We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ZrW2O8 (10-500 K) with a focus on the stiffness of the Zr-O-W linkage. The XPDF is highly sensitive to W-Zr and W-W correlations, but much less so to O-O or W-O correlations. The Zr-W peak in the XPDF data has a weak temperature dependence and, hence, this linkage is relatively stiff and does not permit bending of the Zr-O-W link. We propose that the low energy vibrational modes that lead to negative thermal expansion involve correlated rotations of ZrO6 octahedra that produce large <111> translations of the WO4 tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage.

We studied the temperature dependence of the magnetic properties of VO/Ni bilayers. The Ni films were deposited on either monoclinic or rutile phase VO. The temperature induced VO transformation from a monoclinic to a rutile structure induces strain in the Ni film. Due to an inverse magnetoelastic effect the coercivity of the Ni films is strongly modified. Both Ni films show strong enhancement of the coercivity near the transition temperature. The coercivity enhancement of Ni is associated with the phase coexistence observed in the VO first order phase transition. Above the transition temperature, Ni deposited on monoclinic VO shows a coercivity enhancement whereas Ni deposited on rutile VO shows suppression of the coercivity. The samples were cycled several times to check if the changes in coercivity were reversible. While samples with Ni deposited on rutile VO show reversibility, samples with Ni deposited on monoclinic VO shown an irreversibility after the first structural phase transition. This irreversibility can be associated with cracking of the VO layer as it relieves stress due to the transition and has implications for the resistance versus temperature behavior of the VO.

The local structure of two skutterudite families -CeM4As12 (M = Fe, Ru, Os) and LnCu3Ru4O12 (Ln = La, Pr, and Nd) -have been studied using the Extended X-Ray Absorption Fine Structure (EXAFS) technique with a focus on the lattice vibrations about the rare earth "rattler atoms", and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair-distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the M and As atoms in CeM4As12 and by Cu, O, and Ru atoms in LnCu3Ru4O12. Although some of the bonds within the cage are quite stiff (Correlated Debye temperatures, θcD, are ∼ 500 K for CeM4As12 and above 800 K for LnCu3Ru4O12) we show the structure is not completely rigid. For the rattler atom the nearest neighbor pairs have a relatively low Einstein temperature, θE; ∼ 100-120 K for Ce-As and ∼ 130 K for Ln-O. Surprisingly, the behavior of the second neighbor pairs are quite different; for CeM4As12 the second neighbor pairs (Ce-M have a weaker bond while for LnCu3Ru4O12 the Ln-Ru second neighbor pair has a stiffer effective spring constant than the first neighbor pair. In addition, we show that the As4 or CuO4 rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as "rattlers" and help suppress thermal conductivity. In general neither the rigid cage approximation nor the simple reduced mass approximation are sufficient for describing rattler behavior.

We studied the temperature dependence of the magnetic properties of VO2/Ni bilayers deposited on three different substrates. The temperature induced VO2 transformation from a monoclinic to a rutile structure induces strain in the Ni film. Due to an inverse magnetostrictive effect, the coercivity of the Ni films is strongly modified. The morphology of the films is influenced by the substrate choice and has a strong impact on the magnetic properties. Ni films grown on top of rutile VO2 show a reversible change in the coercivity and a strong enhancement of the coercivity near the transition temperature. The coercivity enhancement of Ni is associated with the phase coexistence observed in the VO2 first order phase transition.

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