| The objective of this study is to improve the quantitative understanding of structural phase transitions in perovskite systems. In this study, the temperature-dependent X-ray Absorption Fine Structure (XAFS) technique was employed because it could provide direct evidence of local structural changes in the materials as a function of temperature. A new analysis technique, the {dollar}sigmasp2{dollar}-derivative method, was developed to examine the singularities occurring in the Debye-Waller factors (DWFs) obtained from the XAFS spectra. This technique can provide information about phase transition temperatures and the nature of the phase transitions. Another analysis technique, the single-shell approximation, was also developed to quantitatively analyze the structural DWFs which were extracted from the experimental XAFS data by an inverse Fourier filtering, a least-squares fitting, and application of the Einstein model approximation. Both analysis techniques were tested by computer simulation.; The results not only verify the existence of phase transitions at 100K and 180K in PbTiO{dollar}sb3{dollar} and at 110K in SrTiO{dollar}sb3{dollar}, but also confirm the presence of the controversial transition at 65K in SrTiO{dollar}sb3{dollar} plus suggest a previously undiscovered transition at 225K. At 110K, SrTiO{dollar}sb3{dollar} undergoes a structural phase transition which is related to the rotation of the neighboring TiO{dollar}sb6{dollar} octahedra about the Ti atoms. The maximum rotation angle calculated from the experimental data is about 1.0{dollar}spcirc{dollar} which is in good agreement with previous results. At 225K, SrTiO{dollar}sb3{dollar} appears to undergo a new structural phase transition which is related to the displacement of the Ti atoms toward (100) directions. The directions of the Ti atom shifts are random and the magnitude of each shift, calculated from the experimental data, is about 0.03A. |
