Rare hearth doped manganites with perovskite structure show peculiar magnetic and electronic transport properties, due to a balance of ferromagnetic double-exchange (DE) and antiferromagnetic superexchange (SE) interaction among Mn ions, and strong electron-phonon interaction (small and/or large polarons)(1). These features are largely determined by the local atomic structure surrounding Mn ions, characterized by Jahn-Teller (JT) distortions of MnO6 octahedra and deeply affected by temperature, composition, pressure and magnetic .elds. Therefore the sensitivity to the short range order of x-ray absorption spectroscopy (XAS) makes this technique largely favorite in studying these compounds. Despite recent scientific literature reports several accurate XAS based studies on these compounds(2), it mainly concerns the analysis of extended region of the XAS spectra, the so called EXAFS region. On the contrary the interpretation of the near edge region of the spectra (XANES) remains qualitative or semi-quantitative(3). Nevertheless structural information in the EXAFS and XANES are complementary. The former mainly probes the atomic radial distribution functions around the absorber, whereas the latter, as it results from the full multiple scattering (FMS) processes and from the details of electronic density of states near the Fermi level, directly probes the topology of local environment surrounding the absorber and its electronic nature. This work reports the .rst attempt to a quantitative analysis of Mn K edge XANES on CaMnO3 and LaMnO3 perovskite samples. The original MXAN program has been used, a recently implemented code(4) which allows ab-initio structural refinement of XANES data in FMS approach. The topology of Mn local atomic environment on CaMnO3 and LaMnO3 samples are reported as derived by the quantitative Mn- K edge XANES fitting. The 3D structures obtained by .tting the XANES data are compared with structural models derived by EXAFS and diffraction (x-ray and neutron). While Mn local structure in CaMnO3 is consistent with the shape and size of MnO6 octahedra obtained by diffraction experiments, in LaMnO3 sample the short and long range information appear quite different. In LaMnO3, in fact, the MnO6 JT distortion determined by XANES is about 15 % reduced with respect to the one determinedby diffraction experiments, confirming and strengthening previous EXAFS results (5).
Monesi, C., Bardelli, F., Meneghini, C., Mobilio, S., Benfatto, M., Manju, U., et al. (2004). Manganese oxide perovskites: quantitative structural refinement of Mn-K- edge XANES data.
Manganese oxide perovskites: quantitative structural refinement of Mn-K- edge XANES data
MONESI, CLAUDIA;MENEGHINI, CARLO;MOBILIO, Settimio;
2004-01-01
Abstract
Rare hearth doped manganites with perovskite structure show peculiar magnetic and electronic transport properties, due to a balance of ferromagnetic double-exchange (DE) and antiferromagnetic superexchange (SE) interaction among Mn ions, and strong electron-phonon interaction (small and/or large polarons)(1). These features are largely determined by the local atomic structure surrounding Mn ions, characterized by Jahn-Teller (JT) distortions of MnO6 octahedra and deeply affected by temperature, composition, pressure and magnetic .elds. Therefore the sensitivity to the short range order of x-ray absorption spectroscopy (XAS) makes this technique largely favorite in studying these compounds. Despite recent scientific literature reports several accurate XAS based studies on these compounds(2), it mainly concerns the analysis of extended region of the XAS spectra, the so called EXAFS region. On the contrary the interpretation of the near edge region of the spectra (XANES) remains qualitative or semi-quantitative(3). Nevertheless structural information in the EXAFS and XANES are complementary. The former mainly probes the atomic radial distribution functions around the absorber, whereas the latter, as it results from the full multiple scattering (FMS) processes and from the details of electronic density of states near the Fermi level, directly probes the topology of local environment surrounding the absorber and its electronic nature. This work reports the .rst attempt to a quantitative analysis of Mn K edge XANES on CaMnO3 and LaMnO3 perovskite samples. The original MXAN program has been used, a recently implemented code(4) which allows ab-initio structural refinement of XANES data in FMS approach. The topology of Mn local atomic environment on CaMnO3 and LaMnO3 samples are reported as derived by the quantitative Mn- K edge XANES fitting. The 3D structures obtained by .tting the XANES data are compared with structural models derived by EXAFS and diffraction (x-ray and neutron). While Mn local structure in CaMnO3 is consistent with the shape and size of MnO6 octahedra obtained by diffraction experiments, in LaMnO3 sample the short and long range information appear quite different. In LaMnO3, in fact, the MnO6 JT distortion determined by XANES is about 15 % reduced with respect to the one determinedby diffraction experiments, confirming and strengthening previous EXAFS results (5).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
