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No abstract available

[en] BaV₁₀O₁₅ can be regarded as a Ba-doped V₂O₃ in which the Ba²⁺ ions substitute in the O^2- close-packed layers. The Ba²⁺ ions order within these layers and direct the occupation of the octahedral sites by V²⁺ and V³⁺ ions resulting in a structure with subtle differences from that of V₂O₃ which can be described in Cmca at room temperature. Magnetic susceptibility data show evidence for two phase transitions at 135 and 40 K. The higher temperature transition at 135 K is shown to be structural in origin to another orthorhombic form, Pbca. The structural transition temperature, T_s, decreases with decreasing V²⁺ content to a minimum value of 105 K. Crystallographic and DSC data support a first-order transition driven by partial bond formation which results in a 7% reduction in the distance between two of the five crystallographically distinct V atoms. There is no conclusive crystallographic evidence for V²⁺/V³⁺ charge ordering in either the Cmca or Pbca forms. Electrical conductivity data show semiconducting behavior above T_s which can be fitted to a small polaron hopping model for the most reduced samples (T_s=135 K). The same sample shows a sharp but not discontinuous decrease in conductivity below T_s, consistent with carrier removal due to bond formation. More oxidized materials with T_s=105 K show a more subtle anomaly. Evidence for correlated (Efros-Shklovskii) variable-range hopping at low temperatures is seen in the T_s=105 K sample from analysis using a Hill-Zabrodski (log dE vs. log T) plot. Thermopower data on the T_s=135 K material show an anomalously small value of S∼+1 μV/K at room temperature which increases to >+200 μV/K upon cooling to 90 K. Plots of dS/dT show evidence for the T_s=135 K phase transition. These results are not consistent with a simple one carrier model for small polaron hopping assuming that the V²⁺ ions are the carriers, which would predict S∼-100 μV/K, but seem to demand a two carrier model with n∼p at room temperature for which n-type carriers are trapped as a result of bond formation at the phase transition as temperature is lowered. The lower temperature phase transition near 40 K is magnetic in origin and will be discussed in a subsequent publication

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[en] The (La/sub x/Y/sub 1-x/)₂Mo₂O₇ system was investigated in the range x = 0.0 to x = 0.5. Single-phase materials exist up to x = 0.4; the x = 0.5 composition has a small impurity contamination. The lattice constants are linear with x and range from 10.224 A (x = 0.0) to 10.461 A (x = 0.5). These lattice constants span the same range as the R₂Mo₂O₇ series from R = Y to R = Nd. In this series, there is a discontinuous change from ferromagnetic long-range order to short-range spin-glass-like order between R = Gd and R = Tb. Yet, the solid solutions all show spin-glass-like properties with maxima in the susceptibility in the 20-25 K range and sample-history-dependent effects at lower temperatures. Deviations from the Curie-Weiss Law occur well above the susceptibility maxima. The Weiss constants change from -61 to + 41 K for x = 0.0 and x = 0.5, respectively, indicating a competition between antiferromagnetic and ferromagnetic exchange interactions. This competition, coupled with the inherent frustration of the Mo⁴⁺ lattice in space group Fd3m is a possible origin of the spin-glass properties

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[en] A phase with the perovskite structure (Pbnm) and a composition YbTiO/sub 2.95/ has been prepared by a high-temperature carbothermic method. Neutron diffraction shows a colinear ferrimagnetic structure at 7 K with Yb and Ti moments antiparallel along the c-axis of the orthorhombic cell and a Yb moment of 1.8(3) μ/sub B/. ¹⁷⁰Yb Moessbauer measurements find a more precise and accurate value of 2.0(1)μ/sub B/ from the maximum hyperfine field. From the temperature dependence of the hyperfine field a T/sub c/ = 42(1) K is found. The Yb sublattice magnetization below T/sub c/ follows a Brillouin function. At low temperature a distribution of hyperfine fields is observed which is attributed to a random distribution of defects surrounding the Yb sites. The magnetic structure is discussed in relation to possible values of the crystal field parameters, especially B₀². 24 references, 4 figures, 3 tables

[en] Crystal structures of CdV₃O₇ and SrV₃O₇ are refined using X-ray powder diffraction data for CdV₃O₇ and a combination of neutron and X-ray powder data for SrV₃O₇. The pseudo two-dimensional feature and possible chain formation of the crystal structures are described. Magnetic properties are examined in the temperature range from 5 to 300 K and magnetic susceptibility data of these insulating antiferromagnets are analyzed according to a linear chain (1-D) Heisenberg model. 19 refs., 10 figs., 5 tabs

[en] The magnetic structure of ScMnO₃ was investigated using polycrystalline samples. Bulk magnetic measurements were carried out revealing a Neel temperature of 130 K, followed by two maxima at 58 K and 40 K. The high-temperature data indicate strong antiferromagnetic coupling between Mn³⁺ ions in the form of a large, negative θ = -943(7) K. Powder neutron diffraction as a function of temperature disclosed magnetic long-range ordering below 130 K, where the chemical and magnetic unit cell have the same volume. A 120 degree magnetic structure was found consistent with the triangular Mn³⁺ sublattice and frustrated antiferromagnetic nearest neighbor coupling. The orientation of the magnetic spins is temperature dependent below 70 K