Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component.

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 M2+/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 study 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. The role and strength of magnetic exchange interactions were characterized using DFT calculations.

Covalency in Actinide Compounds.

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 transition metals. In this concept paper, a general description of covalency in actinide compounds is provided. 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, the current state of the field is illustrated by using several examples from the recent literature. This paper is concluded by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.

Expansion of the Na3 MIII (Ln/An)6 F30 Series: Incorporation of Plutonium into a Highly Robust and Stable Framework.

Nan MAn6 F30 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 Nan MPu6 F30 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 Na3 AlPu6 F30 , Na3 FePu6 F30 , Na3 CoPu6 F30 , and Na2.4 Mn1.6 Pu6 F30 . Surprisingly, in the plutonium series, it was found that Co2+ and Mn2+ precursors oxidized to form Na3 CoIII Pu6 F30 and Na2.4 MnII/III 1.6 Pu6 F30 , whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M3+ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur.

Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho).

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho) were grown as large single crystals (∼0.1-1 mm3) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs2NaLn(PS4)2 series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm3+ and Gd3+ causes a subtle change in coordination number from 9 (for Ln = La-Sm) to 8 (for Ln = Gd-Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs2NaLn(PS4)2 (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.

Crystal Growth of Alkali Uranyl Borates from Molten Salt Fluxes: Characterization and Ion Exchange Behavior of A2(UO2)B2O5 (A = Cs, Rb, K).

A new family of layered alkali uranyl borates, A2(UO2)B2O5 (A = Cs, Rb, K), was synthesized as high quality single crystals via high temperature flux growth methods. At room temperature, the compounds are structurally closely related although they crystallize in different monoclinic space groups, specifically P21/c (Cs), C2/m (Rb), and C2/c (K). At a low temperature (100 K), Cs2(UO2)B2O5 becomes isostructural with K2(UO2)B2O5 as the result of a reversible structure transition by Cs2(UO2)B2O5. The title phases represent the first examples of uranyl borates resulting from high temperature flux growth utilizing alkali halide fluxes. The synthesis, structures, and thermal, optical, and ion exchange properties are reported, and modeling of the atomic structure and disorder of the ion exchanged phases is discussed.

NaGaS2 : An Elusive Layered Compound with Dynamic Water Absorption and Wide-Ranging Ion-Exchange Properties.

Most ternary sulfides belonging to the MGaS2 structure-type have been known for many years and are well-characterized. Surprisingly, there have been no reports of the NaGaS2 composition, which contains Na, a monovalent cation slightly larger in size than Li, found in LiGaS2 , a compound known for its non-linear optical properties. Now it is demonstrated for the first time that the unique reversible water absorption in NaGaS2 has resulted in its absence from previous reports owing to difficulties encountered when characterizing this compound by SC XRD. The layered structure of this compound coupled with uniquely easy migration of water molecules between the layers allows for ion exchange with 3d and 5f metal cations. Some cations, for example, Ni2+ , facilitate exfoliation of the layers, providing a facile synthetic route to a new class of 2D chalcogenide materials and furthermore demonstrating that NaGaS2 can readily uptake uranyl species from aqueous solutions.

3d-Metal Induced Magnetic Ordering on U(IV) Atoms as a Route toward U(IV) Magnetic Materials.

Uranium(IV) 5f2 magnetism is dominated by a transition from a triplet to a singlet ground state at low temperatures. For the first time, we achieved magnetic ordering of U(IV) atoms in a complex fluoride through the incorporation of 3 d transition metal cations. This new route allowed us to obtain an unprecedented series of U(IV) ferrimagnetic materials of the new composition Cs2MU3F16 (M = Mn2+, Co2+, and Ni2+), which were comprehensively characterized with respect to their structural and magnetic properties. Magnetic susceptibility measurements revealed ferromagnetic-like phase transitions at temperatures of ∼14.0, 3.5, and 4.8 K for M = Mn2+, Co2+, and Ni2+, respectively. The transition is not observed when the magnetic M cations are replaced by a diamagnetic cation, Zn2+. Neutron diffraction measurements revealed the magnetic moments of 0.91(6)-1.97(3) µB on the U atoms, which are only partially compensated by antiparallel moments of 1.53(14)-3.26(5) µB on the 3 d cations. This arrangement promotes suppression of the transition to a diamagnetic ground state characteristic of U(IV), and in doing so, induces magnetic ordering on uranium via 3 d-5 f exchange coupling.

