Uptake of Ra during the Recrystallization of Barite: A Microscopic and Time of Flight-Secondary Ion Mass Spectrometry Study

A combined macroscopic and microanalytical approach was applied on two distinct barite samples from Ra uptake batch experiments using time of flight-secondary ion mass spectrometry (ToF-SIMS) and detailed scanning electron microscopy (SEM) investigations. The experiments were set up at near to equilibrium conditions to distinguish between two possible scenarios for the uptake of Ra by already existent barite: (1) formation of a Ba_1–<i>x</i>Ra_<i>x</i>SO₄ solid solution surface layer on the barite or (2) a complete recrystallization, leading to homogeneous Ba_1–<i>x</i>Ra_<i>x</i>SO₄ crystals. It could be clearly shown that Ra uptake in all barite particles analyzed within this study is not limited to the surface but extends to the entire solid. For most grains a homogeneous distribution of Ra could be determined, indicating a complete recrystallization of barite into a Ba_1–<i>x</i>Ra_<i>x</i>SO₄ solid solution. The maxima of the Ra/Ba intensity ratio distribution histograms calculated from ToF-SIMS are identical with the expected Ra/Ba ratios calculated from mass balance assuming a complete recrystallization. In addition, the role of Ra during the recrystallization of barite was examined via detailed SEM investigations. Depending on the type of barite used, an additional coarsening effect or a strong formation of oriented aggregates was observed compared to blank samples without Ra. In conclusion, the addition of Ra to a barite at close to equilibrium conditions has a major impact on the system leading to a fast re-equilibration of the solid to a Ba_1–<i>x</i>Ra_<i>x</i>SO₄ solid solution and visible effects on the particle size distribution, even at room temperature

Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via Mixed Fluxes Techniques

Two uranium and two thorium silicates were obtained using high temperature mixed fluxes methods. K₁₄(UO₂)₃Si₁₀O₃₀ crystallizes in the <i>P</i>2₁/<i>c</i> space group and contains open-branched sechser (six) single silicate chains, whereas K₂(UO₂)­Si₂O₆ crystallizes in the <i>C</i>2/<i>c</i> space group and is built of unbranched achter (eight) silicate chains. The crystals of K₁₄(UO₂)₃Si₁₀O₃₀ and K₂(UO₂)­Si₂O₆ are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi₆] heptamers. The triangle diagram for all known <b>A⁺</b>–UO₂²⁺–SiO₄^4– phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of <b>A⁺</b>–UO₂²⁺–BO₃^3–/BO₄^5– compounds. For both thorium silicates, the transformation of K₂ThSi₂O₇ to K₂ThSi₃O₉ was found to be a factor of the reaction time. K₂ThSi₂O₇ crystallizes in the <i>C</i>2/<i>c</i> space group and belongs to the Na₂Si^VISi₂O₇ structure type. Its 3D framework consists of diorthosilicate Si₂O₇ group and ThO₆ octahedra. Noncentrosymmetric K₂ThSi₃O₉ crystallizes in the hexagonal <i>P</i>6₃ space group and adopts mineral wadeite-type structure based upon triorthosilicate Si₃O₉ rings and ThO₆ octahedra. The coordination environment of thorium for all existing oxo-anion compounds including B, Si/Ge, P/As, Cr/Mo/W, and S/Se/Te are summarized and analyzed. Additionally, spectroscopic properties of all novel materials have been studied

Novel Fundamental Building Blocks and Site Dependent Isomorphism in the First Actinide Borophosphates

Three novel uranyl borophosphates, Ag₂(NH₄)₃[(UO₂)₂{B₃O­(PO₄)₄(PO₄H)₂}]­H₂O (<b>AgNBPU-1</b>), Ag_{(2‑<i>x</i>)}(NH₄)₃[(UO₂)₂­{B₂P₅O_{(20‑<i>x</i>)}(OH)_<i>x</i>}] (<i>x</i> = 1.26) (<b>AgNBPU-2</b>), and Ag_{(2‑<i>x</i>)}(NH₄)₃[(UO₂)₂{B₂P_{(5‑<i>y</i>)}­As_<i>y</i>O_{(20‑<i>x</i>)}(OH)_<i>x</i>}] (<i>x</i> = 1.43, <i>y</i> = 2.24) (<b>AgNBPU-3</b>), have been prepared by the H₃BO₃–NH₄H₂PO₄/NH₄H₂AsO₄ flux method. The structure of <b>AgNBPU-1</b> has an unprecedented fundamental building block (FBB), composed of three BO₄ and six PO₄ tetrahedra which can be written as 9□:[Φ] □⟨3□⟩□|□⟨3□⟩□|□⟨3□⟩□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. <b>AgNBPU-2</b> and <b>AgNBPU-3</b> are isostructural and possess a FBB of two BO₄ and five TO₄ (T = P, As) tetrahedra (7□:□⟨4□⟩□|□). <b>AgNBPU-3</b> is a solid solution with some PO₄ tetrahedra of the <b>AgNBPU-2</b> end-member being substituted by AsO₄. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates

Novel Fundamental Building Blocks and Site Dependent Isomorphism in the First Actinide Borophosphates

Three novel uranyl borophosphates, Ag₂(NH₄)₃[(UO₂)₂{B₃O­(PO₄)₄(PO₄H)₂}]­H₂O (<b>AgNBPU-1</b>), Ag_{(2‑<i>x</i>)}(NH₄)₃[(UO₂)₂­{B₂P₅O_{(20‑<i>x</i>)}(OH)_<i>x</i>}] (<i>x</i> = 1.26) (<b>AgNBPU-2</b>), and Ag_{(2‑<i>x</i>)}(NH₄)₃[(UO₂)₂{B₂P_{(5‑<i>y</i>)}­As_<i>y</i>O_{(20‑<i>x</i>)}(OH)_<i>x</i>}] (<i>x</i> = 1.43, <i>y</i> = 2.24) (<b>AgNBPU-3</b>), have been prepared by the H₃BO₃–NH₄H₂PO₄/NH₄H₂AsO₄ flux method. The structure of <b>AgNBPU-1</b> has an unprecedented fundamental building block (FBB), composed of three BO₄ and six PO₄ tetrahedra which can be written as 9□:[Φ] □⟨3□⟩□|□⟨3□⟩□|□⟨3□⟩□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. <b>AgNBPU-2</b> and <b>AgNBPU-3</b> are isostructural and possess a FBB of two BO₄ and five TO₄ (T = P, As) tetrahedra (7□:□⟨4□⟩□|□). <b>AgNBPU-3</b> is a solid solution with some PO₄ tetrahedra of the <b>AgNBPU-2</b> end-member being substituted by AsO₄. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates