Graphical Correlation of the Néel Temperatures of Chlorides, Bromides, and Iodides of Divalent 3d Transition Metal Ions

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Abstract
Indirect exchange in isostructural compounds of divalent Mn, Fe, Co, and Ni increases with the electron affinities of the cations and the polarizabilities of the anions. Spin‐orbital coupling can increase TN (for Fe) in a strong trigonal field, and decrease TN in a cubic field. Exchange interactions in the chlorides, bromides, and iodides take place via double layers of anions. As a result, a higher threshold cation electron affinity is required to effect spin alignment than for single anion intermediaries. A graphical comparison of the TN values of the chlorides, bromides, and iodides as a function of the ionization potentials of the cations demonstrates the consistency of these data with those of cubic oxides and fluorides and with simple principles.