Crystal structure and magnetism of actinide oxides: A review.

In actinide systems, the 5f electrons experience a uniquely delicate balance of effects and interactions having similar energy scales, which are often difficult to properly disentangle. The interplay of factors such as the dual character of 5f-states, competing interactions, and strong spin-orbit coupling results in magnetically unusual and intriguing behavior: multi-k antiferromagnetic ordering, multipolar ordering, mixed valence configurations, and more. Despite the inherent allure of their exotic properties, the exploratory science of even the more basic, binary systems like the actinide oxides has been extremely limited due to their toxicity, radioactivity, and reactivity. In this article, we provide an overview of the available synthesis techniques for selected binary actinide oxides, including the actinide dioxides, sesquioxides, and a selection of the higher oxides. For these oxides, we review and evaluate the current state of knowledge on their crystal structures and magnetic properties. In many aspects, substantial knowledge gaps exist in the current body of research on actinide oxides related to understanding their magnetic ground states. Bridging these gaps is vital for improving not only a fundamental understanding of these systems but also of future nuclear technologies. To this end, we note the experimental techniques and necessary future investigations which may aid in better elucidating the nature of these fascinating systems.

Synthesis, Characterization, and Magnetic Properties of Uranium-Containing 2H-Perovskite-Related Sulfides: Ba3MUS6 (M = Mn, Fe, Co, Ni) and Ba3Co0.858(5)Mg0.142(5)US6.

The uranium-containing 2H-perovskite-related chalcogenide family of compounds was revisited using the recently developed boron-chalcogen mixture (BCM) method for actinides to aid in their syntheses and to obtain magnetic measurements. Two known 2H-perovskite-related structures, Ba3MnUS6 and Ba3FeUS6, were synthesized using the BCM method and were found to exhibit antiferromagnetic transitions at TN = ∼7.6 and 10.8 K, respectively. Combining the BCM method with the molten flux crystal growth technique resulted in single crystals of three new compositions, Ba3NiUS6, Ba3CoUS6, and Ba3Co0.858(5)Mg0.142(5)US6, the synthesis and characterization of which is reported. Magnetic measurements of Ba3NiUS6 revealed a complex magnetic susceptibility consisting of a weak, glassy, antiferromagnetic transition near 65 K followed by an antiferromagnetic transition at TN = ∼18 K. A reduced radius ratio plot for the existing chalcogenide compositions and new additions to this structure type reported herein is presented to aid in the search for additional 2H-perovskite-related sulfides.

Rb2Ln[Si2O6]F (Ln = Y, Eu-Lu): Flux Synthesis, Structure Determination, and DFT-Calculated Formation Enthalpies of a Series of Mixed-Anion Rare Earth Cyclosilicates.

A new series of mixed-anion alkali rare earth silicate fluorides with composition Rb2Ln[Si2O6]F (Ln = Y, Eu-Lu) has been synthesized via an alkali chloride/fluoride eutectic flux synthetic route. All synthesized compositions crystallize in the tetragonal space group P42/mnm with a 3D framework consisting of LnO4F2 octahedra, tetrasilicate rings, and 1D channels containing alkali metals. A combination of powder X-ray diffraction, single-crystal X-ray diffraction, and luminescence emission spectroscopy was performed to characterize the reaction products. In addition, density functional theory (DFT) calculations were utilized to calculate the 0 K formation enthalpies of the synthesized phases and of hypothetical trivalent actinide analogues to probe the likelihood of the successful synthesis of such trivalent transuranic containing phases, specifically Am and Cm, in the future.

Boron Chalcogen Mixture Method-Assisted Alkali Halide Flux Crystal Growth of an Extensive Family of Quaternary Rare Earth Transition-Metal Thiosilicates: Investigation of Their Structures and Magnetic Properties.

A series of transition-metal-containing rare earth thiosilicates, RE3TM0.5SiS7 (RE = Gd-Yb; TM = Fe, Co, Ni), was obtained via flux crystal growth utilizing the boron chalcogen mixture (BCM) method. The series includes the first reported ytterbium-containing thiosilicates crystallizing in this structure type. The thiosilicates crystallize in the hexagonal crystal system in space group P63. The use of the BCM method to synthesize phase-pure samples of the title compounds for magnetic measurements is discussed, highlighting how the approach avoids some of the difficulties that plague typical chalcogenide syntheses. Magnetic measurements demonstrate that some of the compounds order antiferromagnetically and exhibit transition temperatures below 15 K.