Size-Driven Stability of Lanthanide Thiophosphates Grown from an Iodide Flux.

To determine the influence of the lanthanide size on the structures and properties of thiophosphates, a thiophosphate series containing different lanthanides was synthesized via high temperature flux crystal growth and their structures and physical properties analyzed and compared. Layered thiohypophosphates NaLnP2S6 (Ln = La, Ce, Pr) and thiopyrophosphates CsLnP2S7 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, Y) were grown out of an iodide flux using consistent reaction conditions across both series. Under the mildly reducing iodide flux reaction conditions, a rather rare example of phosphorus reduction from the +5 to the +4 oxidation state was observed. Both resultant structure types are based on lanthanide thiophosphate sheets with the alkali cations located between them. Magnetic susceptibility measurements were conducted and revealed Curie-Weiss behavior of the samples, with a Van Vleck contribution in the CsSmP2S7 sample. UV-vis data was found to be in good agreement with the literature, indicating little influence of the sulfide environment on the localized 4f orbitals.

Na2(UO2)(BO3): An All-Uranium(V) Borate Synthesized under Mild Hydrothermal Conditions.

The first entirely pentavalent uranium borate, Na2(UO2)(BO3), was synthesized under mild hydrothermal conditions. The single-crystal structure was solved in the orthorhombic space group Cmcm with a = 10.0472(3) Å, b = 6.5942(2) Å, and c = 6.9569(2) Å. Magnetic susceptibility measurements revealed an antiferromagnetic transition at 12 K and an effective magnetic moment of 2.33 µB. Density functional theory calculations indicated dynamic stability of the structure above 0 K.

Chirality and polarity in the f-block borates M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, and Cf).

The reactions of trivalent lanthanides and actinides with molten boric acid in high chloride concentrations result in the formation of M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, Cf). This cubic structure type is remarkably complex and displays both chirality and polarity. The polymeric borate network forms helical features that are linked via two different types of nine-coordinate f-element environments. The f-f transitions are unusually intense and result in dark coloration of these compounds with actinides.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions.

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCln; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mn+) on redox reactions accessible to lanthanide ions; Ln3+ + e1- → Ln2+ (Ln = Eu, Yb, Sm; e1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu2+ instantaneously formed when Eu3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca2+ from CaCl2) via the Eu3+ + Cl1- → Eu2+ + ½Cl2 reaction. Conversely, molten salts with less polarizing outer-sphere M1+ cations (e.g., K1+ in KCl) stabilized Ln3+. For instance, the Eu3+/Eu2+ reduction potential was >0.5 V more positive in CaCl2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mn+ cations (high polarization power) inductively removed electron density from Lnn+ across Ln-Cl⋯Mn+ networks and stabilized electron-rich and low oxidation state Ln2+ ions. Conversely, less polarizing Mn+ cations (like K1+) left electron density on Lnn+ and stabilized electron-deficient and high-oxidation state Ln3+ ions.

Quaternary cerium(iv) containing fluorides exhibiting Ce3F16 sheets and Ce6F30 frameworks.

