RESUMEN

The conductive properties of fluorite-like structures KLn4Mo3O15F (Ln = La, Pr, Nd: KLM, KPM, KNM) have been studied theoretically and experimentally. Theoretical studies included the geometrical-topological analysis of voids and channels available for migration of working ions; bond valence site energy calculations of the oxygen ions' migration energy; quantum-chemical calculations for the estimation of the oxygen vacancies formation energy. Experimental measurements of conductivity were made using impedance spectroscopy and as a function of oxygen partial pressure. The total conductivity was ∼10-3 S cm-1 for KLM and ∼10-2 S cm-1 for KPM and KNM at 800 °C. Measurements with changes in partial pressure proved the mixed nature of electric transport in KLM, KPM, and KNM phases, with predominantly ionic conductivity. The measured ion transference numbers in air reached approximately 0.9 at 800 °C for the KPM and KNM ceramics. Also, evaluated proton transfer numbers were less than 10%, indicating a small contribution to the total conductivity.

RESUMEN

Herein, neptunium(V) carbonates containing sodium or potassium cations were synthesized via chemical precipitation. Various techniques such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetry combined with differential scanning calorimetry, X-ray diffraction, and X-ray absorption spectroscopy were used to analyze the microstructures and elemental compositions of these samples. The crystal structures of hydrated NaNpO2CO3·3H2O (P1, a = 4.3420(2) Å, b = 4.8962(2) Å, c = 10.0933(11) Å, α = 91.014(7)°, ß = 77.834(11)°, and γ = 90.004(10)°) and KNpO2CO3 (P63/mmc, a = b = 5.0994(2) Å, c = 10.2210(15) Å) were determined for the first time using the Rietveld method. The synthesized carbonates exhibited distinct structural features and decomposition behaviors, as demonstrated through thermogravimetry analysis, which revealed the presence of crystalline hydrate water in sodium neptunium(V) carbonate. Furthermore, calcium-containing neptunium(V) carbonates were synthesized and characterized. Samples with the general composition Ca0.5NpO2CO3 were obtained using the ion exchange method and chemical precipitation from solutions containing competing cations (Ca2+, Na+, K+, and Mg2+). The synthesis conditions notably affected the diffraction patterns of the obtained calcium neptunium(V) carbonates. This investigation enhances our understanding of the structural properties and thermodynamic stability of neptunium(V) carbonates in the presence of diverse cations commonly found under radioactive waste disposal conditions.

RESUMEN

TmMgB5O10 spontaneous crystals were synthesized via the flux-growth technique from a K2Mo3O10-based solvent. The crystal structure of the compound was solved and refined within the space group P21/n. The first principles calculations of the electronic structure reveal that TmMg-pentaborate with an ideal not defected crystal structure is an insulator with an indirect energy band gap of approximately 6.37 eV. Differential scanning calorimetry measurements and powder X-ray diffraction studies of heat-treated solids show that TmMgB5O10 is an incongruent melting compound. A characteristic band of the Tm3+ cation corresponding to the 3H6 → 1D2 transition is observed in the photoluminescence excitation spectra of TmMg-borate. The as-obtained crystals exhibit intense blue emission with the emission peaks centered at 455, 479, 667, and 753 nm. The most intensive band corresponds to the 1D2 → 3F4 transition. TmMgB5O10 solids demonstrate the thermal stability of photoluminescence.

RESUMEN

Using a high-temperature ampoule technique and lead metal as a flux, we have grown single crystals and determined crystal structures from single-crystal X-ray diffraction data of two metal-rich phosphides, Sr5Pt12P9 (P 21/m, a = 6.1472(3) Å, b = 25.1713(13) Å, c = 6.4635(3) Å, ß = 99.604(2)°, Z = 2, R1 = 0.0326, wR2 = 0.0786) and BaPt3P2 (P 212121, a = 6.3605(6) Å, b = 6.8541(7) Å, c = 11.3493(12) Å, Z = 4, R1 = 0.0231, wR2 = 0.0501). Both compounds belong to their own structure types and feature 3D networks of Pt and P atoms, with the channels occupied by alkaline earth metal cations. Density functional theory calculations reveal Sr5Pt12P9 to be a metal, while BaPt3P2 is a narrow-gap semiconductor with a band gap of 0.24 eV. Bonding analysis shows that both compounds feature networks of prominent covalent localized Pt-P bonds, responsible for their structural stability, as well as additional weaker and, likely, less localized Pt-Pt interactions.

RESUMEN

Sequential extraction tests were used to study partitioning of U in the bottom sediments of two reservoirs that have been used for the temporary storage of nuclear waste at the "Mining and Chemical Combine" (Zheleznogorsk, Krasnoyarsk region, Russia). Various sequential extraction protocols were applied to the bottom sediment samples and the results compared with those obtained for laboratory-prepared simulated samples with different speciation and partitioning, e.g., U(VI) sorbed onto various inorganic minerals and organic matter, as well as uranium oxides. The distributions of uranium in fractions extracted from simulated and actual contaminated samples were compared to shed light on the speciation of U in the bottom sediments. X-ray absorption spectroscopy, X-ray diffraction, and scanning electron microscopy were also used to analyze the partitioning of U in contaminated sediments. We also compared the results obtained using the spectroscopic and microscopic techniques, as well as sequential extraction.

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