Electron Paramagnetic Resonance of Gd3+ in Single Crystals of La(C2H5SO4)3· 9H2O, Bi2Mg3(NO3)12· 24H2O, and Sm(C2H5SO4)3· 9H2O Below 40 G

Electron paramagnetic resonance studies of Gd³⁺ ion in three types of magnetically dilute single crystals possessing C₃ site symmetry have been carried out at magnetic fields below 40 G and at 77°K. The zero‐field splittings and g factors of Gd³⁺ obtained from these studies are compared with those obtained from: (a) a previous low‐field EPR study of Gd³⁺ in $\mathrm{La}({\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{SO}}_4{)}_3\text{· 9}{\mathrm{H}}_2\mathrm{O}$ , (b) conventional (high‐field) EPR studies in this laboratory of Gd³⁺ in the same three host‐crystal systems; (c) previous conventional (high‐field) EPR studies of Gd³⁺ in $\mathrm{La}({\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{SO}}_4{)}_3\text{· 9}{\mathrm{H}}_2\mathrm{O}$ and in ${\mathrm{Bi}}_2{\mathrm{Mg}}_3({\mathrm{NO}}_3{)}_{12}\text{· 24}{\mathrm{H}}_2\mathrm{O}$ . While all possible such comparisons involving ±7/2, ±5/2 and ±5/2, ±3/2 transitions showed agreement within the stated uncertainties of the various data sets, those involving ±3/2, ±1/2 transitions did not. Moreover, anomalous absorptions of lower intensity consistently were observed to the low‐field side of the +3/2, +1/2 and the −3/2, −1/2 transitions only. These anomalous signals are shown to be interpretable as arising from the ``forbidden'' +3/2, −1/2 and −3/2, +1/2 transitions. Detailed study of the normal and forbidden transitions permitted the discrepancy between the energies of the +3/2, +1/2 and the −3/2, −1/2 transitions (and between the energies of the +3/2, −1/2 and the −3/2, +1/2 transitions) to be rationalized in terms of a postulated nonlinearity in the field‐dependence of the energies of these spin states at magnetic fields below ∼10 G, the lower limit of the present observations.