Nondipolar Interaction Between Nearest-Neighbor Neodymium Ions in the Ethyl Sulfate

Measurements made on the EPR spectrum of a pair of interacting nearest-neighbor neodymium ions in lanthanum ethyl sulfate show that the interaction is predominantly dipolar, but that there is an additional term AᵢⱼSizSjz+1/2Bᵢⱼ(Sᵢ₊S_j-+S_i-Sⱼ₊), with Aᵢⱼ=(+32±6)×10^-4 cm^-1 and Bᵢⱼ=(-3±3)×10^-4 cm^-1. A semiquantitative discussion is given of the possible origins of this nondipolar contribution including super-exchange, electric quadrupole-quadrupole, and virtual-phonon exchange interactions.