Measurement of the local Jahn-Teller distortion in LaMnO_3.006

The atomic pair distribution function (PDF) of stoichiometric LaMnO_3 has been measured. This has been fit with a structural model to extract the local Jahn-Teller distortion for an ideal Mn(3+)O_6 octahedron. These results are compared to Rietveld refinements of the same data which give the average structure. Since the local structure is being measured in the PDF there is no assumption of long-range orbital order and the real, local, Jahn-Teller distortion is measured directly. We find good agreement both with published crystallographic results and our own Rietveld refinements suggesting that in an accurately stoichiometric material there is long range orbital order as expected. The local Jahn-Teller distortion has 2 short, 2 medium and 2 long bonds.Comment: 5 pages, 3 postscript figures, minor change

Estimates of electronic interaction parameters for LaMMO3_3 compounds (MM=Ti-Ni) from ab-initio approaches

We have analyzed the ab-initio local density approximation band structure calculations for the family of perovskite oxides, La$M$O$_3$ with $M$=Ti-Ni within a parametrized nearest neighbor tight-binding model and extracted various interaction strengths. We study the systematics in these interaction parameters across the transition metal series and discuss the relevance of these in a many-body description of these oxides. The results obtained here compare well with estimates of these parameters obtained via analysis of electron spectroscopic results in conjunction with the Anderson impurity model. The dependence of the hopping interaction strength, t, is found to be approximately $r^{-3}$.Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct 15,1996

COVALENCY EFFECTS IN MAGNETIC INTERACTIONS

On présente une revue des interactions magnétiques dans le but de dégager les informations sur la covalence qui peuvent en être déduites. Les interactions hyperfines sur des ligands, les champs hyperfins dans les métaux mesurés par spectrométrie Mössbauer et diffraction neutronique sont présentées en détail, en insistant sur la dernière approche qui est la plus productive en ce domaine. Les informations déjà recueillies sont confrontées aux connaissances en chimie et on essaie de dégager les axes intéressants pour de futures études.Magnetic interactions, categorised into ligand based, metal based and neutron diffraction, are reviewed with regard to the information they give on covalency. The ligand hyperfine interaction, the metal hyperfine field measured by the Mössbauer effect and neutron diffraction measurements are mentioned in some detail, with particular emphasis on the latter as being the most productive experimental technique in this field. The information already gathered is assessed with reference to chemical knowledge, and useful avenues for future investigation are suggested