Electronic structure study of YNbTiO6_6 vs. CaNb2_2O6_6 with U, Pu and minor actinide substitutions using compound-tunable embedding potential method

The compound-tunable embedding potential (CTEP) method is applied to study actinide substitutions in the niobate crystals YNbTiO$_6$ and CaNb$_2$O$_6$. Two one-center clusters centered on Ca and Y are built and 20 substitutions of Ca and Y with U, Np, Pu, Am, and Cm in four different oxidation states were made for each cluster. Geometry relaxation is performed for each resulting structure, and electronic properties are analyzed by evaluating the spin density distribution and X-ray emission spectra chemical shifts. Though the studied embedded clusters with actinides having the same oxidation state are found in general to yield similar local structure distortions, for Am and Cm in high "starting" oxidation states the electron transfer from the environment was found, resulting in decrease of their oxidation states, while for "starting" U$^{\rm III}$ state the electron transfer goes in the opposite direction, resulting in increase of its oxidation state to U$^{\rm IV}$. The U substitutions are additionally studied with the use of multi-center models, which can provide both more structural and electronic relaxation and also include charge-compensating vacancies. For "starting" U$^{\rm VI}$ case, the decrease in oxidation state similar to that of Am$^{\rm VI}$ and Cm$^{\rm VI}$ in one-center clusters is observed in our calculations but in a different way. Since the really synthesized YNbTiO$_6$ structures can not be considered as perfect (periodic) crystals because the Nb and Ti atoms are statistically distributed within them occupying the same Wyckoff positions, different Ti $\leftrightarrow$ Nb substitutions are studied and corresponding structural changes are estimated

A CONCEPT OF EFFECTIVE STATE OF ATOMS-IN-COMPOUNDS TO DESCRIBE PROPERTIES DETERMINED BY THE VALENCE ELECTRON'S DENSITIES IN ATOMIC CORES

Author Institution: B.P. Konstantinov Petersburg Nuclear Physics Institute, Gatchina, Leningrad district 188300; Department of Physics, Saint Petersburg State University, Petrodvoretz 198904, RUSSIAA new method of circumscribing the effective electronic states of "atoms-in-compounds'' to study the properties of molecules and solids which are described by the operators heavily concentrated in atomic cores is discussed. Among the properties are hyperfine structure, P,T-parity nonconservation effects, chemical shifts of X-ray emission and Mossbauer lines, etc. Advantage of the approach is that a good quantitative agreement of predicted and experimental data can be attained. From computational point of view the method is based on the relativistic pseudopotential theory and procedures of a posteriori recovery of wave functions (which are smoothed near atomic nuclei at the molecular calculation stage with using the pseudopotential method) in the atomic cores. We report results of our recent investigations of a number of diatomic molecules

Calculations of Chemical Shifts of X-ray Emission Spectra and Effective States of Nb Atom in the Niobates

A computational approach to evaluation of chemical shifts of characteristic X-ray emission lines of d and f elements in different chemical compounds is proposed. It is based on modeling the embedded cluster electronic structure using relativistic pseudopotential approximation that is followed by restoration of (all-electron) wavefunction in the core region. The proposed approach is applied to the Nb Kα lines in fersmite. The line positions and their shifts with respect to those of metallic Nb were evaluated via the restoration of the all-electron wavefunction in the Nb core region. The results are in satisfactory agreement with the experiment, overestimating the measured value by ∼20 %

Chemical Shifts of Kα1 and Kα2 Lines in X-Ray Emission Spectra of Yb(II)/Yb(III) Fluorides: a Quantum Chemical Study

Chemical shifts of Kα1 (2p3/2-1s1/2) and Kα2 (2p1/2-1s1/2) lines X-ray emission spectra (XES) of Yb in its fluorine compounds were calculated. Valence transition Yb(II)→Yb(III) was considered on examples of YbF2, Yb2F4, and YbF3 molecules and YbF+ 2 cation. Relativistic pseudopotential and corresponding basis sets for ytterbium have been generated and used in Hartree–Fock and DFT PBE0 calculations. The results for a threefold coordinated Yb(II) in the FYbF2YbF dimer indicate a very weak dependence of the chemical shifts on the Yb coordination number as well as on association of ytterbium difluoride molecules. The XES chemical shifts in ytterbium compounds are primarily associated with different population of the 4f-shell

Calculations of Chemical Shifts of X-ray Emission Spectra and Effective States of Nb Atom in the Niobates

A computational approach to evaluation of chemical shifts of characteristic X-ray emission lines of d and f elements in different chemical compounds is proposed. It is based on modeling the embedded cluster electronic structure using relativistic pseudopotential approximation that is followed by restoration of (all-electron) wavefunction in the core region. The proposed approach is applied to the Nb Kα lines in fersmite. The line positions and their shifts with respect to those of metallic Nb were evaluated via the restoration of the all-electron wavefunction in the Nb core region. The results are in satisfactory agreement with the experiment, overestimating the measured value by ∼20 %

Chemical Shifts of Kα1 and Kα2 Lines in X-Ray Emission Spectra of Yb(II)/Yb(III) Fluorides: a Quantum Chemical Study

Chemical shifts of Kα1 (2p3/2-1s1/2) and Kα2 (2p1/2-1s1/2) lines X-ray emission spectra (XES) of Yb in its fluorine compounds were calculated. Valence transition Yb(II)→Yb(III) was considered on examples of YbF2, Yb2F4, and YbF3 molecules and YbF+ 2 cation. Relativistic pseudopotential and corresponding basis sets for ytterbium have been generated and used in Hartree–Fock and DFT PBE0 calculations. The results for a threefold coordinated Yb(II) in the FYbF2YbF dimer indicate a very weak dependence of the chemical shifts on the Yb coordination number as well as on association of ytterbium difluoride molecules. The XES chemical shifts in ytterbium compounds are primarily associated with different population of the 4f-shell