Precision measurements and analysis have been made of the 290 °K, 10 GHz EPR spectra of the Gd3+ [4f7, 8S7/2] ion dilutely substituted (~5:103) into the C3h-site symmetry of the lanthanide host lattices Ln(C2H5SO4)3∙9H2O, where Ln ≡ La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Er, Tm, Yb, and Lu as well as Y which is chemically lanthanide-like. It is found that empirical, linear relationships exist between the magnitude of the Gd3+ spin-Hamiltonian parameter B20 and (i) the ionic radius of the lanthanide host; (ii) the unit cell dimensions of the host lattice; (iii) the molar volume of the host lattice; and (iv) the estimated crystalline potential parameter [Formula: see text]. A similar linear relationship is found between the ionic radius of the lanthanide host and B60 but not B40. These spin-Hamiltonian parameters are related theoretically to the crystalline electric field parameters through the perturbation interactions of the ground state with the excited states. It is concluded from a review of these interactions that the empirical relations reported in this paper cannot be explained using the values of the crystalline electric field parameters calculated from either excited state optical data or lattice summations. The EPR spin-Hamiltonian parameters for Gd3+ in the Y lattice confirm the latter's lanthanide behavior and assign it an effective ionic radius of (0.0896 ± 0.0001) nm which locates it very close to Ho, (0.0894 ± 0.0001) nm, confirming the results of Von Hevesy, Zachariasen et al., and Ketelaar.
