Local and average crystal structure and displacements of La11B6 and EuB6 as a function of temperature

Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction data and the local structure from La LIII-edge x-ray-absorption fine structure (XAFS) are presented for a La11B6 sample as a function of temperature (∼10 - 320K). These data are compared to XAFS results on a EuB6 sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites obtained from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual 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 the ferromagnetic transition temperature for EuB6 place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites