Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV, V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

Theoretical Prediction and Interpretation of 237Np Mössbauer Isomer Shifts

“…The 237 Np isomer shift has a wide known range, about −70 to +40 mm/s between Np(VI) and Np(III), relative to the NpAl 2 standard, and it can be used to examine covalent effects in Np organometallic complexes and ionic solids. 3,6 The present study focuses on the isomer shift of Np compounds for which experimental Mossbauer data are already available. The isomer shift arises from the electrostatic interaction between the electronic and nuclear charge distributions, specifically from the differential change of these interactions upon the nuclear transitions in the γ-ray source versus the absorbing nucleus in the probe ("absorber").…”

“…In other words, donation bonding into the 5f shell will increase the 5f electron density and result in a more positive isomer shift relative to a purely ionic shielding for the same oxidation state. For example, neptunocene 3 [Np(IV)] has δ = +19 mm/s, whereas the ionic compound 6 NpF 4 has δ = −6 mm/s. The magnitude of the covalency effects on the isomer shift depends on the Np oxidation state.…”

X-ray Diffraction, Mössbauer Spectroscopy, Magnetic Susceptibility, and Specific Heat Investigations of Na4NpO5 and Na5NpO6

“…Jovéet al, who have investigated correlations among the isomer shift, electric field gradient, and local neptunium structure in crystallized and amorphous neptunium compounds covering the Np(III) to Np(VII) valence states, 23,24 have pointed out the existence of a linear relationship between the average neptunium−ligand distance and isomer shift for hexavalent neptunium compounds. 23,24 The values reported herein for Na 4 NpO 5 fit very well with this trend, as shown in 24 The authors also have emphasized a correlation between the values of the quadrupole coupling constant |e 2 qQ| and isomer shift, with a separate region for the non-neptunyl compounds NpF 6 , Ba 2 CoNpO 6 , Ba 2 CuNpO 6 , and Li 4 NpO 5 . 24 The Mossbauer parameters found herein for Na 4 NpO 5 satisfy the criteria for this group of compounds, i.e., 0 < |e 2 qQ| < 50 mm s −1 and −63 < δ IS < −52 mm s −1 .…”

“…The red line is a linear fit of the experimental points. Asterisk indicates that, when not available, the mean (Np−L) distance was approximated with the mean (U−L) distance reported by Jovéet al 23 and was corrected for the difference in ionic radius between Np 6+ and U 6+ according to Shannon's tabulated data, 19 i.e., 0.01 Å. 24 but the corresponding spectrum was, to the authors' knowledge, never published.…”

Theoretical Evaluation of Metal–Ligand Bonding in Neptunium Compounds in Relation to 237Np Mössbauer Spectroscopy

“…Therefore, the QS and the EFG tensor asymmetry parameter η reflect the local electronic environment of a metal ion in a system in general, 5 and 237 Np in particular. 2,6 Although 237 Np Mossbauer spectroscopy has proven to be a valuable technique for extracting chemical bonding information in Np compounds 3,13,19−22 (representing early actinide complexes more generally), the isomer shifts and QS trends have proven difficult to interpret. Regarding the isomer shift, difficulties in their interpretation arise from our limited understanding of the extent of the donation bonding in the different oxidation states.…”