Electronic structure and properties of EuO and EuS in the molecular-cluster approximation

Molecular-cluster models are developed to describe ground-state properties of EuO and EuS in the Hartree-Fock-Slater one-electron approximation. Spin-polarized (EuX₆)^10- complexes are examined using both neutral- and ionic-model potentials which incorporate a part of the effect of the crystalline environment. Self-consistent calculations are made for (EuO₆)^10-. From the charge and spin densities, the transferred hyperfine field at the O site in EuO is found to be -8 ±{} 2 kG and a small solid-state bonding effect is predicted for the neutron magnetic form factor. The pressure dependence of the charge density at the Eu nucleus in EuS is determined as a function of bond length in the (EuS₆)^10- cluster and used to obtain from the experimental data an isomer-shift calibration constant α=-0.48a₀³ mm/sec. The one-electron energy levels of the (EuO₆)^10- cluster are found to be in good agreement with the augmented-plane-wave results of Cho when both calculations are performed with similar model potentials. The extension to self-consistency leads to significant energy-level rearrangement which indicates the importance of final-state relaxation and Coulomb correlation effects in the interpretation of experimental spectra.