Local Vibrations and Negative Thermal Expansion in ZrW sub(2)O sub(8)

We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ZrW sub(2)O sub(8) (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 ZrO sub(6) octahedra that produce large [left angle bracket]111[right angle bracket] translations of the WO sub(4) tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage.