Test of Pseudo-Spin Theory of Ammonium Scheelites
The properties of crystalline ammonium salts are, in many cases, anomalous when compared to those of isostructural alkali metal salts of the same anion. This is thought to be the result of a cooperative interaction between the rotating ammonium ions and the lattice. The ammonium scheelites NH4XO4 with X = Re, Tc and I show strongly anisotropic thermal expansion, highly anomalous NQR behaviour, and large diffuse peaks in the heat capacity near 200 K. A pseudo-spin theory has been proposed which accounts satisfactorily for some of the anomalies 1, 2). Results of a direct test of the theory using neutron diffraction will be reported in this paper.