Crystal Growth and Magnetism of Transition Metal Pyrochlore Fluorides.

Geometric magnetic frustration arises when the geometry of a structure prevents the simultaneous fulfillment of nearest-neighbor antiferromagnetic interactions and is commonly observed in lattices that exhibit a triangular topology, such as those found in the pyrochlore structure. Via a mild hydrothermal route, we have synthesized seven quaternary ß-pyrochlore-related fluorides AxM2+xM3+(2-x)F6 (A = Cs and Rb; M2+ = Co2+, Ni2+, and Zn2+; and M3+ = V3+ and Fe3+). Crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy. After adjusting the reaction conditions, phase-pure products of AxM2+xM3+(2-x)F6 were obtained. The magnetic susceptibility and isothermal magnetization data for all seven compounds were collected to interpret the magnetic behavior, which ranged from paramagnetic to antiferromagnetic with and without a ferromagnetic component. We found that the magnetic behavior of the AxM2+xV3+(2-x)F6 pyrochlore structures strongly depends on the presence or absence of unpaired electrons on the M2+ position. The titled pyrochlore compounds, with the exception of the Zn-analogue, can be considered frustrated materials, with frustration indices in the range of 6-13.

Composition Based Oxidation State Prediction of Materials Using Deep Learning Language Models.

Oxidation states (OS) are the charges on atoms due to electrons gained or lost upon applying an ionic approximation to their bonds. As a fundamental property, OS has been widely used in charge-neutrality verification, crystal structure determination, and reaction estimation. Currently, only heuristic rules exist for guessing the oxidation states of a given compound with many exceptions. Recent work has developed machine learning models based on heuristic structural features for predicting the oxidation states of metal ions. However, composition-based oxidation state prediction still remains elusive so far, which has significant implications for the discovery of new materials for which the structures have not been determined. This work proposes a novel deep learning-based BERT transformer language model BERTOS for predicting the oxidation states for all elements of inorganic compounds given only their chemical composition. This model achieves 96.82% accuracy for all-element oxidation states prediction benchmarked on the cleaned ICSD dataset and achieves 97.61% accuracy for oxide materials. It is also demonstrated how it can be used to conduct large-scale screening of hypothetical material compositions for materials discovery.

Flux Synthesis, UV-vis Absorbance, and Magnetism of Cesium Copper Silicates with an Isolated Super-Super Exchange Spin Dimer in Cs6Cu2Si9O23.

Single crystals of four new cesium copper silicates were grown from CsCl/CsF flux. [CsCs4Cl][Cu2Si8O20] is a salt-inclusion compound that crystallizes in space group P4/m with lattice parameters a = 12.2768(3) Å and c = 8.6470(2) Å; Cs6Cu2Si9O23 crystallizes in space group P21/n with lattice parameters a = 15.0763(9) Å, b = 6.9654(4) Å, c = 26.9511(17) Å, and ß = 99.240(2)°; Cs8Cu3Si14O35 crystallizes in space group C2/c with a = 39.2236(13) Å, b = 6.9658(2) Å, c = 14.9115(5) Å, and ß = 97.1950(10)°; and Cs2CuSi3O8 is a member of the stuffed tridymite family and crystallizes in a monoclinic distortion of the CsAlSiO4 structure type with space group C2/m and a = 12.8587(3) Å, b = 5.38510(10) Å, c = 9.0440(2) Å, and ß = 133.2580(10)°. All four compounds contain CuO4-flattened tetrahedra. The degree of flattening can be correlated with the UV-vis spectra. Cs6Cu2Si9O23 exhibits spin dimer magnetism that can be attributed to super-super-exchange between two Cu(II) ions connected by a silicate tetrahedron. The other three compounds all show paramagnetic behavior down to 2 K.

Polymorphism in A3MF6 (A = Rb, Cs; M = Al, Ga) grown using mixed halide fluxes.

