Abstract
Some components of the Aij interaction tensors for 3rd to 7th neighbouring Nd3+ ions in LaCl3 and LaBr3 have been measured by means of e. p. r. There are some appreciable super­ exchange contributions to these interactions. The trends in the variation from one neigh­bour to another were unexpected, 4th and 5th neighbours having much larger superexchange than 3rd neighbours. LaBr3 is similar to LaCl3 except that the exchange contributions are somewhat larger in LaBr3, corresponding to larger covalent overlap. The qualitative features of the measured exchange interactions are explained in terms of the details of the super­exchange paths. For 7th neighbours the interaction is purely dipolar, and it seems likely that more distant neighbours will also have purely dipolar interactions.

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