Electron spin resonance in a single crystal of LaF3 containing 0.01% Gd3+ shows three magnetically distinguishable ions per unit cell in a rhombic crystalline electric field which is consistent with a hexamolecular unit cell of symmetry C6/mcm. This is in disagreement with recent crystallographic measurements, but in agreement with the interpretation of the Faraday rotation given by Van Vleck and Hebb. The parameters of the spin Hamiltonian are given in table 1. The crystal field splitting in zero magnetic field is of the order of 0.3 cm-1 which makes this a possible salt for maser operation in low magnetic field.

The paramagnetic resonance spectrum in crystals of GdCl3 diluted with LaCl3 and CeCl3 has been examined. The splitting of the ground level 8S7,2 of Gd3+ in zero magnetic field is small, and has a remarkable temperature dependence. A theory to account for the experimental results is developed, and it is shown that perturbation mechanisms both linear and quadratic in the crystalline field potential are important.