Interactions between Ce 3 + pairs and Nd 3 + pairs in LaCl 3 and LaBr 3

Abstract

The e.p.r. pair spectra have been measured of nearest neighbour and second neighbour pairs of Ce 3 + ions in LaBr 3 , and of Nd 3 + ions in LaCl 3 and LaBr 3 . Shifts in g value from those of the isolated ion are shown to be due to lattice distortion for N d pairs and to electric quadrupole-quadrupole interaction (e.q.q.) for Ce pairs. The interaction tensors are shown to have large contributions from magnetic dipole-dipole and superexchange interactions for both ions, and there is an appreciable e.q.q. contribution for Ce pairs. It is shown that the superexchange has to be of the generalized form discussed by Levy (1964), and that the usual form of exchange Hamiltonian which is bilinear in components of J will not explain our results. Measurement in two isomorphous host lattices has been very valuable in reaching the above conclusions. All of the systematic changes between LaCl 3 and LaBr 3 are well, accounted for in terms of differences in lattice constant, covalency and polarizability of the anions. Our results demonstrate the important role played by the anions in the interaction processes.