Single crystals of A3MF6 (A = Rb, Cs; M = Al, Ga) were grown from mixed alkali chloride/fluoride fluxes in sealed silver tubes. For Cs3AlF6 and Cs3GaF6, two polymorphs were observed at room temperature: m-Cs3MF6 and o-Cs3MF6. For the two Rb containing compositions, only one room temperature polymorph was observed: o-Rb3AlF6 and t-Rb3GaF6, respectively. Simultaneous TGA/DSC and high temperature SCXRD/PXRD were used to study the high temperature behavior of A3MF6. The compounds of all four compositions were found to undergo structure transitions upon heating to the same cubic structure type, c-A3MF6.

Flux Crystal Growth of the Extended Structure Pu(V) Borate Na2(PuO2)(BO3).

As part of our exploration of plutonium-containing materials as potential nuclear waste forms, we report the first extended structure Pu(V) material and the first Pu(V) borate. Crystals of Na2(PuO2)(BO3) were grown out of mixed hydroxide/boric acid flux and found to crystallize in the orthorhombic space group Cmcm with lattice parameters of a = 9.9067(4) Å, b = 6.5909(2) Å, and c = 6.9724(2) Å. Na2(PuO2)(BO3) adopts a layered structure in which layers of PuO2(BO3)2- are separated by sodium cations. Plutonium is found in a pentagonal bipyramidal coordination environment, with axial Pu(V)-O plutonyl bond lengths of 1.876(3) Å and equatorial Pu-O bond lengths ranging from 2.325(5) to 2.467(3) Å. We find that the Pu(V)-O plutonyl bond lengths are approximately 0.1 Å longer than the reported Pu(VI)-O plutonyl bond lengths and shorter by approximately 0.033 Å than the corresponding U(V) uranyl bond lengths. Raman spectroscopy on single crystals was used to determine the PuO2+ plutonyl stretching and the equatorial breathing mode frequencies of the pentagonal bipyramidal coordination environment around plutonium. Density functional theory calculations were used to calculate the Raman spectrum to help identify the Raman bands at 690 and 630 cm-1 as corresponding to the plutonyl(V) ν1 stretch and the equatorial PuO5 breathing mode, respectively. UV-vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.

Developing Waste Forms for Transuranic Elements: Quaternary Neptunium Fluorides of the Type NaxMNp6F30 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Al, Ga).

A series of quaternary Np(IV) fluorides was synthesized using a mild hydrothermal synthesis approach. The compositions are all of the type NaxMNp6F30, where M = Ti(III), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Al(III), and Ga(III) and x = 4 for divalent metals, x = 3 for trivalent metals. The compounds all crystallize in the P-3c1 space group and are isotypic with actinide analogues NaxMAn6F30 (An = Ce, U, Th, Pu). Structure data from the neptunium crystals were combined with data from the other actinide analogues to establish the tetravalent, nine-coordinated ionic radii of neptunium (1.030(2) Å), plutonium (1.014(1) Å), and cerium (1.012(2) Å). Radiation damage studies were also carried out on a surrogate material, the cerium analogue Na3AlCe6F30, which indicates that the structure type has low resistance to amorphization. Density functional theory calculations were carried out to compute the band gaps and enthalpies of formation variations among the isotypic cerium and actinide structures to compare the stability of the structures.

f-block MOFs: A Pathway to Heterometallic Transuranics.

A novel series of heterometallic f-block-frameworks including the first examples of transuranic heterometallic 238 U/239 Pu-metal-organic frameworks (MOFs) and a novel monometallic 239 Pu-analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)-MOF formation and reported the first value of a U-to-Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid-state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An-integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An-containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.

Systematic literature review of the performance characteristics of Chebyshev polynomials in machine learning applications for economic forecasting in low-income communities in sub-Saharan Africa.

