SIMULATION STUDY OF OFF-CENTER SHIFTS IN PEROVSKITE SYSTEMS

An orientational order-disorder phase transition in perovskites has be en studied by simulation using the FEFF-5 ab initio program. Off-cente r shifts of the central atoms toward the [100] directions in perovskit e systems have been modeled for three different phases: (i) totally di sordered, (ii) partially ordered, and (iii) totally ordered structures . XAFS spectra have been simulated as a function of temperature from t hese modeled structures. The relative Debye-Waller factors extracted f rom the simulated XAFS spectra have been quantitatively analyzed using the Einstein model and a single-shell approximation.