Electron Spin Resonance in Two Groups of Lanthanon Salts

Electron spin resonance measurements have been made on lanthanon ions in the hexa-antipyrene iodide (HAPI) salts and also in lanthanum trifluoride. At room temperature the HAPI have trigonal symmetry with one lanthanon ion per unit cell but at low temperatures there is a change of crystal structure. For all ions except Ce 3+ and Gd 3+ electron spin resonance is observed only at temperatures below the transition. The lanthanon ion in the HAPI at low temperatures and in the trifluoride have rhombohedral point symmetry and there are several ions in the unit cell. Not all ions with Kramers degeneracy exhibited resonance in both salts, and the few non-Kramers cases investigated also exhibited no resonance. In both salts the most extensive measurements were made on Ce 3+ . In the HAPI Ce 3+ has a remarkably long relaxation time and the spectrum is visible at 90°K. The measurements on Ce 3+ in LaF 3 are correlated with the theoretical discussion given by Van Vleck and Hebb in 1934.