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 ${\left(\mathrm{Eu}{X}_6\right)}^{10-}$ 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 ${\left(\mathrm{E}\mathrm{u}{\mathrm{O}}_6\right)}^{10-}$. 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 ${\left(\mathrm{E}\mathrm{u}{\mathrm{S}}_6\right)}^{10-}$ cluster and used to obtain from the experimental data an isomer-shift calibration constant $\alpha =-0.48\mathrm{}{a}_0^3$ mm/sec. The one-electron energy levels of the ${\left(\mathrm{E}\mathrm{u}{\mathrm{O}}_6\right)}^{10-}$ 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.