We use a combination of x-ray diffraction, total scattering, and quantum mechanical calculations to determine the mechanism responsible for hydration-driven contraction in ${\mathrm{ZrW}}_2{\mathrm{O}}_8$. The inclusion of ${\mathrm{H}}_2\mathrm{O}$ molecules within the ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ network drives the concerted formation of new $\mathrm{W}─\mathrm{O}$ bonds to give one-dimensional $(─\mathrm{W}─\mathrm{O}─{)}_n$ strings. The topology of the ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ 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, ${\mathrm{ZrW}}_2{\mathrm{O}}_8·{\mathrm{H}}_2\mathrm{O}$ is heavily disordered, with each configuration of strings forming a dense aperiodic “spaghetti.” This new connectivity contracts the unit cell via large shifts in the Zr and W atom positions. Fluctuations of the undistorted parent structure towards this spaghetti phase emerge as the key negative thermal expansion (NTE) phonon modes in ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ itself. The large relative density of NTE phonon modes in ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ actually reflects the degeneracy of volume-contracting spaghetti excitations, itself a function of the particular topology of this remarkable material.

• Received 13 April 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.265501