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EXAFS Study of the Local Environment of Pb Impurity in CaTiO3, SrTiO3 and BaTiO3
A.A. Veligzhanin2, A.I. Lebedev1, V.V. Mischenko1, I.A. Sluchinskaya1 and A.A. Chernyshov2 1 Moscow State University, Moscow, 119992, Russia 2 RRC Kurchatov Institute, Moscow, 123182, Russia e-mail: swan@scon155.phys.msu.su Ferroelectric properties of perovskite solid solutions are studied extensively for many years. It was known that substitution of the A atoms in the perovskite ABO3 structure for Pb strongly affects the ferroelectric properties of crystals. For example, doping of BaTiO3 by Pb increases the phase transition temperature [1] and the doping of SrTiO3 [2] and CaTiO3 by Pb results in the appearance of ferroelectricity in these incipient ferroelectrics. To understand better the microscopic mechanisms of these effects, the local environment of Pb impurity atoms in CaTiO3, SrTiO3 and BaTiO3 was studied in this work by EXAFS technique. EXAFS measurements were made on the K1.3b station of synchrotron radiation source "Sibiria-2" at Kurchatov Institute. Spectra were collected at 300 K in fluorescence mode at the LIII-edge of Pb (13055 eV) and K-edge of Sr (16115 eV). The samples of Ba0.9Pb0.1TiO3, Sr0.8Pb0.2TiO3, Sr0.98Pb0.02TiO3 and Ca0.9Pb0.1TiO3 were prepared by thermal decomposition of coprecipitated oxalate complexes. The EXAFS spectra were analyzed in the conventional way [3]. To find the local distortion about the Pb atom, a model was used, in which the (100) displacement of the central Pb atom and Debye-Waller factors for atoms in 3­5 nearest shells were varied with taking into account the lattice parameter obtained from independent X-ray diffraction experiment. Strong distortion of the local environment of Pb impurity atoms was revealed by the data analysis. Debye-Waller factors for the first (Pb-O) shell in all three perovskites were much larger than this factor for the first Sr-O shell in SrTiO3. Large Debye-Waller factors resulted in disappearance of the contribution of the first shell to the EXAFS spectra already at k > 4 е-1. At the same time, the contribution from the second (Pb-Ti) shell was clearly visible in spectra up to 10 е-1. This means that doping by Pb results in strong random tilting of neighboring TiO6 octahedra without changing the Pb-Ti interatomic distance. Moreover, the data analysis revealed noticeable (0.1­0.15 е) off-center displacement of the Pb atoms from the A sites in SrTiO3 and BaTiO3 crystals, the magnitude of the displacement being much larger than the relative displacement of Ti and Ba atoms in tetragonal BaTiO3 (0.05 е [1]). Therefore, the doping of perovskite crystals by Pb results in two effects that can alter the phase transition temperature: 1) the appearance of dipole moments of off-center Pb atoms, and 2) the appearance of strong rotational disorder of TiO6 octahedra.
References 1. F. Jona, G. Shirane. Ferroelectric crystals. Pergamon Press, 1962. 2. V.V. Lemanov, E.P. Smirnova, E.A. Tarakanov. Fizika Tverdogo Tela. 39, 714 (1997). 3. A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin, I.H. Munro. Phys. Rev. B 55, 14770 (1997).