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ISSN 1062 8738, Bulletin of the Russian Academy of Sciences: Physics, 2010, Vol. 74, No. 9, pp. 1235­1237. © Allerton Press, Inc., 2010. Original Russian Text © I.A. Sluchinskaya, A.I. Lebedev, A. Erko, 2010, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2010, Vol. 74, No. 9, pp. 1289­ 1291.

Local Environment and Oxidation State of a Mn Impurity in SrTiO3 Determined from XAFS Data
I. A. Sluchinskayaa, A. I. Lebedeva, and A. Erko
a

b

Physics Department, Moscow State University, Moscow, 119991 Russia b BESSY II, Berlin, 12489 Germany e mail: irinasluch@nm.ru

Abstract--The local environment and oxidation state of a Mn impurity in strontium titanate doped with 3% Mn were studied by X ray absorption fine structure spectroscopy. The effect of the synthesis conditions on the incorporation of the impurity into A and B sites was studied. It was established that Mn ions substituting for Ti are in the Mn4+ oxidation state and are on center. Mn ions substituting for Sr are in the Mn2+ oxidation state and off center, and are displaced from the lattice site by 0.32 å. It is shown that studies of the X ray absorption near the edge structure can be used to determine the ratio of Mn atoms incorporated into A and B sites in the lattice. DOI: 10.3103/S1062873810090145

INTRODUCTION It was long considered that upon the doping of SrTiO3 with manganese, the impurity atoms substitute for the Ti atoms and are in the Mn4+ oxidation state [1]. Annealing of samples in a reduction atmosphere can move the Mn ions to a lower valence state [2]. Recently, Lemanov et al. [3, 4] found strong dielectric relaxation in SrTiO3(Mn) at < 77 K. They explained the observed phenomena as reorientation of the dipoles of the "polaron" defects and dipole moments of off center Mn 4+ ions; however, they did not relate Ti them to the appearance of ferroelectricity in the crys tal. Tkach et al. [5] later found the conditions for impurity Mn atoms to substitute for Sr atoms in the Mn2+ oxidation state. Unusual dielectric phenomena were observed in all such samples. In samples where the Mn atoms were found in Ti sites, these effects were absent. To explain the dielectric properties of samples doped with Mn, the authors assumed that Mn2+ ions are off center when substituting for the Sr sites and the observed relaxation phenomena are due to the slowing down of the jumps between potential minima upon a decrease in temperature [6]. This assumption was con firmed by analyzing the EPR spectrum in [7]. This aim of this work was the X ray absorption fine structure (XAFS) determination of the structural posi tion and oxidation state of impurity Mn atoms in SrTiO3 samples prepared in different ways. EXPERIMENTAL The studied samples had nominal composition (Sr0.97Mn0.03)TiO3 and Sr(Ti0.97Mn0.03)O3, and were pre pared by solid phase synthesis. The initial compo

nents were SrCO3, nanocrystalline TiO2 obtained by hydrolysis of tetrapropylortotitanate and dried at 500°C, and Mn(CH3COO)2 · 4H2O. The components were weighed in the required proportions, ground in an acetone layer until the mixture was completely dry, and annealed in air at 1100°C for 8 h. The obtained powders were once again ground and annealed under the same conditions. Some of the samples were addi tionally annealed in air at 1350°C for 2.5 h and at 1500°C for 1 h. It should be noted that the use of the soluble manganese salt and its impregnation in the mixture of SrCO3 and mamocrystalline TiO2 led to the partial interaction of the salt with SrCO3 and the adsorption of the manganese and strontium acetates on the developed surface of the TiO2 nanoparticles. In the end, this ensured the uniform distribution of the impurity in the obtained samples. Extended X ray absorption fine structure (EXAFS) and X ray absorption near edge structure (XANES) spectra were recorded by X ray fluorescence on the KMC 2 station of the BESSY synchrotron radiation source in the vicinity of the K edge of Mn absorption (6.539 keV) at 300 K. EXAFS spectra were processed conventionally. The experimental methods are described in more detail in [8]. EXAFS spectra are shown in Fig. 1 for two samples: Sr(Ti0.97Mn0.03)O3 annealed at 1100°C, and (Sr0.97Mn0.03)TiO3 annealed at 1500°C. An analysis of the EXAFS spectra for the Sr(Ti0.97Mn0.03)O3 sample shows that the spectra correspond completely to the model in which the Mn atoms substitute for the Ti atoms and are on center (see table). For the (Sr0.97Mn0.03)TiO3 sample, good agreement of the experimental and calculated EXAFS spectra (Fig. 1b)

