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Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM²⁺M³⁺F₆ offer a convenient platform for probing the interplay between electron distribution over M²⁺ and M³⁺ sites and structural distortions; however, they are limited to compounds with M^2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl– anions under hydrothermal conditions for the mild reduction of Mn₂O₃ in concentrated HF to obtain the CsMn²⁺Mn³⁺F₆ composition as a phase pure sample and studymore » its properties. The magnetism of CsMn₂F₆ was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn₂F₆ using powder neutron diffraction, which revealed successive long-range ordering of the Mn²⁺ and Mn³⁺ sites that is accompanied by a second transition. As a result, the role and strength of magnetic exchange interactions were characterized using DFT calculations.« less

The relevance of multidimensional and porous crystalline materials to nuclear waste remediation and storage applications has motivated exploratory research focused on materials discovery of compounds, such as actinide mixed-oxoanion phases, which exhibit rich structural chemistry. The novel phase K1.8Na1.2[(UO2)BSi4O12] has been synthesized using hydrothermal methods, representing the first example of a uranyl borosilicate. The three-dimensional structure crystallizes in the orthorhombic space group Cmce with lattice parameters a = 15.5471(19) Å, b = 14.3403(17) Å, c = 11.7315(15) Å, and V = 2615.5(6) Å3, and is composed of UO6 octahedra linked by [BSi4O12]5− chains to form a [(UO2)BSi4O12]3− framework. The synthesismore » method, structure, results of Raman, IR, and X-ray absorption spectroscopy, and thermal stability are discussed.« less

Abstract Na _n MAn ₆ F ₃₀ is an extremely versatile framework structure for incorporating tetravalent actinides (An) and cerium along with divalent or trivalent d‐metals (M); moreover, the structure exhibits a high resistance to harsh chemical conditions. This extreme robustness can potentially be exploited for the sequestration of plutonium in a stable matrix; however, no Na _n MPu ₆ F ₃₀ compounds have been reported so far. Herein, we present four new plutonium fluorides that have been prepared as single crystals by mild hydrothermal synthesis methods. Structural characterizations revealed their compositions to be Na ₃more » AlPu ₆ F ₃₀ , Na ₃ FePu ₆ F ₃₀ , Na ₃ CoPu ₆ F ₃₀ , and Na _2.4 Mn _1.6 Pu ₆ F ₃₀ . Surprisingly, in the plutonium series, it was found that Co ²⁺ and Mn ²⁺ precursors oxidized to form Na ₃ Co ^III Pu ₆ F ₃₀ and Na _2.4 Mn ^II/III _1.6 Pu ₆ F ₃₀ , whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M ³⁺ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur.« less

Covalency in actinides has emerged as a resounding research topic on account of the technological importance in separating minor actinides from lanthanides for spent nuclear fuel processing, and utilization of their distinct bonding properties has been realized as a route towards overcoming this challenge. Because of the limited radial extent of the 4f orbitals, there is almost no 4f electron participation in bonding in lanthanides; this is not the case for the actinides, which have extended 5f orbitals that are capable of overlapping with ligand orbitals, although not to the degree of overlap as in the d orbitals of transitionmore » metals. In this concept paper, we provide a general description of covalency in actinide compounds. After introducing two main approaches to enhance covalency, either by exploiting increased orbital overlap or decreasing energy differences between the orbitals causing orbital energy degeneracy, we show the current state of the field using several examples from the recent literature. Here, we will conclude by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.« less

The stability of the novel Pu( iv ) silicate, Cs ₂ PuSi ₆ O ₁₅ , was predicted from a combination of crystal chemical reasoning and DFT calculations and confirmed by its synthesis via flux crystal growth.

Molten flux crystal growth is a thriving field for the discovery of uranium oxides.

The synthesis of five new U( iv , v ) and U( v , vi ) silicates points toward moderate supercritical synthesis as a new route to the stabilization of mixed-valent uranium compounds.