L. Soderholm

The oxidation state of Pr in PrBa2Cu3O7

Abstract PrBa2Cu3O7 is isostructural with YBa2Cu3O7, yet it is not a superconductor. We have performed a variety of experiments to determine the valence of Pr in this material. We report on the results of synthetic trend studies, powder neutron diffraction, X-ray absorption spectra, and magnetic susceptibility experiments and offer our analysis of the contribution that each makes to the overall understanding of the electronic behavior of Pr in this compound. Since Pr4+ is relatively stable, and since Pr is known to form mixed-valent alloys, the suggestion had been made that Pr4+ might be inhibiting superconductivity through an in situ oxidation reduction reaction with the Cuue5f8O bands. We conclude that Pr is essentially trivalent in PrBa2Cu3O7.

Unusual structure, bonding and properties in a californium borate

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

In situ studies of a platform for metastable inorganic crystal growth and materials discovery

Significance Dense inorganic materials comprise most functional electronic, optical, and magnetic devices. Whereas the discovery of new inorganic materials can increase our technical capabilities and uncover new phenomena, the search is difficult due to their formation at high temperatures where only the most stable (often known) materials can be isolated postreaction. We find a variety of unexpected and unknown materials nucleating at moderate temperatures in molten salts. By probing these processes with in situ diffraction, we are able to identify a large variety of new phases quickly and pave a path to more efficient materials discovery. Rapid shifts in the energy, technological, and environmental demands of materials science call for focused and efficient expansion of the library of functional inorganic compounds. To achieve the requisite efficiency, we need a materials discovery and optimization paradigm that can rapidly reveal all possible compounds for a given reaction and composition space. Here we provide such a paradigm via in situ X-ray diffraction measurements spanning solid, liquid flux, and recrystallization processes. We identify four new ternary sulfides from reactive salt fluxes in a matter of hours, simultaneously revealing routes for ex situ synthesis and crystal growth. Changing the flux chemistry, here accomplished by increasing sulfur content, permits comparison of the allowable crystalline building blocks in each reaction space. The speed and structural information inherent to this method of in situ synthesis provide an experimental complement to computational efforts to predict new compounds and uncover routes to targeted materials by design.

Understanding the role of aqueous solution speciation and its application to the directed syntheses of complex oxidic zr chlorides and sulfates

The lack of an in-depth understanding of solution-phase speciation and its relationship to solid-state phase formation is a grand challenge in synthesis science. It has severely limited the ability of inorganic chemists to predict or rationalize the formation of compounds from solutions. The need to investigate mechanisms that underlie self-assembly has motivated this study of aqueous Zr-sulfate chemistry as a model system, with the goal of understanding the structures of oligomeric clusters present in solution. We used high-energy X-ray scattering (HEXS) data to quantify Zr correlations in a series of solutions as a function of sulfate concentration. The pair distribution function (PDF) from the sulfate-free sample reveals that the average oligomeric Zr moiety is larger than the tetrameric building unit, [Zr4(OH)8(H2O)16](8+), generally understood to dominate its solution speciation. At sulfate concentrations greater than 1 m (molal), bidentate sulfate is observed, a coordination not seen in Zr(SO4)2·4H2O (2), which forms upon evaporation. Also seen in solution are correlations consistent with sulfate-bridged Zr dimers and the higher-order oligomers seen in 2. At intermediate sulfate concentrations there are correlations consistent with large Zr hydroxo-/oxo-bridged clusters. Crystals of [Zr18(OH)26O20(H2O)23.2(SO4)12.7]Cl0.6·nH2O (3) precipitate from these solutions. The Raman spectrum of 3 has a peak at 1017 cm(-1) that can be used as a signature for its presence in solution. Raman studies on deuterated solutions point to the important role of sulfate in the crystallization process. These solution results emphasize the presence of well-defined prenucleation correlations on length scales of <1 nm, often considered to be within the structurally amorphous regime.

Tetraalkylammonium uranyl isothiocyanates

Three tetraalkylammonium uranyl isothiocyanates, [(CH(3))(4)N](3)UO(2)(NCS)(5) (1), [(C(2)H(5))(4)N](3)UO(2)(NCS)(5) (2), and [(C(3)H(7))(4)N](3)UO(2)(NCS)(5) (3), have been synthesized from aqueous solution and their structures determined by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO(2)(NCS)(5)(3-), and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds 1-3, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate. By tracking the Raman signatures of thiocyanate, the presence of both free and bound thiocyanate is confirmed in solution. The shift in the Raman signal of the uranyl symmetric stretching mode suggests the formation of higher-order uranyl thiocyanate complexes in solution, while the solid-state Raman data support homoleptic isothiocyanate coordination about the uranyl cation. Presented here are the syntheses and crystal structures of 1-3, pertinent Raman spectra, and a discussion regarding the relationship of these isothiocyanates to previously described uranyl halide phases, UO(2)X(4)(2-).

The magnetic susceptibility of Pr4+ in BaPrO3: Evidence of long-range magnetic order☆

We have reexamined the magnetic behavior of BaPrO3 in the temperature range 4.2 < T < 300 K. An anomaly in the susceptibility at 11.6(1) K has been observed, and is attributed to the onset of long-range ordering of the Pr4+ f-electrons. Above the critical temperature, the paramagnetism is composed of a temperature-dependent term with an effective moment of 0.7(1) μB and a temperature-independent term of 6.9(2) × 10−4 emu/mole. Calculations based on a simple crystal field model explain the low effective moment found experimentally.

Understanding fluxes as media for directed synthesis: in situ local structure of molten potassium polysulfides.

Rational exploratory synthesis of new materials requires routes to discover novel phases and systematic methods to tailor their structures and properties. Synthetic reactions in molten fluxes have proven to be an excellent route to new inorganic materials because they promote diffusion and can serve as an additional reactant, but little is known about the mechanisms of compound formation, crystal precipitation, or behavior of fluxes themselves at conditions relevant to synthesis. In this study we examine the properties of a salt flux system that has proven extremely fertile for growth of new materials: the potassium polysulfides spanning K(2)S(3) and K(2)S(5), which melt between 302 and 206 °C. We present in situ Raman spectroscopy of melts between K(2)S(3) and K(2)S(5) and find strong coupling between n in K(2)S(n) and the molten local structure, implying that the S(n)(2-) chains in the crystalline state are mirrored in the melt. In any reactive flux system, K(2)S(n) included, a signature of changing species in the melt implies that their evolution during a reaction can be characterized and eventually controlled for selective formation of compounds. We use in situ X-ray total scattering to obtain the pair distribution function of molten K(2)S(5) and model the length of S(n)(2-) chains in the melt using reverse Monte Carlo simulations. Combining in situ Raman and total scattering provides a path to understanding the behavior of reactive media and should be broadly applied for more informed, targeted synthesis of compounds in a wide variety of inorganic fluxes.

Oxidation state of uranium in A6Cu12U2S15 (A = K, Rb, Cs) compounds.

Black single crystals of A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia ̅3d at room temperature. The structure comprises a three-dimensional framework built from US(6) octahedra and CuS(3) trigonal planar units with A cations residing in the cavities. There are no S-S bonds in the structure. To elucidate the oxidation state of U in these compounds, various physical property measurements and characterization methods were carried out. Temperature-dependent electrical resistivity measurement on a single crystal of K(6)Cu(12)U(2)S(15) showed it to be a semiconductor. These three A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) compounds all exhibit small effective magnetic moments, < 0.58 μ(B)/U and band gaps of about 0.55(2) eV in their optical absorption spectra. From X-ray absorption near edge spectroscopy (XANES), the absorption edge of A(6)Cu(12)U(2)S(15) is very close to that of UO(3). Electronic band structure calculations at the density functional theory (DFT) level indicate a strong degree of covalency between U and S atoms, but theory was not conclusive about the formal oxidation state of U. All experimental data suggest that the A(6)Cu(12)U(2)S(15) family is best described as an intermediate U(5+)/U(6+) sulfide system of (A(+))(6)(Cu(+))(12)(U(5+))(2)(S(2-))(13)(S(-))(2) and (A(+))(6)(Cu(+))(12)(U(6+))(2)(S(2-))(15).

Changing Hafnium Speciation in Aqueous Sulfate Solutions: A High-Energy X-ray Scattering Study

The relationship of solution speciation and the structures of corresponding precipitates is examined for an aqueous Hf(4+) sulfate series. High-energy X-ray scattering (HEXS) and Raman spectroscopy data are used to probe atomic correlations in solutions. Hf(4+) in acidic perchlorate solution shows no evidence of a mononuclear metal species but instead has a peak in the pair-distribution function (PDF), generated from the HEXS data, at 3.55 Å, indicating Hf(4+)-Hf(4+) solution correlations. The peak intensity is consistent with clusters that are, on average, larger than the tetramic unit [M4(OH)8(H2O)16](8+) usually attributed to Zr(4+) and Hf(4+) solution speciation under these conditions. Addition of sulfate results in a breakup of hydroxo-bridged oligomers into sulfate-capped dimers and, for higher concentrations, Hf-sulfate monomers. The bidentate coordination mode of sulfate dominates the dissolved precursors, although it is not found in the structure of the final crystallized product, which instead is comprised of bridging-bidentate sulfate ligation. Neither the PDF patterns nor the Raman spectra show any evidence of the larger oligomers, such as the octadecameric metal clusters, found in similar Zr(4+) solutions. The oligomeric units found in solution provide insights into possible assembly routes for crystallization. In addition to expanding our understanding of synthesis science this study also reveals differences in the aqueous chemistries between Hf and Zr, two elements with ostensibly very similar chemical behavior.

Dichalcogenide bonding in seven alkali-metal actinide chalcogenides of the KTh2Se6 structure type.

The solid-state compounds CsTh(2)Se(6), Rb(0.85)U(1.74)S(6), RbU(2)Se(6), TlU(2)Se(6), Cs(0.88)(La(0.68)U(1.32))Se(6), KNp(2)Se(6), and CsNp(2)Se(6) of the AAn(2)Q(6) family (A = alkali metal or Tl; An = Th, U, Np; Q = S, Se, Te) have been synthesized by high-temperature techniques. All seven crystallize in space group Immm of the orthorhombic system in the KTh(2)Se(6) structure type. Evidence of long-range order and modulation were found in the X-ray diffraction patterns of TlU(2)Se(6) and CsNp(2)Se(6). A 4a × 4b supercell was found for TlU(2)Se(6) whereas a 5a × 5b × 5c supercell was found for CsNp(2)Se(6). All seven compounds exhibit Q-Q interactions and, depending on the radius ratio R(An)/R(A), disorder of the A cation over two sites. The electrical conductivity of RbU(2)Se(6), measured along [100], is 6 × 10(-5) S cm(-1) at 298 K. The interatomic distances, including those in the modulated structure of TlU(2)Se(6), and physical properties suggest the compounds may be formulated as containing tetravalent Th or U, but the formal oxidation state of Np in the modulated structure of CsNp(2)Se(6) is less certain. The actinide contraction from Th to U to Np is apparent in the interatomic distances.