Ab initio study of a Bi~(3+) impurity in Cs_2NaYCl_6 and Y_2O_3:Comparison of perturbative and variational electron correlation methods

摘要

Ab initio study of excitation energies and oscillator strengths for absorption towards the ~3P_1 and ~1P_1 states of the Bi~(3+) ion has been performed for the Bi~(3+) ion in gas phase and as a dopant of the cubic elpasolite Cs_2NaYCl_6 and the yttria Y_2O_3 crystal using the ab initio embedded-cluster method.The ground and excited states were computed with a relativistic spin-orbit configuration interaction approach suited for heavy elements.Electron correlation was treated in the scalar relativistic scheme with perturbative,variational,and coupled-cluster methods.Intermediate coupling is included via an effective-Hamiltonian based spin-orbit configuration interaction approach.Small-core (60 electrons) and large-core (78 electrons) relativistic effective core potentials (ECPs) have been used to describe the bismuth ion.The best match with experiment was obtained with the small-core ECP.The accuracy of excitation energies strongly depends on the electron correlation method used.The agreement between experimental data and the results obtained using second-order multiconfigurational perturbation theory is greatly improved with the shifted zeroth-order Hamiltonian proposed by Ghido et al.[Chem.Phys.Lett.396,142 (2004)].Although quite time consuming,coupled-cluster and variational methods yield good agreement with experimental data.The first absorption band recorded for the doped elpasolite crystal is positioned with an excellent accuracy while the computed energy of the second absorbing manifold is in poorer agreement with experimental data.This suggests that interactions with neglected close-lying excited states with a ligand-to-metal charge transfer character may be significant.Calculations of the spectrum of Bi~(3+) doping yttria in both the S_6 and C_2 site symmetries indicate that the absorbing manifold arises from electronic excitations localized on the Bi~(3+) doping ion with main triplet 6s6p character.Our results predict the first absorbing peak to lie about 0.5 eV lower for the S_6 sites than for the C_2 site,thus attributing the violet and the green emission wavelengths to the S_6 and C_2 sites,respectively.A subsequent study of Stokes shift and emission wavelength should hopefully lead to a final assignment of the measured excitation spectra.