A series of new Ce(iv) based fluorides with two different compositions, Cs2MCe3F16 (M = Ni2+, Co2+, Mn2+, and Zn2+) and Na3MCe6F30 (M = Al3+, Ga3+, Fe3+, and Cr3+) were synthesized as high quality single crystals via a mild hydrothermal route. The compounds with the composition Cs2MCe3F16 (M = Ni2+, Co2+, Mn2+, and Zn2+) crystallize in the hexagonal crystal system with space group P63/mmc and are isotypic with the uranium analogs, whereas the Na3MCe6F30 (M = Al3+, Ga3+, Fe3+, and Cr3+) compounds crystallize in the trigonal space group P3[combining macron]c1 and are isotypic with the uranium and thorium analogs NaxMM'6F30 (M' = Th, U). The Cs2MCe3F16 compounds exhibit a complex 3D crystal structure constructed of edge-sharing cerium trimers, in which all three Ce atoms share a common µ3-F unit. The Na3MCe6F30 compounds are constructed of edge- and vertex-sharing cerium polyhedra connected to each other to form Ce6F306- framework, which can accommodate only relatively smaller trivalent cations (M3+ = Al3+, Ga3+, Fe3+, and Cr3+) as compared to uranium and thorium analogs. Magnetic susceptibility measurements were carried out on the samples of Cs2MCe3F16 (M = Ni2+ and Co2+), which exhibit paramagnetic behavior.

"Soft" Alkali Bromide and Iodide Fluxes for Crystal Growth.

In this review we discuss general trends in the use of alkali bromide and iodide (ABI) fluxes for exploratory crystal growth. The ABI fluxes are ionic solution fluxes at moderate to high temperatures, 207 to ~1,300°C, which offer a good degree of flexibility in the selection of the temperature profile and solubility. Although their main use is to dissolve and recrystallize "soft" species such as chalcogenides, many compositions with "hard" anions, including oxides and nitrides, have been obtained from the ABI fluxes, highlighting their unique versatility. ABI fluxes can serve to provide a reaction and crystallization medium for different types of starting materials, mostly the elemental and binary compounds. As the use of alkali halide fluxes creates an excess of the alkali cations, these fluxes are often reactive, incorporating one of its components to the final compositions, although some examples of non-reactive ABI fluxes are known.

Targeting complex plutonium oxides by combining crystal chemical reasoning with density-functional theory calculations: the quaternary plutonium oxide Cs2PuSi6O15.

The stability of the novel Pu(iv) silicate, Cs2PuSi6O15, was predicted from a combination of crystal chemical reasoning and DFT calculations and confirmed by its synthesis via flux crystal growth. Formation enthalpies of the A2MSi6O15 (A = Na-Cs; M = Ce, Th, U-Pu) compositional family were calculated and indicated the Cs-containing phases should preferentially form in the Cmc21 structure type, consistent with previous experimental findings and the novel phases produced in this work, Cs2PuSi6O15 and Cs2CeSi6O15. The formation enthalpies of a second set of compositions, A2MSi3O9, were also calculated and a comparison between the two compositional families correctly predicted A2MSi6O15 to be on average more stable than A2MSi3O9.

Flux crystal growth: a versatile technique to reveal the crystal chemistry of complex uranium oxides.

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide containing materials, reviews the history of flux crystal growth of uranium containing phases, and highlights the recent advances in the field. Specifically, we discuss how recent developments in f-element materials, fueled by accelerated materials discovery via crystal growth, have led to the synthesis and characterization of new families of complex uranium containing oxides, namely alkali/alkaline uranates, oxychlorides, oxychalcogenides, tellurites, molybdates, tungstates, chromates, phosphates, arsenates, vanadates, niobates, silicates, germanates, and borates. An overview of flux crystal growth is presented and specific crystal growth approaches are described with an emphasis on how and why they - versus some other method - are used and how they enable the preparation of specific classes of new materials.

Moderate supercritical synthesis as a facile route to mixed-valent uranium(iv,v) and (v,vi) silicates.

Mixed-valent uranium(iv,v) and (v,vi) phases represent a unique subset of known uranium compounds. Efforts to develop our current understanding of these materials have pointed to hydrothermal methods as effective preparative techniques. Herein we report the successful use of moderate supercritical conditions for the synthesis of five new U(v) containing phases.