Physica Scripta. Vol. T115, 359–361, 2005 XAFS Study of Local Structure Change in Perovskite Titanates K. Sato 1 , T. Miyanaga 1 , S. Ikeda 1 and D. Diop 2 1 Department of Materials Science and Technology, Faculty of Science and Technology, Hirosaki University,Aomori 036-8561, Japan 2 Department of Physics, Faculty of Science and Technique, UniversityAnta Cheikh Diop of Dakar, Dakar, Senegal Received June 26, 2003; accepted in revised form June 23, 2004 pacs numbers: 02.40.−k, 12.39.Dc, 14.80.Hv Abstract The Ti K-edge EXAFS for PbTiO 3 measured in the temperature range including the critical temperature (763 K) due to the phase transition from tetragonal to cubic phase is discussed from the local point of view. Three kinds of atomic pairs, shorter Ti-O(1), medium Ti-O(2) and longer Ti-O(3) were analyzed independently. The interatomic distance of Ti-O(3) changes at T c discontinuously and shows a “displacive” type transition. On the other hand, EXAFS shows that the lattice distortion remains even in cubic phase, which indicates “order-disorder” type transition. The peak intensity at the pre-edge region in XANES, which indicates the distortion of Ti atoms along the c-axis of the oxygen octahedron, corresponds to the distortion obtained from EXAFS analysis. 1. Introduction X-ray absorption fine structure (XAFS) has become a technique to study local structures as a complementary technique to diffraction methods which are based on long-range order [1]. PbTiO 3 has long been considered as “displacive” type ferroelectrics: The perovskite PbTiO 3 shows tetragonal structure at T<T c (=763 K) (lattice parameters a = b = 3.89 Å and c = 4.15 Å at 300 K [2]) and cubic structure at T>T c (a = b = c = 3.905 Å at 773 K). On the other hand, if the local distortion does not change with temperature across T c the transition is “order-disorder” type. In this case, only the orientations of the distortion within a small domain change with temperature; the orientation is arranged along the preferable direction below T c , but it is disordered above T c . The local structural change of PbTiO 3 has been extensively studied by Sicron et al. using XAFS of both Pb L III and Ti K- edges [3, 4]. These results show that an essential element of “order-disorder” is present even in this normally pure ferroelectric crystal, which displays a soft mode and a dielectric constant of typical “displacive” type ferroelectrics. Recently, a more detailed study by Pb L III edge EXAFS for PbTiO 3 has been carried out [5] where three kinds of atomic pairs Pb-O(1,2,3) are analyzed independently. Shorter atomic pairs of Pb-O(1,2) show “order- disorder” type behavior, on the other hand, the longest Pb-O(3) shows “soft mode” behavior [5]. In this paper, we study the local structural change around Ti atoms in PbTiO 3 crystal through T c in more detail. Actually, there exist three types of Ti-O pairs by tetragonal distortion; longer and shorter interatomic distances along the c-axis and a medium Ti-O pairs perpendicular to the c-axis. Our results show that the local distortion remains above T c in which PbTiO 3 shows cubic phase. 2. Experimental and Data Analyses The powder sample of PbTiO 3 (99.99%) was purchased from Soeakwa Co. Ltd. and was checked by X-ray powder diffraction to assure its crystal structure. The powder was mixed with BN and was dry pressed for producing a pellet that could be easily handled and placed within the furnace. X-ray absorption spectra of Ti K- edge (4.95 keV) were measured at BL9A of Photon Factory (KEK) in transmission mode. A Si(111) double crystal monochrometer was used. The temperature range for measurement was from 300 K to 923 K. The EXAFS analysis was performed according to XANADU code [6] and it is given only briefly here. The normalized EXAFS interference function, (k), was multiplied by a Hamming window function to reduce the ripples in the Fourier-transformed spectra in r-space. The k-range for the Fourier transform was from 2.95 to 8.8 Å −1 . 3. Results and Discussion We discuss here the result for Ti K-edge XAFS. Figure 1 shows the EXAFS oscillation functions, k 2 (k), of Ti K EXAFS for PbTiO 3 crystal at various temperatures obtained by transmission measurement. Although the signal in the transmitted ionization chamber is not so large due to the absorption by Pb atoms, satisfactory data quality has been obtained up to 8.5 Å −1 for EXAFS analyses even at high temperatures. As the temperature increases the amplitude of the EXAFS function decreases gradually. Figure 2 shows the Fourier transforms of Ti K EXAFS for PbTiO 3 crystals at various temperatures as shown in Fig. 1. The peak intensity decreases as temperature increases and change of the fine structure is observed. The atoms are distorted along the c-axis at tetragonal phase: There are three kinds of Ti-O atomic pairs; shorter Ti-O(1), medium Ti-O(2) and longer Ti-O(3) as shown in Fig. 3. The peaks from 1 to 2 Å include three Ti-O pairs. In the case of Ti K-edge we tried to perform 3 shell-fitting in order to obtain the structural parameters and reasonable results were obtained. Our unknown parameters for the fitting are r and  2 for each shell and the total number of them is 6. Fig. 1. EXAFS oscillation functions, k 2 (k), of Ti K-edge EXAFS for PbTiO 3 crystal at various temperatures. C  Physica Scripta 2005 Physica Scripta T115