Two- and Three-Dimensional Open-Framework Uranium Arsenates: Synthesis, Structure, and Characterization

Hydrothermal reactions between uranium salts and arsenic pentoxide in the presence of two different amines yielded six new uranium arsenate phases exhibiting open-framework structures, ethylenediamine (en): [C2N2H9]-[(UO2)(ASO(4))] I; [C2N2H10][(UO2)F(HASO(4))]2 center dot 4H(2)O, II; [C2N2H9][U2F5(HASO(4))(2)], III; [C2N2H9][UF2(ASO(4))], IV; diethylenetriamine (DETA), [C4N3H16][U2F3(ASO(4))(2)(HAsO4)] V; and [C4N3H16][U2F6(AsO4)(HAsO4)], VI. The structures were determined using single crystal studies, which revealed two- (I, II, V) and three-dimensional (III, IV, VI) structures for the uranium arsenates. The uranium atom, in these compounds, exhibits considerable variations in the coordination (6 to 9) that appears to have some correlation with the synthetic conditions. The water molecules in [C2N2H10][(UO2)F(HAsO4)](2 center dot)4H(2)O, II, could be reversibly removed, and the dehydrated phase, [C2N2H10][(UO2)F(HAsO4)](2), IIa, was also characterized using single crystal studies. The observation of many mineralogical structures in the present compounds suggests that the hydrothermal method could successfully replicate the geothermal conditions. As part of this study, we have observed autunite, Ca[(UO2)(PO4)](2)(H2O)(11), metavauxite, [Fe(H2O)(6)][Al(OH)(H2O)(PO4)](2), finarite, PbCU(SO4)(OH)(2), and tancoite, LiNa2H[Al(PO4)(2)(OH)], structures. The repeated observation of the secondary building unit, SBU-4, in many of the uranium arsenate structures suggests that these are viable building units. Optical studies on the uranium arsenate compound, [C4N3H16][U2F6(AsO4)(HASO(4))), VI, containing uranium in the +4 oxidation state indicates a blue emission through an upconversion process. The compound also exhibits antiferromagnetic behavior.

Doubly spectral stochastic finite element method (DSSFEM) for random field problems

Uncertainties in complex dynamic systems play an important role in the prediction of a dynamic response in the mid- and high-frequency ranges. For distributed parameter systems, parametric uncertainties can be represented by random fields leading to stochastic partial differential equations. Over the past two decades, the spectral stochastic finite-element method has been developed to discretize the random fields and solve such problems. On the other hand, for deterministic distributed parameter linear dynamic systems, the spectral finite-element method has been developed to efficiently solve the problem in the frequency domain. In spite of the fact that both approaches use spectral decomposition (one for the random fields and the other for the dynamic displacement fields), very little overlap between them has been reported in literature. In this paper, these two spectral techniques are unified with the aim that the unified approach would outperform any of the spectral methods considered on their own. An exponential autocorrelation function for the random fields, a frequency-dependent stochastic element stiffness, and mass matrices are derived for the axial and bending vibration of rods. Closed-form exact expressions are derived by using the Karhunen-Loève expansion. Numerical examples are given to illustrate the unified spectral approach.

Evolution of superconductivity in PrFe1-xCoxAsO (x=0.0-1.0)

We report the synthesis and physical property characterization of Prfe(1-x)Co(x)AsO (x=0.0-1.0). The studied samples are synthesized by through the solid state reaction route via the vacuum encapsulation method. The pristine compound PrFeAsO does not show superconductivity, but rather exhibits a metallic step like transition due to spin density wave (SOW) ordering of Fe moments (Fe-SDW) below 150 K, Followed by another upward step due to anomalous ordering of Pr moments (Pr-TN) at 12 K. Both the Fe-SDW and Pr-TN temperatures decrease monotonically with Co substitution at Fe site Superconductivity appears in a narrow range of x from 0.07 to 0.25 with maximum T-c at 11.12 K for x=0.15. Samples with x >= 0.25 exhibit metallic behavior right from 300 K down to 2 K, without any Fe-SDW or Pr-TN steps in resistivity. In fact, though Fe-SDW decreases monotonically, the pr(TN) disappeared even with x=0.02. The magneto transport measurements below 14 Ton superconducting polycrystalline Co doped Pi FeAs0 lead to extrapolated values of the upper critical fields H-c2(0)] of up to 60 T. (C) 2014 Elsevier Ltd. All rights reserved.