Facile Oxide to Chalcogenide Conversion for Actinides Using the Boron–Chalcogen Mixture Method

Abstract

Actinide chalcogenides are of interest for fundamental studies of the behavior of 5f electrons in actinides located in a soft ligand coordination environment. As actinides exhibit an extremely high affinity for oxygen, the synthesis of phase-pure actinide chalcogenide materials free of oxide impurities is a great challenge and, moreover, requires the availability and use of oxygen-free starting materials. Herein, we report a new method, the boron–chalcogen mixture (BCM) method, for the synthesis of phase-pure uranium chalcogenides based on the use of a boron–chalcogen mixture, where boron functions as an “oxygen sponge” to remove oxygen from an oxide precursor and where the elemental chalcogen effects transformation of the oxide precursor into an oxygen-free chalcogenide reagent. The boron oxide can be separated from the reaction mixture that is left to react to form the desired chalcogenide product. Several syntheses are presented that demonstrate the broad functionality of the technique, and thermodynamic calculations that show the underlying driving force are discussed. Specifically, three classes of chalcogenides that include both new (rare earth uranium sulfides and alkali–thorium thiophosphates) and previously reported compounds were prepared to validate the approach: binary uranium and thorium sulfides, oxide to sulfide transformation in solid-state reactions, and in situ generation of actinide chalcogenides in flux crystal growth reactions.

Covalency in Actinide Compounds

“…Considering U as an example, one needs to achieve its highest +6 oxidation state while using low electronegative anions, such as sulfide or selenide, as counter‐ions. At first glance this appears to be a relatively straightforward approach, however, it is indeed challenging to stabilize uranium in the +6 oxidation state in a sulfide environment owing to the tendency of sulfides to reduce uranium from +6 to +4, [22, 23] complicating the rational design of inorganic solid‐state materials with covalent An−Q bonding (An=actinide, Q=chalcogenide).…”

“…The orbital degeneracy,o nt he contrary,c an be relatively easily tuned through the metal atom oxidation state and the nature of the atoms bound to it. To achieve higher covalency,o ne could take advantage of using am etal atom in its highest oxidation states to increasei ts electronegativity,a nd a" ligand" atom that haslow electronegativity.Considering Uasa nexample, one needs to achieve its highest + 6o xidations tate while using low electronegative anions, such as sulfide or selenide, as counter-ions.A tf irst glance this appearst ob ear elatively straightforward approach, however,i ti si ndeed challenging to stabilizeu ranium in the + 6o xidation state in as ulfide environment owing to the tendency of sulfides to reduce uranium from + 6t o+ 4, [22,23] complicating the rational design of inorganic solid-state materials with covalentA n ÀQb onding( An = actinide,Q= chalcogenide).…”

Trends in rare earth thiophosphate syntheses: Rb3Ln(PS4)2 (Ln = La, Ce, Pr), Rb3−xNaxLn(PS4)2 (Ln = Ce, Pr; x = 0.50, 0.55), and RbEuPS4 obtained by molten flux crystal growth

“…Molten flux crystal growth is a popular synthetic method that has become a pillar of modern solid state chemistry due to its continued success in the production of X-ray diffraction quality single crystals of a wide variety of structural families, including metal oxides, [1][2][3] halides, 4,5 and chalcogenides. [6][7][8][9][10][11] The use of alkali halide fluxes in particular has resulted in many new single crystalline compounds, while also having the added benefit of the desired crystalline products being easily removed from the product mixture using common solvents, such as water or methanol.…”

“…Molten flux synthesis has been shown to be an effective approach for the synthesis of thiophosphate compounds and, over the past decades, has resulted in the successful crystal growth of various thiophosphate compositions incorporating many elements, including transition metals, [12][13][14] rare earth metals, [15][16][17][18] and the actinides. 11,19 The thiophosphates have garnered attention in recent years for their high ionic conductivities 20,21 and for their non-linear optical properties. 22,23 In addition, numerous studies have focused simply on investigating the rich structural chemistry of thiophosphates, which were found to exhibit great diversity and modularity in composition and structure due to the many thiophosphate motifs that exists and that have been incorporated into new crystal structures, such as the thiopyrophosphate building block P(V) 2 S 7 4− , 24,25 the ortho-thiophosphate building block P(V)S 4 3− , [26][27][28][29][30][31][32] the hexathiometadiphosphate building block P(V) 2 S 6 4− , 18,33 and the hexathiohypophosphate building block P(IV) 2 S 6 2− .…”

Lanthanide thioborates, an emerging class of nonlinear optical materials, efficiently synthesized using the boron–chalcogen mixture method

“…We recently reported the BCM method as a facile approach for the synthesis of actinide chalcogenides from their oxides by incorporating a mixture of boron and the desired chalcogen into the reagent mixture. 8 The large difference in formation energy of B 2 O 3 ( i.e. Δ f G °(vitreous-B 2 O 3 ) = −1182.5 kJ mol −1 ) over the boron chalcogenide ( e.g.…”