[en] (La_1-xPb_x)_1-y□_yMnO₃ with x=0.05-0.5 and y=0, 0.05, 0.1 (where □ is a vacancy) was studied to evaluate the effects of A-site vacancies on the physical properties. In this system manganese perovskites form with tolerance factors close to 1 and low A-site cation size mismatch due to similarities in the effective ionic radii of La³⁺ and Pb²⁺. Increasing vacancy concentration indicates no significant effect on the lattice parameters or volume. However, the vacancies introduce a greater A-site cation size mismatch, which leads to a lowering of the ferromagnetic and metal-insulator transition temperatures, although the transitions are not broadened with increasing vacancy content. Due to the vacancies a distribution of Mn-O-Mn angles and Mn-O distances are created, and long range order in (La_1-xPb_x)_1-y□_yMnO₃ appears to be determined by Mn-O-Mn angles and Mn-O distances which most distort from 180 deg. and are the longest, respectively, in the structure

[en] This article reviews primarily the quasi-low dimensional behavior of molybdenum bronzes, but also some new results on other transition metal bronzes, particularly of the vanadium and tungsten phosphate bronzes are discussed. (author). 168 refs.; 29 figs.; 5 tabs

[en] The series of La_1-xSr_xCoO_3-δ (0.5 ≤ x ≤ 0.9) perovskites have been prepared using the Pechini gel technique. The products were chemically oxidized by stirring in a sodium hypobromite solution. The samples have been characterized by powder X-ray diffraction, thermal analysis, iodometric titration, Co K edge X-ray absorption spectroscopy, temperature-dependent SQUID magnetic susceptibility, and temperature-dependent electrical resistivity. The Sr-rich samples (x > 0.7) have the brownmillerite-type structure prior to oxidation and the cubic perovskite structure after treatment with sodium hypobromite. Iodometric titration shows as much as a ∼14% increase in Co(IV) concentration in the Sr-rich samples after chemical oxidation. The Co K edge spectra show that there is very little change in the formal cobalt valence with increasing Sr content. The effective magnetic moments of the oxidized compounds level off with increasing Sr concentration. All of the chemically oxidized samples exhibit small-bandgap semiconducting behavior. The data lead to the postulation of an equilibrium between Co(IV) and O^- in the series

[en] The crystal and magnetic structures of the ALaMnMoO₆ (A=Ba,Sr) double perovskites have been studied. The Sr compound is monoclinic (space-group P2₁/n) and does not exhibit long-range magnetic order. The crystal lattice of the Ba compound is pseudocubic, although detailed analysis of neutron diffraction spectra together with the synchrotron diffraction data reveal a slight triclinic distortion (space-group I1-bar). The magnitude of the tilting distortion is significantly reduced compared to the Sr compound. Ferrimagnetic order is observed below 80(10) K. The refined Mn moment is 1.35(7)μ_B at 4 K, suggesting that the ordered moment is reduced by strong frustration effects. This frustration is discussed in terms of competition between nearest-neighbor and next-nearest-neighbor superexchange interactions. Frequency dependent ac susceptibility measurements rule out the existence of a spin-glass state

[en] Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0=< x=<1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75< x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2-xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25K, respectively, for 0.75=< x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2-xLa_xMnWO₆

[en] The quasi-one-dimensional conductor BaVS₃ undergoes a metal-insulator transition at T_MI=70 K accompanied by a formation of superstructure doubling the chain periodicity and a magnetic transition at T_X=30 K. We present an anomalous x-ray scattering investigation of the charge-density wave (CDW) satellite reflections at the V K-edge. Our study shows that no charge disproportionation larger than 0.01 electron occurs at T_MI and T_X. We interpret this result by the stabilization of two out-of phase CDW's for the d_z² and e(t_2g) electrons, which induces an orbital ordering while reducing the intrachain Coulomb repulsions

[en] Quantitative study of thermodynamic properties of solid solutions of UO_2+x with divalent and trivalent oxides is important for predicting the behavior of oxide fuel. Although early literature work measured vapor pressure in some of these solid solutions, direct calorimetric measurements of enthalpies of formation have been hampered by the refractory nature of such oxides. First measurements of the enthalpies of formation in the systems UO_2+x-CaO and UO_2+x-YO_1.5, obtained by high-temperature oxide melt solution calorimetry, are reported. Both systems show significantly negative (exothermic) heats of formation from binary oxides (UO₂, plus O₂ and CaO or YO_1.5, as well as from UO₂ plus UO₃ and CaO or YO_1.5), consistent with reported free energy measurements in the urania-yttria system. The energetic contributions of oxygen content (oxidation of U⁴⁺) and of charge balanced ionic substitution as well as defect clustering are discussed. Behavior of urania-yttria is compared to that of corresponding systems in which the tetravalent ion is Ce, Zr, or Hf. The substantial additional stability in the solid solutions compared to pure UO_2+x may retard, in both thermodynamic and kinetic sense, the oxidation and leaching of spent fuel to form aqueous U⁶⁺ and solid uranyl phases

[en] Powder neutron diffraction studies show that CaLaMnMoO₆ double perovskite crystallizes in monoclinic P2₁/n, with a=5.56961(9), b=5.71514(9), c=7.9358(1)A and β=90.043(1)^o. Mn and Mo occupy the 2c and 2d positions, respectively, with 6.0(4)% Mn/Mo anti-site mixing. Temperature-dependent magnetic susceptibility measurements reveal that CaLaMnMoO₆ is ferrimagnetic, with T_N=92(3)K, below which large magnetic frustration is detected. The zero-field magnetic moment measured at 5K is about 1.2μ_B, comparable to that of ALaMnMoO₆ (A=Ba and Sr), but much lower than expected for antiparallel ordering of formally Mn²⁺ (d⁵) and Mo⁵⁺ (d¹). Moreover, no long-range magnetic ordering is observed in neutron diffraction data down to 4K. The magnetic frustration is discussed in the framework of nearest-neighbors next-nearest-neighbors magnetic frustration

[en] A₂MnB'O₆ (A=Ca, Sr; B=Sb, Ta) double perovskites have been synthesized and their structural and magnetic properties have been investigated. Rietveld refinement of the powder X-ray diffraction data for Sr₂MnSbO₆ indicated significant ordering of Mn and Sb at the B-site while all other phases showed mostly a random distribution of the B-site cations. X-ray absorption spectroscopic data established the presence of Mn in the 3+ and Sb/Ta in the 5+ oxidation states in all the phases. Magnetic susceptibility data indicated ferromagnetic correlations for all the A₂MnB'O₆ phases with Weiss temperatures varying from 64 to 107 K. - Graphical abstract: The M vs. H plots for the ordered Sr₂MnSbO₆. The magnetization data at 5 K shows hysteresis loop (inset) with a Brillouin-like curvature indicating significant ferromagnetic correlations in the system

[en] The D₂ clathrate hydrate crystal structure was determined as a function of temperature and pressure by neutron diffraction for the first time. The hydrogen occupancy in the (32+X)H₂·136H₂O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between two and four molecules, while remaining single occupancy of the small (dodecahedral) cage. Above 130-160 K, the guest D₂ molecules were found in the delocalized state, rotating around the centers of the cages. Decrease of temperature results in rotation freezing followed by a complete localization below 50 K