5. A simplified cartoon model of the vibrations of the W and Zr atoms in ZrW2O8 inferred from X-ray Absorption Fine Structure (XAFS) measurements. The XAFS results show that the fluctuations of the W-Zr distance is very small - the W-Zr linkage is quite stiff. Consequently as the W atom vibrates vertically to the plane formed by the three Zr atoms (in a (1,1,1) plane) the Zr atoms must move together to keep the W-Zr distance approximately constant. This produces a negative thermal contraction of the lattice (the Zr-Zr distance rZr-Zr which decreases, is directly related to the cubic lattice constant a; i.e. rZr-Zr = a ⁄ √2).