ABSTRACT
The compound-tunable embedding potential (CTEP) method developed in [Lomachuk et al., Phys. Chem. Chem. Phys., 2020, 22, 17922; Maltsev et al., Phys. Rev. B, 2021, 103, 205105] to describe the electronic structure of fragments and point defects in materials is applied to crystals containing periodically arranged lanthanide atoms, which can have an open 4f-shell. We consider YbF2, YbF3, YbCl2, and YbCl3 crystals for the pilot CTEP studies such that 4f-electrons are not treated explicitly at the CTEP generation stages. Instead, the pseudopotentials with 60 and 59 electrons in the core for Yb(II) and Yb(III), correspondingly, are applied and the latter treats the "4f-hole-in-core". At the final stage, the two-component embedded cluster study of fragments of YbHaln crystals (Hal = F, Cl; n = 2, 3) is performed using the CTEP method and a relativistic pseudopotential with 28 electrons in the core for the central Yb atom. Remarkable agreement of the electronic densities within the YbHal2 fragments with those of the original periodic crystal calculation is demonstrated. The calculated interatomic distances between the central Yb and nearest halide atoms are in pretty good agreement with the experimental data, the deviations are within 0.015 Å for all the studied crystals. Thus, the overall accuracy for the crystal characteristics evaluated using CTEP in the combined periodic-structure and embedded cluster study is comparable with that of Yb-containing molecular calculations.
ABSTRACT
Modern strategies for the safe handling of high level waste (HLW) and its long-term disposal in deep geological formations include the immobilization of radionuclides in the form of mineral-like matrices. The most promising matrices for the immobilization of actinides are ceramic forms of waste based on phosphate minerals such as monazite, xenotime, and cheralite. However, the mechanism of substitution of lanthanides and Y by actinides in phosphate minerals is not entirely clear. We formulated a theoretical model, compound-tunable embedding potential (CTEP), that allows one to predict properties of such crystals with point defects. The reliability of the model is validated by a good agreement of calculated geometry parameters with available experimental data. The substitution of Y in the xenotime crystal by Th and U is studied by relativistic DFT in the framework of the CTEP method, based on constructing the embedding potential as the linear combination of short-range "electron-free" spherical "tunable" pseudopotentials. It is shown on the basis of the proposed model that oxidation state +3 is energetically more profitable than +4 not only for thorium but also for uranium as solitary point defects. This atypical oxidation state of U in the mineral is discussed.
ABSTRACT
Reaction of [99Tc(CO)6]ClO4 with alkali in aqueous solutions yields yellow 99Tc3H(CO)14 as the major product. On the other hand, [99TcH(CO)5] becomes the major product when the reaction with alkali is combined with the extraction into hexane. The molecular structure of 99Tc3H(CO)14, determined by SCXRD, is composed of the 99Tc2(CO)9 fragment bound to the 99Tc(CO)5 fragment via the hydrogen bridge and weak metal-metal bond. This compound crystallizes in the monoclinic system, space group P21/n, a = 9.6954(2) Å, b = 15.0985(3) Å, c = 14.5090(3) Å, and ß = 104.925(2)°. 99Tc3H(CO)14 was additionally characterized by IR spectroscopy. The mechanism of hydrolysis of [99Tc(CO)6]ClO4 was suggested.
ABSTRACT
Technetium(I) and rhenium(I) pentacarbonyl complexes with ethyl 2-isocyanoacetate and methyl 11-isocyanoundecanoate, [M(CO)5(CNCH2COOEt)]ClO4 (M = Tc (1) and Re (2)) and [M(CO)5(CN(CH2)10COOMe)]ClO4 (M = Tc (3) and Re (4)), were prepared and characterized by IR, (1)H NMR, and (13)C{(1)H} NMR spectroscopy. The crystal structures of 1 and 2 were determined using single-crystal X-ray diffraction. The kinetics of thermal decarbonylation of technetium complexes 1 and 3 in ethylene glycol was studied by IR spectroscopy. The rate constants and activation parameters of this reaction were determined and compared with those for [Tc(CO)6](+). It was found that rhenium complexes 2 and 4 were stable with respect to thermal decarbonylation. Histidine challenge reaction of complexes 1 and 2 in phosphate buffer was examined by IR spectroscopy. In the presence of histidine, the rhenium pentacarbonyl isocyanide complex partially decomposes to form an unidentified yellow precipitate. Technetium analogue 1 is more stable under these conditions.
