temperature of the structural phase transition. In order to explain the observed anomalies in the dielectric constant, the following models were proposed in [2­6]: (1) the displacive phase transition, (2) the distortion of TiO6 octahedra due to the replace ment of strontium atoms by praseodymium atoms, and (3) the formation of polar nanoregions either as a result of the formation of "defect states" or as a result of the off centering of Pr atoms.

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976 (3/2, 1/2, 1/2) (111) (200) (3/2, 1/2, 3/2) (210) (110) (211) 400 (100)

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300 I, counts/s

200

100

0 10

20

30

40 50 2, deg

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Fig. 1. X ray diffraction pattern of the (Sr0.85Pr0.15)TiO3 sample annealed at 1400°C. Points represent fragments of the X ray diffraction pattern that are shifted along the ver tical axis and recorded for an accumulation time of 300 s. Arrows indicate the calculated positions of the superstruc ture reflections.

Extended X ray absorption fine structure (EXAFS) and X ray absorption near edge structure (XANES) spectra were recorded by detecting X ray fluorescence with a RæNTEC energy dispersive detector. The investigations were performed at the BESSY synchro tron radiation source on the KMC 2 station at the Pr LII absorption edge (6.440 keV) and the Ti K absorp tion edge (4.966 keV) at 300 K. The choice of the LII edge, instead of the LIII edge, was motivated by the fact that the Pr L1 fluorescence line excited at the LIII edge almost completely coincides with the Ti K1 fluores cence line, which significantly complicates the detec tion of the signal from Pr atoms. For the chosen Pr LII edge, the L1 fluorescence line is exited and separated by 560 eV from the Ti fluorescence line. The EXAFS spectra were processed using the conventional method [7]. 3. RESULTS AND DISCUSSION The X ray diffraction analysis has revealed that the Sr(Ti1­ xPrx)O3 (x = 0.05) samples synthesized at annealing temperatures in the range from 1300 to 1600°C are two phase and, as can be judged from additional lines in the X ray diffraction pattern, con tain the Ruddlesden­Popper phases Sr3Ti2O7 and Pr6O11; the (Sr1­ xPrx)TiO3 (x = 0.05­0.15) samples are single phase; and the sample with x = 0.3 contains a small amount of the second phase PrO2­ . The X ray diffraction patterns of the (Sr1­ xPrx)TiO3 samples with x = 0.15­0.30, along with the reflections characteristic of the cubic perovskite phase, exhibit additional superstructure reflections whose intensity increases with an increase in the Pr concentration. The superstructure reflections indicated by arrows in Fig. 1 can be indexed in terms of the cubic lattice as the lines with the (3/2, 1/2, 1/2), (3/2, 1/2, 3/2), and (5/2, 1/2, 1/2) indices. In accordance with the results reported in the paper by Glazer [8], the appearance of these reflections suggests that the oxygen octahedra are rotated around the c axis according to the scheme a0a0c­; in this case, the symmetry of the crystal lattice is reduced from cubic to tetragonal (space group I4/mcm). Since the structural phase transition in undoped SrTiO3 occurs at 105 K, it can be concluded that, at x 0.15, the temperature of this phase transi tion exceeds 300 K. Our conclusions agree with the conclusions drawn from the neutron diffraction stud ies [6]; however, it should be noted that, in our work, the complete set of superstructure reflections was observed using X ray diffraction for the first time. An increase in the temperature of the structural phase transition upon doping with praseodymium is not sur prising, because the ionic radius of praseodymium is considerably smaller than the ionic radius of stron tium.
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Another reason for the interest expressed by researchers in SrTiO3(Pr) solid solutions is that the Pr impurity atoms have magnetic moments, which makes possible the appearance of magnetic ordering and properties characteristic of multiferroics in the sam ples. X ray absorption fine structure (XAFS) spectros copy studies of the local structure and charge state of the Pr impurity in SrTiO3 are important because of the contradictory data on the crystal structure, the loca tion of Pr atoms, and their charge state, as well as because of the lack of reliable data on the microscopic structure and mechanisms providing electroneutrality upon the heterovalent substitution. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Samples of the nominal composition (Sr1­ xPrx)TiO3 with x = 0.05­0.30, and Sr(Ti1­ xPrx)O3 with x = 0.05 were prepared by the solid phase reaction method. The initial components were as follows: SrCO3, nanocrystalline TiO2 synthesized by the hydrolysis of tetrapropylorthotitanate and dried at 500°C, and Pr6O11. The components were weighed in the required proportions, ground under acetone until the mixture was completely dried, and annealed in air at 1100°C for 8 h. The obtained powders were ground once again and additionally annealed at temperatures in the range from 1100 to 1600°C. The annealing times were as fol lows: 8 h at 1100°C, 4 h at 1300 and 1400°C, and 2 h at 1600°C. In the samples annealed at 1100°C, praseodymium had not yet entered into the reaction; solid solutions were formed beginning from 1300°C.

