Negative-thermal-expansion ZrW$_{2}$O$_{8}$. Elasticity and pressure

The elasticity of the negative thermal expansion (NTE) compound ZrW$_{2}$O$_{8}$ is rather strange: the solid softens as its volume decreases on warming. Does ZrW$_{2}$O$_{8}$ also soften when pressure alone is applied? Using pulse-echo ultrasound in a large-volume moissanite anvil cell, we find an unusual decrease in bulk modulus with pressure at 300K. Our results are inconsistent with conventional lattice dynamics, but a framework-solid-based non-linear model with many degrees of freedom predicts elastic softening as increases in either temperature or pressure reduce volume. The pressure-induced phase transition from $\alpha $-ZrW$_{2}$O$_{8}$ (cubic) to $\gamma $-ZrW$_{2}$O$_{8}$ (orthorhombic) is found to take place at $\approx $ 0.5 GPa, result confirmed by Raman spectroscopy.