A neutron powder diffraction study of Rb_x(NH₄)_1-xAlF₄ mixed compounds

The neutron powder profile refinement method has been used to determine the structures of mixed compounds Rb_x(NH₄)_1-xAlF₄ for x<or=0.10 at 290 and 5 K. At 290 K, the average structure for x <or=0.10 is refined in the space group I4/mcm as is pure NH₄AlF₄. The introduction of rubidium ions leads essentially to a variation in the cell parameters and to a slight decrease in the fluorine octahedra tilts. At 5 K for x=0.03, the long-range order of the ammonium ions settles as in NH₄AlF₄. For x=0.07 and x=0.10, it is shown that the low-temperature phase does not exhibit any 3D order in the ammonium network. This result precludes the hypothesis of A3NNI phases and tends to suggest the existence of a structural spin-glass behaviour. Then very small rubidium concentrations are sufficient to destroy the long-range order in the ammonium sublattice.