(5/2, 1/2, 1/2)

(220)

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XAFS STUDIES OF THE LOCAL STRUCTURE AND CHARGE STATE 0.3 3 XANES signal 1 2 3 4 5 6 7 0.2 0.1 k(k) 0

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-0.1 -0.2 6440 6460 Energy, eV 6480 3 4 5 6 k, å-1 7 8 9

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Fig. 2. Pr LII XANES spectra of the SrTiO3(Pr) samples and reference compounds of trivalent and tetravalent praseodymium: (1) Pr2Ti2O7; (2) BaPrO3; (3­5) (Sr0.95Pr0.05)TiO3 samples annealed at 1300, 1400, and 1600°C, respectively; (6) (Sr0.85Pr0.15)TiO3 annealed at 1400°C; and (7) (Sr0.7Pr0.3)TiO3 sample annealed at 1400°C. The spectra were recorded at 300 K.

Fig. 3. Pr LII EXAFS spectrum of the (Sr0.85Pr0.15)TiO3 sample annealed at 1400°C. Points are experimental data, and the solid line shows their best theoretical approxima tion. The spectrum was recorded at 300 K.

The parameters of pseudocubic lattice1 for the (Sr1­ xPrx)TiO3 samples annealed at 1400°C are as fol lows: a = 3.897 å for x = 0.05, a = 3.898 å for x = 0.15, and a = 3.894 å for x = 0.30. It is seen that the lattice parameter decreases only slightly with an increase in the Pr concentration. According to the data reported in [2, 5, 6], the lattice parameter of the (Sr1­ xPrx)TiO3 compounds after doping remains almost unchanged. In order to determine the charge state of the Pr impurity, the XANES spectra of the samples under investigation were compared with those of the refer ence compounds. The XANES spectra for five samples of the (Sr1­ xPrx)TiO3 solid solutions and reference compounds BaPrO3 and Pr2Ti2O7 are shown in Fig. 2. From a comparison of these spectra, it can be con cluded that, irrespective of the praseodymium con centration and the preparation conditions, the Pr ions are predominantly in the charge state 3+. The structural positions of the impurity ions were determined by analyzing the EXAFS spectra. The characteristic spectrum of the extended X ray absorp tion fine structure as a function of the photoelectron wave vector k for the (Sr0.85Pr0.15)TiO3 sample and its best theoretical approximation are shown in Fig. 3. The best agreement between the calculated and exper imental spectra is achieved in the model in which strontium atoms are replaced by praseodymium
1

The lattice parameters a and c in the tetragonal phase are so close to each other that, in our experiments, they could not be measured independently. Therefore, the lattice of our samples is considered to be pseudocubic. PHYSICS OF THE SOLID STATE Vol. 54 No. 5

atoms. The interatomic distances and Debye­Waller factors for the three nearest shells are presented in the table. A comparison of the obtained distances with the average interatomic distances calculated from the measured lattice parameter has demonstrated that strong relaxations around the impurity atom manifest themselves only in the first shell (R ­0.136 å). In the second shell, by contrast, there is a slight increase in the average interatomic distance (R +0.02 å). The accuracy in determining the parameters of the third shell is insufficient to draw any conclusions, but the distance to this shell is in agreement with the X ray diffraction data. The relatively low values of the Debye­Waller fac tors for the second shell, which correspond to typical values of the amplitude of thermal vibrations at 300 K in perovskites, allow us to exclude almost completely the possibility of manifesting the off centering of Pr atoms. The unexpectedly high Debye­Waller factor for the first shell in the sample with x = 0.15 is explained by the rotations of the octahedra revealed from the X ray measurements, when the lengths of twelve Pr­O bonds become different. In the sample with x = 0.05, the high value of the Debye­Waller fac tor can indicate a large amplitude of thermal rotations of the octahedra, because, according to [5], the tem perature of the structural phase transition at this praseodymium concentration is close to room tem perature. The possibility of incorporating Pr atoms into the B sites of the perovskite structure was verified by com paring the experimental EXAFS spectra with the cal culated spectra obtained in the model allowing for the simultaneous incorporation of the impurity atoms into the A and B sites. With an increase in the fraction of the

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2 i

Structural parameters obtained from the processing of the EXAFS spectra (Ri is the distance to the ith shell, and is the Debye­Waller factor for this shell) Sample (Sr0.95Pr0.05)TiO3, annealing at 1300°C Shell Pr­O Pr­Ti Pr­Sr (Sr0.95Pr0.05)TiO3, annealing at 1600°C Pr­O Pr­Ti Pr­Sr (Sr0.85Pr0.15)TiO3, annealing at 1400°C Pr­O Pr­Ti Pr­Sr (Sr0.95Pr0.05)TiO3, (a = 3.897 å) Sr­O Sr­Ti Sr­Sr Ri, å 2.629(9) 3.394(5) 3.887(34) 2.623(7) 3.388(4) 3.926(13) 2.608(15) 3.404(8) 3.930(38) 2.756 3.375 3.897 i , å2 0.017(1) 0.003(1) 0.022(5) 0.014(1) 0.003(1) 0.012(2) 0.020(2) 0.004(1) 0.018(5)
2