1235


1236 k(k) 0.4 0.2 0 -0.2 -0.4 2 k(k) 0.10 0.05 0 -0.05 -0.10 2 4 6 8 10 (b) 4 6 8 10 k , å- 1 (a)

SLUCHINSKAYA et al. XANES signal 1.6 1.2 0.8 0.4 0 3 6 1 2 7 5 4 6560 6580 6600 Energy, eV

6540

Fig. 2. XANES spectra of the SrTiO3(Mn) samples and reference compounds of two and four valence manga nese: (1) (Sr0.97Mn0.03)TiO3 sample annealed at 1500°C; (2) (Sr0.97Mn0.03)TiO3 sample annealed at 1350°C; (3) (Sr0.97Mn0.03)TiO3 sample annealed at 1100°C; (4) Sr(Ti0.97Mn0.03)O3 sample annealed at 1100°C; (5) Sr(Ti0.97Mn0.03)O3 sample annealed at 1500°C; (6) Mn(CH3COO)2 · 4H2O, (7) MnO2.

12 k, å-1

Fig. 1. EXAFS spectra of two SrTiO3(Mn) samples recorded at the K edge of Mn absorption: (a) Sr(Ti0.97Mn0.03)O3 sample annealed at 1100°C; (b) (Sr0.97Mn0.03)TiO3 sample annealed at 1500°C. The points represent the experimental data; the lines represent their best theoretical approximation.

is obtained only in the model with the off center shift of the Mn atom from the Sr site. This is revealed as two Mn­Ti distances (3.095 and 3.467 å, see table). The shift of the Mn atom from the A site was estimated from the Mn­Ti distances (~0.32 å). Our data on the on center position of the Mn4+ ion upon its substitut ing for titanium and the off center position of the Mn2+ ion upon its substituting for strontium agree with the results from the theoretical calculations of Kvyatkovskii [9].

The XANES spectra for five SrTiO3(Mn) samples and two reference samples (Mn(CH3COO)2 · 4H2O and MnO2) are shown in Fig. 2. A comparison of them shows that the edges of absorption in the spectra of the (Sr0.97Mn0.03)TiO3 sample annealed at 1500°C (curve 1) and Sr(Ti0.97Mn0.03)O3 sample annealed at 1100°C (curve 4) are displaced with respect to one another by about ~7 eV. It is conventionally thought that the oxi dation state of an atom in the crystal can be deter mined from the position of the steepest region on the edge of absorption of the atom in the XANES spectra, and the shift of the edge of the absorption therefore directly indicates that in the above two samples the Mn atoms, being located (according to the EXAFS data) in two different sites in the lattice, are in two different oxidation states. By comparing our spectra with those of the reference samples, we may conclude that the Mn ions in SrTiO3 located in the site are in oxidation state +2, and the Mn ions located in the site are in oxidation state +4. The difference in the form of the

Structural parameters obtained from processing EXAFS spectra of the two studied samples (Ri is the distance to the ith co ordination sphere, and i2 is the sphere's Debye Waller coefficient) Sample Sr(Ti0.97Mn0.03)O3, annealing 1100°C (Sr0.97Mn0.03)TiO3, annealing 1500°C Coordination sphere 1 2 3 1 2 3 Ri, å 1.914 3.328 3.895* 2.32; 2.86 3.095; 3.467 3.84
2 i , å

2

Atom O Sr Ti O Ti Sr

0.0012 0.0015 0.0061* 0.040; 0.021 0.007; 0.008 0.014

* The given value is for the channel of single scattering with account of the focal effects (two and threefold scattering). BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS Vol. 74 No. 9 2010


