Negative Hydration Expansion in ZrW2O8: Microscopic Mechanism, Spaghetti Dynamics, and Negative Thermal Expansion

Mia Baise; Phillip M. Maffettone; Fabien Trousselet; Nicholas P. Funnell; François-Xavier Coudert; Andrew L. Goodwin

doi:10.1103/PhysRevLett.120.265501

paper

Negative Hydration Expansion in ZrW2O8: Microscopic Mechanism, Spaghetti Dynamics, and Negative Thermal Expansion

arXiv:1804.04906 · doi:10.1103/PhysRevLett.120.265501

Abstract

We use a combination of X-ray diffraction, total scattering and quantum mechanical calculations to determine the mechanism responsible for hydration-driven contraction in ZrW $_{2}$ O $_{8}$ . Inclusion of H $_{2}$ O molecules within the ZrW $_{2}$ O $_{8}$ network drives the concerted formation of new W--O bonds to give one-dimensional (--W--O--) $_{n}$ strings. The topology of the ZrW $_{2}$ O $_{8}$ network is such that there is no unique choice for the string trajectories: the same local changes in coordination can propagate with a large number of different periodicities. Consequently, ZrW $_{2}$ O $_{8}$ is heavily disordered, with each configuration of strings forming a dense aperiodic `spaghetti'. This new connectivity contracts the unit cell \emph{via} large shifts in the Zr and W atom positions. Fluctuations of the undistorted parent structure towards this spaghetti phase emerge as the key NTE phonon modes in ZrW $_{2}$ O $_{8}$ itself. The large relative density of NTE phonon modes in ZrW $_{2}$ O $_{8}$ actually reflect the degeneracy of volume-contracting spaghetti excitations, itself a function of the particular topology of this remarkable material.

5 pages, 4 figures