Correlated atomic motions in the negative thermal expansion material ZrW 2 O 8 : A local structure study

Recent studies of zirconium tungstate, ZrW 2 O 8 , show an isotropic negative thermal expansion (NTE) over a wide temperature range. It has been proposed that the low-energy phonon vibrational modes observed in both specific heat and phonon density-of-states measurements, are responsible for this unusual NTE. We have carried out x-ray-absorption tine-structure (XAFS) experiments at both the W L 1 1 1 edge and Zr K edge to study the detailed local structure in ZrW 2 O 8 . Our XAFS results show a very small temperature dependence of the broadening parameter, σ, for the W-Zr atom pair and the W-O-Zr linkage; consequently, the displacements of the W, O, and Zr atoms must he correlated. The data show a much larger temperature dependence of σ for the nearest W 1 -W 2 pair as well as for the nearest Zr-Zr pair. These combined results indicate that it is the correlated motion of a WO 4 tetrahedron and its three nearest ZrO 6 octahedra that leads to the NTE effect in this material instead of primarily transverse vibrations of the middle O atom in the W-O-Zr linkage. The data for both W-W and Zr-Zr atom pairs also indicate a hardening of the effective spring constant near 100 K, which is consistent with the shift of the lowest mode with T in the phonon density of states. A simple model is developed to explain the NTE in terms of the local structure results; it also provides a natural explanation for the lack of a soft-mode phase transition.