Chebyshev polynomials have unique properties that place them in a class of functions that are highly efficient in the approximation of non-linear functions. Machine learning techniques are being applied to solve complex non-linear problems in the financial markets where there is a proliferation of financial products. The techniques for valuing diverse portfolios of these products can be time consuming and expensive. Formal research has been conducted to determine how machine learning can considerably reduce the computational effort without losing accuracy. The objective of this systematic literature review is to discover evidence of research on the optimal use of Chebyshev polynomials in machine learning and neural networks that may be used for the estimation of generalized financial outcomes of large clusters of small economic units in low-income communities in sub-Saharan Africa. Scopus, ProQuest, and Web of Science databases were queried with search criteria designed to recover peer-reviewed research articles that addressed this objective. Many articles discussing broader applications in engineering, computer science, and applied mathematics were found. Several articles provided insights into the challenges of forecasting stock price outcomes from unpredictable market activities, and in investment portfolio valuations. One article addressed specific environmental issues relating to energy, biology, and ecological situations, and presented encouraging results. While the literature search did not find any similar articles that address economic forecasting for low-income communities, the applications and techniques used in stock market forecasting and portfolio valuations can contribute to formative theory on sustainable development. There is currently no theoretical underpinning of sustainable development initiatives in developing countries. A framework for small business structures, data collection, and near real-time processing is proposed as a potential data-driven approach to guide policy decisions and private sector involvement. Supplementary Information: The online version contains supplementary material available at 10.1007/s43546-022-00328-w.

Crystallization of A3Ln(BO3)2 (A = Na, K; Ln = Lanthanide) from a Boric Acid Containing Hydroxide Melt: Synthesis and Investigation of Lanthanide Borates as Potential Nuclear Waste Forms.

A series of alkali metal rare-earth borates were prepared via high-temperature flux crystal growth, and their structures were characterized by single crystal X-ray diffraction (SXRD). Na3Ln(BO3)2 (Ln = La-Lu) crystallize in the monoclinic space group P21/n, the potassium series K3Ln(BO3)2 (Ln = La-Tb) crystallize in the orthorhombic space group Pnma, while the Ln = Dy, Ho, Tm, Yb analogues crystallize in the orthorhombic space group Pnnm. To demonstrate the generality of this synthetic technique, high-entropy oxide (HEO) compositions K3Nd0.15(1)Eu0.20(1)Gd0.20(1)Dy0.22(1)Ho0.23(1)(BO3)2 and K3Nd0.26(1)Eu0.29(1)Ho0.22(1)Tm0.14(1)Yb0.10(1)(BO3)2 were obtained in single crystal form. Radiation damage investigations determined that these borates have a high radiation damage tolerance. To assess whether trivalent actinide analogues of Na3Ln(BO3)2 and K3Ln(BO3)2 would be stable, density functional theory was used to calculate their enthalpies of formation, which are favorable.

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

The Boron-Chalcogen Mixture method was used to obtain single crystals of the previously extremely difficult to synthesize lanthanide orthothioborates to investigate their structures and their structurally connected optical behavior, such as second harmonic generation. Using a combined halide and polychalcogenide flux, the BCM method yielded single crystals of LnBS3 (Ln = La, Ce, Pr, Nd), which are isostructural and crystallize in the non-centrosymmetric space group, Pna21. Second harmonic generation measurements confirmed the expectation that LaBS3 would exhibit a strong SHG response, measured at 1.5 × KDP.

Luminescence and Scintillation of [Nb2O2F9]3--Dimer-Containing Oxide-Fluorides: Cs10(Nb2O2F9)3F, Cs9.4K0.6(Nb2O2F9)3F, and Cs10(Nb2O2F9)3Cl.

We report three novel Nb-containing oxide-fluorides, Cs10(Nb2O2F9)3F, Cs9.4K0.6(Nb2O2F9)3F, and Cs10(Nb2O2F9)3Cl, which were prepared as high-quality single crystals via a HF-based mild hydrothermal route. The compounds all crystallize in the trigonal crystal system with space group P3̅m1. All three compositions form the same framework structure consisting of isolated [Nb2O2F9]3- dimers that create hexagonal channels that are occupied by disordered halide species. Upon excitation by UV light at room temperature, these compounds display broad band emission with a maximum at 440 nm for Cs10(Nb2O2F9)3F. The broad band emission of these compounds is attributed to the charge-transfer transitions of Nb-O bonds within the [Nb2O2F9]3- dimers. All three compounds scintillate blue under X-ray irradiation. Radioluminescence (RL) measurements performed on Cs10(Nb2O2F9)3F demonstrate that the RL emission intensity decreases with increasing temperature and that the integrated RL emission (300-750 nm) is 4% of Bi4Ge3O12 (BGO) powder. Thermogravimetric analysis confirms that Cs10(Nb2O2F9)3F has excellent thermal stability up to 600 °C and no structural phase transition is observed prior to sample decomposition.