Because there are no data for the Pr3+ ion in the twelvefold coordination, we used the data obtained for the eightfold coordination: RSr = 1.26 å and RPr = 1.126 å [9]. The difference between the ionic radii of these ions is equal to 0.134 å, which coincides with the value found in our study for the relaxation (­0.136 å). At the same time, our distances to the second shell turned out to be even slightly overestimated. In our opinion, this can be explained by the fact that the charge of the Pr impurity ion exceeds the charge of the Sr ion, which enhances the repulsion between the Pr3+ and Ti4+ ions and increases the Pr­Ti distance. The fact that we did not reveal significant incorporation of praseodymium atoms into the B sites of the perovskite structure makes impossible the explanation of the constancy of the lattice parameter by partial substitu tion of the B sites [5]. In our opinion, the weak depen dence of the lattice parameter on the praseodymium concentration is explained by the repulsion between the Pr and Ti atoms. Although, in this paper, we concerned only the structural aspects of SrTiO3(Pr), nonetheless, we found that praseodymium is not an off center impu rity and does not cause a significant distortion of the TiO6 octahedra. The model of ferroelectricity in the system of frozen electric dipoles, in our opinion, is also untenable; therefore, none of the models listed in the Introduction can explain the appearance of the dielectric anomalies. The absence of an influence of the Pr impurity on the soft mode [4] suggests that the observed dielectric anomalies are not related to the bulk of the crystal. In our opinion, the possible cause of their occurrence can be the emergence of electrical conductivity due to hopping of electrons between Pr atoms, which can exist in two charge states (Pr3+ and Pr4+). Even at a very low concentration of Pr4+ centers, because of the large distances over which electrons are transferred, the changes in dipole moments can be large enough, thus leading to significant observable effects. ACKNOWLEDGMENTS The authors are grateful to V.F. Kozlovskii for his assistance in performing the X ray measurements. REFERENCES
1. V. V. Lemanov, Ferroelectrics 226, 133 (1999). 2. A. DurÀn, E. MartÌnez, J. A. DÌaz, and J. M. Siqueiros, J. Appl. Phys. 97 (10), 104109 (2005). 3. A. DurÀn, F. Morales, L. Fuentes, and J. M. Siqueiros, J. Phys.: Condens. Matter 20 (8), 085219 (2008).
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impurity atoms in the B sites, the agreement between the experimental and calculated curves becomes worse. This means that the probability of the incorpo ration of impurity atoms into the B sites is relatively low, which, most likely, is associated with a very strong difference between the ionic radii of Ti4+ (0.605 å) and Pr3+ (0.990 å) in the octahedral coordination [9]. In this work, it was established that the Pr3+ ions are located in the A sites of the perovskite structure and are on center; therefore, their charge state 3+ suggests the appearance of vacancies in order to ensure the electro neutrality. These vacancies can be both VSr and VO. The possibility of reducing titanium to the Ti3+ state can be excluded, because, otherwise, the samples would acquire a dark color. The investigation of the pre edge structure at the Ti K absorption edge for the (Sr0.95Pr0.05)TiO3 sample did not reveal an increase in the intensity of the transition 1s 3d (eg), which is forbidden in the dipole approximation [10]. This indi cates that oxygen vacancies in the octahedra are almost completely absent. On the other hand, if the electroneutrality would be provided by the formation of VSr, the excess of Sr atoms in the sample should lead to the formation of Ruddlesden­Popper phases at approximately the same intensity as in the case of Sr(Ti0.95Pr0.05)O3. The X ray diffraction patterns did not exhibit any indications of these phases; however, this can be associated with very small sizes of the cor responding precipitates. The observed decrease in the interatomic distance in the first shell is completely consistent with the dif ference between the ionic radii of Pr3+ and Sr2+.

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XAFS STUDIES OF THE LOCAL STRUCTURE AND CHARGE STATE 4. R. Ranjan, R. Hackl, A. Chandra, E. Schmidbauer, D. Trots, and H. Boysen, Phys. Rev. B: Condens. Mat ter 76 (22), 224109 (2007). 5. R. Ranjan, R. Garg, R. Hackl, A. Senyshyn, E. Schmid bauer, D. Trots, and H. Boysen, Phys. Rev. B: Condens. Matter 78 (9), 092102 (2008). 6. R. Garg, A. Senyshyn, H. Boysen, and R. Ranjan, Phys. Rev. B: Condens. Matter 79 (14), 144122 (2009). 7. A. I. Lebedev, I. A. Sluchinskaya, V. N. Demin, and I. Manro, Izv. Akad. Nauk, Ser. Fiz. 60 (10), 46 (1996).

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8. A. M. Glazer, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 31, 756 (1975). 9. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 32, 751 (1976); http://abulafia.mt.ic.ac.uk/shannon/ 10. A. I. Lebedev, I. A. Sluchinskaya, A. Erko, A. A. Velig zhanin, and A. A. Chernyshov, Phys. Solid State 51 (5), 991 (2009).

Translated by O. Borovik Romanova

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