Local and average crystal structure and displacements of (LaB6)-B-11 and EuB6 as a function of temperature - art. no. 224302

Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction data and the local structure from La L-III-e dge x-ray-absorption fine structure (XAFS) are presented for a (LaB6)-B-11 sample as a function of temperature (similar to 10-320 K). These data are c ompared to XAFS results on a EuB6 sample. The single-site La and B position al distribution widths and the La-B and La-La bond length distribution widt hs and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublat tices are only slightly correlated with each other. Moreover, while the tem perature dependence of the displacement parameters of the average sites obt ained from diffraction fit an Einstein model well, the temperature dependen ce of the La-B bond length distribution width requires at least two vibrati onal frequencies, corresponding to the La and B frequencies of the individu al sites. XAFS data on EuB6 indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above th e ferromagnetic transition temperature for EuB6 place stringent limits on t he lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement i n the magnetoresistance transition is in sharp contrast to the strong latti ce involvement observed in the colossal magnetoresistance lanthanum mangane se perovskites.