Investigation of Metastable Low Dimensional Halometallates.

The solvothermal synthesis, structure determination and optical characterization of five new metastable halometallate compounds, [1,10-phenH][Pb3.5I8] (1), [1,10-phenH2][Pb5I12]·(H2O) (2), [1,10-phen][Pb2I4] (3), [1,10-phen]2[Pb5Br10] (4) and [1,10-phenH][SbI4]·(H2O) (5), are reported. The materials exhibit rich structural diversity and exhibit structural dimensionalities that include 1D chains, 2D sheets and 3D frameworks. The optical spectra of these materials are consistent with bandgaps ranging from 2.70 to 3.44 eV. We show that the optical behavior depends on the structural dimensionality of the reported materials, which are potential candidates for semiconductor applications.

Synthesis of Hydrated Ternary Lanthanide-Containing Chlorides Exhibiting X-ray Scintillation and Luminescence.

A series of new ternary lanthanide-based chlorides, Cs2EuCl5(H2O)10, Cs7LnCl10(H2O)8 (Ln = Gd or Ho), Cs10Tb2Cl17(H2O)14(H3O), Cs2DyCl5(H2O)6, Cs8Er3Cl17(H2O)25, and Cs5Ln2Cl11(H2O)17 (Ln = Y, Lu, or Yb), were prepared as single crystals via a facile solution route. The compounds with compositions of Cs7LnCl10(H2O)8 (Ln = Gd or Ho) and Cs5Ln2Cl11(H2O)17 (Ln = Y, Lu, or Yb) crystallize in a monoclinic crystal system in space groups C2 and P21/c, respectively, whereas Cs2EuCl5(H2O)10, Cs10Tb2Cl17(H2O)14(H3O), and Cs8Er3Cl17(H2O)25 crystallize in orthorhombic space groups Pbcm, Pnma, and P212121, respectively. Cs2DyCl5(H2O)6 crystallizes with triclinic symmetry in space group P1̅. All of these compounds exhibit complex three-dimensional structures built of isolated lanthanide polyhedral units that are linked together by extensive hydrogen bonds. Cs2EuCl5(H2O)10 and Cs10Tb2Cl17(H2O)14(H3O) luminesce upon irradiation with 375 nm ultraviolet light, emitting intense orange-red and green color, respectively, and Cs10Tb2Cl17(H2O)14(H3O) scintillates when exposed to X-rays. Radioluminescence (RL) measurement of Cs10Tb2Cl17(H2O)14(H3O) in powder form shows that the RL emission integrated in the range of 300-750 nm was ∼16% of BGO powder.

A Geometrically Frustrated Family of MIIMIIIF5(H2O)2 Mixed-Metal Fluorides with Complex Magnetic Interactions.

Inverse weberites are of interest as geometrically frustrated magnetic materials due to their unique cation arrangement. We have synthesized nine isostructural materials that adopt the inverse weberite crystal structure, which consists of cross-linked kagome layers. These materials, having the general formula MIIMIIIF5(H2O)2 (MII = Co, Mn, Ni, Zn; MIII = Ga, Cr, Fe, V), were synthesized using mild hydrothermal conditions, which yielded phase-pure samples after optimization of the reaction conditions. Their crystal structures and optical, thermal, and magnetic behavior were characterized using single-crystal X-ray diffraction, UV-vis spectroscopy, thermogravimetric analysis, and measurement of the magnetic susceptibility and isothermal magnetization data, respectively. Three distinct types of magnetism were observed, including simple paramagnetism, antiferromagnetism, and canted antiferromagnetism; the last type is accompanied by a high frustration index fin the range 4.16-8.09. We demonstrated that the magnetic behavior of inverse weberites depends on the presence or absence of unpaired-electron-containing cations on the two distinct crystallographic sites, which can be employed for the prediction of the magnetic properties of other compounds in this rich and diverse family.

Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles.

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu3 S4 and EuSm2 S4 . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu3 S4 , EuSm2 S4 , SmS1.9 , and Sm3 S4 . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl2 formed EuS while EuI2 formed Eu3 S4 , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI2 initially forms EuS, and converts over time to Eu3 S4 .

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.