AN EXPLANATION OF ELECTRON PARAMAGNETIC TRANSITIONS AND LOW TEMPERATURE ENTROPY ANOMALIES OF HEXAMMINE NICKEL HALIDES

A point charge electrostatic model is used to explain the sudden increase in electron paramagnetic resonance linewidth at critical temperatures T c and the low temperature entropy anomalies exhibited by hexammine nickel halides. It is suggested that these results are due to hindered rotation of the H3 groups between different equilibria arrangements of H 3 protons within the solids. Barriers within the crystals are computed by assigning charges β|e| to each proton site, and summing the β2e2/rij terms between protons i and j on the same and on neighbouring Ni(NH3)6 clusters. Low and high energy arrangements of protons within each cluster are obtained. The model explains the electron paramagnetic resonance results by predicting that the nickel ion experiences a change from trigonal to cubic symmetry, occurring when the nearest neighbour electrostatic interaction between protons on neighbouring clusters is thermally overcome. The entropy anomalies, corresponding to a depopulation of one member of a ground state split by rotation of individual H3 groups, give values for β|e|. Reasonably consistent values for the three halides (similar 1/4|e|) lend support to the validity of the main predictions.