The phase-derivative method in EXAFS applied to the study of rare earth nickel perovskites

Rare earth nickel perovskites (RNiO3) undergo a first-order metal to insulator transition as temperature decreases. They present also an unusual antiferromagnetic order in the insulating low temperature state. These transport and magnetic properties are strongly linked to the structure. Modifications in the macroscopic properties results from very subtle changes in the atomic arrangement and electronic states. We have used EXAFS spectroscopy to study the complex interplay among short-range structure and the transport and magnetic properties. The Ni K-edge EXAFS spectra of rare earth nickel perovskites (RNiO3) were measured at several temperatures. A splitting into two bond lengths in the Ni coordination shell, corresponding to two different Ni sites, has been observed in the insulating and metallic states of PrNiO3, NdNiO3 and EuNiO3. The existence of two Ni sites in these light rare earth compounds provides a common ground to understand the behavior of the whole family of nickelates. The splitting has been analyzed by the phase-derivative method, since standard fitting procedures were not able to produce reliable results. The phase derivative is a fit-independent method that can be applied to obtain quantitative structural parameters in EXAFS spectroscopy. It has proven to be a reliable and useful tool to be applied in cases where standard fitting procedures lead to unstable and unreliable parameters.