  The hydration and ammonization of ZrW2O8 is known to lead to positive and
negative thermal expansion behaviour respectively. We report ab-initio
calculations to understand this anomalous behaviour. We identify the crucial
low energy phonon modes involving translations, rotations and distortions of
WO4 and ZrO6 polyhedra, which lead to NTE in ZrW2O8 in pure and ammoniated
forms; however, the rotation and distortion motions get inhibited on hydration
and lead to positive thermal expansion. We demonstrate that the thermal
expansion coefficient could be tailored by engineering the phonon dynamics of a
material.
