Molten Salt Synthesis and Structural Characterization of Novel Salt-Inclusion Vanadium Bronze Cs5FeV5O13Cl6

Flux Method Growth and Structural Analysis of Two Tetravalent Uranium Orthophosphate Single Crystals, Cs2UIV(PO4)2 and Isostructural Cs2(UIV0.75CeIV0.25)(PO4)2

Two novel uranium(IV) orthophosphate framework compounds were obtained by the high-temperature flux method in CsCl-CsF eutectic salt. Cs2UIV(PO4)2 (1) and isostructural Cs2(UIV0.75CeIV0.25)(PO4)2 (2) are tetragonal structures bridged by (U/Ce)IV-O octacoordinated dodecahedra and PO4 tetrahedra, with Cs+ cations filling in the channels. The crystal structures exhibit good structural and thermal stability with a potential capacity to immobilize tetravalent radionuclides.

Understanding the Stability of Salt-Inclusion Phases for Nuclear Waste-forms through Volume-based Thermodynamics

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.

New Lanthanide Mixed-Valent Vanadium(III/IV) Oxosilicates, Ln4V(5-x)Zn(x)Si4O22 (Ln = La, Ce, Pr, and Nd), Crystallizing in a Quasi Two-Dimensional Rutile-Based Structure

A family of lanthanide mixed-valent vanadium(III/IV) oxosilicates Ln4V(5-x)Zn(x)Si4O22 (Ln = La, Ce, Pr, and Nd) was synthesized as high-quality single crystals via a high-temperature molten salt method. An in situ reduction of V(V) to V(III/IV) as well as of Ce(IV) to Ce(III) was achieved utilizing Zn metal as the reducing agent, some of which is incorporated into the crystal structure. Ce4V(4.77)Zn(0.23)Si4O22, to the best of our knowledge, is the first example of a cerium-containing mixed-valent vanadium silicate. The crystal structures were determined by single-crystal X-ray diffraction, and the four isostructural oxosilicates were determined to crystallize in the chevkinite-structure type in the monoclinic space group I2/a. The unit cells of the Ln4V(5-x)Zn(x)Si4O22 (Ln = La, Ce, Pr, and Nd) series are related to the reported C2/m phases Ln4V5Si4O22 (Ln = La, Pr, and Nd) by a doubling of the c-axis and a loss of a mirror plane. The three-dimensional crystal structure consist of two-dimensional rutile-based vanadium oxide and lanthanide oxide layers linked via Si2O7 groups. The temperature dependence of the magnetic susceptibility of these compounds was measured, and only the Nd analogue exhibited a magnetic transition at 5 K; all samples displayed a discontinuity or deviation from linearity at ca. 130-150 K.