A neutron powder diffraction study of Rbx(NH4)1-xAlF4 mixed compounds

The neutron powder profile refinement method has been used to determine the structures of mixed compounds Rbx(NH4)1-xAlF4 for xrubidium 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.