LOCAL ENVIRONMENT AND OXIDATION STATE

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edge of the absorption in the studied samples, in which the Mn ions are in oxidation state +4, and that of the MnO2 sample is due to the position and the form of the edge of the absorption in XANES spectra being deter mined not only by the oxidation state of the atom, but also by the band and crystalline structure of the studied object. At room temperature, MnO2 is a metal with rutile structure, while the studied samples are dielec trics with a cubic lattice of the perovskite type. The weak near edge structure observed in all of the samples is due to forbidden transitions from the 1s level of the Mn atom to the conductivity band (i.e., it is also deter mined by the band structure). From a comparison of the XANES spectra for the samples of nominal composition (Sr0.97Mn0.03)TiO3 annealed at 1350°C (curve 2 in Fig. 2) and 1100°C (curve 3 in Fig. 2) with the spectra of samples for which the EXAFS analysis was performed (curves 1 and 4 in Fig. 2), it follows that spectra 2 and 3 are a superposition of spectra 1 and 4. This means that in these samples, Mn is in both lattice sites and in both oxidation states. It follows from these data that in the sample with a deliberate shift away from Sr/Ti stoichi ometry to the deficiency in Sr (the (Sr0.97Mn0.03)TiO3) type sample), an increase in the temperature of the final annealing leads to a systematic increase in the concentration of manganese in oxidation state +2 in the A sites. In the sample annealed at 1100°C (curve 3 in Fig. 2), a large number of the Mn atoms are in the sites, despite the composition's deliberate shift away from stoichiometry in order to introduce the admix ture in the A site. Thermal treatment of sample 3 at 1500°C transfers the impurity Mn atoms from the Ti sites to the Sr sites, in which their oxidation state is +2. As for the samples with the deliberate shift from sto ichiometry to the deficiency in Ti (the Sr(Ti0.97Mn0.03)O3)­type samples), an increase in the temperature of the final annealing also leads to increase in the concentration of manganese in oxida tion state +4 located in the B site (curves 4 and 5). CONCLUSIONS Our studies of the XANES and EXAFS spectra in SrTiO3(Mn) show that, depending on the preparation

conditions, Mn atoms can be incorporated into the and sites of the perovskite lattice and remain there in different oxidation states. The redistribution of the impurity atoms between two sites can be controlled by changing the annealing temperature and creating a deliberate shift away from stoichiometry in the com position of the crystal. It was established that the impurity Mn atoms substituting for Ti atoms occupy the on center position in the lattice and are in oxida tion state +4, while the Mn atoms substituting for Sr atoms are off center, displaced from the lattice sites by a distance of about ~0.32 å, and in oxidation state +2. Our direct data confirm the assumption of the authors [5, 6] about the possibility of incorporating impurity manganese into the A sites. It has been shown that XANES studies of the edge structure can be used to determine the ratio of the manganese atoms occupy ing different crystallographic positions in the lattice. REFERENCES
1. MÝller, K.A., Phys. Rev. Lett., 1959, vol. 2, no. 8, p. 341. 2. Serway, R.A., Berlinger, W., MÝller, K.A., and Collins, R.W., Phys. Rev. B, 1977, vol. 16, no. 11, p. 4761. 3. Lemanov, V.V., Smirnova, E.P., Sotnikov, A.V., and Weihnacht, M., Fiz. Tverd. Tela, 2004, vol. 46, issue 8, p. 1402. 4. Lemanov, V.V., Sotnikov, A.V., Smirnova, E.P., and Weihnacht, M., J. Appl. Phys., 2005, vol. 98, no. 5, p. 056 102. 5. Tkach, A., Vilarinho, P.M., and Kholkin, A.L., Appl. Phys. Lett., 2005, vol. 86, no. 17, p. 172 902. 6. Tkach, A., Vilarinho, P.M., Kholkin, A.L., et al., Phys. Rev. B, 2006, vol. 73, no. 10, p. 104 113. 7. Laguta, V.V., Kondakova, I.V., Bykov, I.P., et al., Phys. Rev. B, 2007, vol. 6, no. 5, p. 054 104. 8. Lebedev, A.I., Sluchinskaya, I.A., Erko, A., and Kozlovskii, V.F., Pis'ma Zh. Eksperim. Teoret. Fiz., 2009, vol. 89, issue 9, p. 545. 9. Kvyatkovskii, O.E., Fiz. Tverd. Tela, 2009, vol. 51, issue 5, p. 932.

BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS

Vol. 74

No. 9

2010