ORIGIN OF THE NEGATIVE THERMAL-EXPANSION IN ZRW2O8 AND ZRV2O7

The negative thermal expansion recently observed over a wide range of temperatures in ZrW2O8 may be attributed to the existence of low-frequ ency phonon modes which can propagate with no distortions of the WO4 t etrahedra and ZrO6 octahedra, the so-called 'rigid unit modes'. Using methods developed for the study of similar modes in silicates we have located the surfaces of these modes in wave-vector space. The rigid-un it mode interpretation accounts for the weak effect of the 430 K struc tural phase transition on the negative thermal expansion, provided tha t the disordered phase does not involve formation of W2O7 and W2O9 com plexes. On the other hand, the crystal structure of the related materi al ZrV2O7 is cross braced by pairs of linked tetrahedra and is therefo re significantly less flexible. In this case a qualitatively different mechanism may be responsible for the negative thermal expansion obser ved in ZrV2O7.