THE EFFECT OF ANION SUBSTITUTION IN NICKEL HEXAMMINE SALTS

Abstract
Several metal hexammine salts show rotational tunnelling of the NH₃ groups in their low-temperature phases /1,2/. This NH₃ tunnelling on one hexammine cation is strongly influenced by NH₃ groups of the neighbouring cations. It is important to elucidate whether the anions play a direct rôle in determining the rotational barrier or whether changes in the barrier result simply from changes in the lattice parameter. To this end we have started a study of Ni(NH₃)₆l₂ in which we replaced varying amounts of the iodide ions with bromides as substitutional impurities. We synthesised mixtures of Ni(NH₃)₆(l,Br)₂ with 1, 5, 10, 20 and 50 atom% Br. A normal distribution of next neighbours fits the low-resolution neutron scattering data well. The presence of the bromide ions as substitutional impurities creates regions of "negative pressure" in the nickel hexammine iodide lattice.