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Local Vibrations and Negative Thermal Expansion in ZrW 2 O 8

2014; American Physical Society; Volume: 112; Issue: 4 Linguagem: Inglês

10.1103/physrevlett.112.045505

1092-0145

F. Bridges, Trevor Keiber, Pavol Juhás, Simon J. L. Billinge, Logan Sutton, John M. Wilde, Glen R. Kowach,

Ferroelectric and Piezoelectric Materials

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.