Temperature dependence of the crystal-field splittings and hyperfine structure in Eu2+:CaWO4 and evidence for nuclear quadrupole interactions

Abstract

The crystal-field splittings and hyperfine-structure constants of Eu2+ in CaWO4 have been accurately measured at 4.2 °K and at 78 °K using the method of electron paramagnetic resonance. These measurements were intended to provide liquid-helium temperature parameters for ENDOR experiments presently in progress but showed significant differences from the earlier measurements of Bronstein and Volterra. Because of these differences and the considerable current interest in the values of these parameters for S-state ions, z-axis measurements were extended to cover the temperature range from 1.8 °K to room temperature.A very good fit to the measured line positions has been obtained by carrying out an exact diagonalization of the spin Hamiltonian within the 8S7/2 manifold. The parameters fitted are [Formula: see text], [Formula: see text], b20, b40, b44, b60, b44, A, and B. Residual discrepancies, of the order of a tenth of a line width, are shown to be consistent with the presence of a purely ionic quadrupole interaction and an interaction between the nuclear quadrupole moment and the crystalline electric field. The temperature variation of the dipolar hyperfine-coupling constant is shown to be consistent with the theory of Šimánek and Orbach as extended to rare earth ions in cubic lattices by Menne, Ames, and Lee if a Θ of 150 °K is assumed. Within the restrictions imposed by the lack of thermodynamic data and the assumption of a point-charge lattice, it is shown that the temperature dependence of the fourth- and sixth-order crystal-field parameters is consistent with the predominance of a splitting mechanism at least quadratic and more probably of fourth power in the crystalline potential. The second-order axial parameter is found to have a temperature dependence which can not be explained in terms of a single mechanism.