Ammonium-ion ordering in a scheelite lattice:NH4ReO4

The anomalous heat capacity and thermal expansion of ${\mathrm{NH}}_4$ ${\mathrm{ReO}}_4$ near 200 K are attributed to cooperative ordering of ammonium ions, similar to that which occurs in ammonium halides. No phase transition takes place because the ammonium site symmetry in this lattice favors one of the two ammonium orientations. A model of Ising pseudospins in a longitudinal field provides a satisfactory description of the observed lattice strains, heat capacity, and nuclear-quadrupole-resonance and Raman spectra. There is strong coupling between ammonium ions and lattice strain in ${\mathrm{NH}}_4$ ${\mathrm{ReO}}_4$, so that mean-field theory should be an excellent approximation.