RIGID UNIT MODES AND THE NEGATIVE THERMAL-EXPANSION IN ZRW2O8

ZrW2O8 has a negative coefficient of thermal expansion from 0.3 K to i ts decomposition temperature around 1050K. The negative thermal expans ion is isotropic and is not disrupted by the structural phase transiti on at 430 K. A complicated distribution in reciprocal space of Rigid U nit Mode (RUM) phonons has been found for ZrW2O8 using Lattice Dynamic s. The RUMs are very low energy phonons which cause collective rotatio ns of ZrO6 octahedra and WO4 tetrahedra within the crystal structure. These distortions enable ZrW2O8 to contract upon heating.