Local Vibrations and Negative Thermal Expansion inZrW2O8

Abstract

We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ (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 ${\mathrm{ZrO}}_6$ octahedra that produce large $⟨111⟩$ translations of the ${\mathrm{WO}}_4$ tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage. DOI: http://dx.doi.org/10.1103/PhysRevLett.112.045505 © 2014 Published by the American Physical Society