Low-Lying Crystal-Field Spectra of Rare Earth Compounds

The crystal-field splitting of the degenerate ground state of rare earth ions produces energy differences which correspond to far-infrared frequencies (10–200 cm−1). Absorption spectra of certain rare earth double nitrates, ethyl sulphates, and oxides, cooled to liquid helium temperatures, have been observed in this frequency range. The positions of the absorption lines have been compared with values deduced from optical and other measurements. In general the agreement is good. Measurements of linewidths with different concentrations of paramagnetic ions indicate that spin-lattice relaxation and electric quadrupole interaction are the predominant line-broadening mechanisms. In the case of Pr3+ in La ethyl sulphate splitting of non-Kramers degenerate levels leads to further broadening. The splitting of the crystal-field levels of Er2O3 in the antiferromagnetic state have been observed, but not yet fully analyzed.