Rigid Unit Modes and the negative thermal expansion in ZrW2O8

ZrW2O8 has a negative coefficient of thermal expansion from 0.3 K to its decomposition temperature around 1050 K. The negative thermal expansion is isotropic and is not disrupted by the structural phase transition at 430 K. A complicated distribution in reciprocal space of Rigid Unit Mode (RUM) phonons has been found for ZrW2O8 using Lattice Dynamics. The RUMs are very low energy phonons which cause collective rotations of ZrO6 octahedra and WO4 tetrahedra within the crystal structure. These distortions enable ZrW2O8 to contract upon heating