Effect of Hydration and Ammonization on the Thermal Expansion Behaviour of ZrW2O8: Ab-initio Lattice Dynamical Perspective

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.