A COMPUTER-SIMULATION STUDY OF THE DISORDER IN AMMONIUM PERRHENATE

Ammonium scheelites (of which ammonium perrhenate is an example) show several anomalies in their behaviour (including thermal contraction of the a lattice parameter, large thermal expansion of the c lattice par ameter, enhanced nuclear quadrupolar relaxation, increased specific he at) which has been attributed to the onset of orientational disorder o f the ammonium ions. Unlike in the ammonium halides there is no actual phase transition. Extensive molecular dynamics simulations of this ma terial are reported. The model shows anomalies in the thermal expansio n which are associated with a broadening and shift of the orientationa l distribution of the ammonium ions. There is no evidence of more than one preferred orientation. The ammonium ions also begin to jump betwe en the twelve equivalent orientations, and we present evidence that th is is primarily by rotation about one of the bonds. The activation ene rgy for this agrees well with experiment. Angular and linear velocity- time correlation functions for the ammonium ion show rapid dephasing a t higher temperatures, which is consistent with the observed broadenin g of the corresponding lines in the Raman spectrum. The origin of the anomalous thermal expansion is discussed in terms of various contribut ions to the entropy of the crystal.