Roman V. Shpanchenko

On the stability region and structure of the Nd1+xBa2−xCu3Oy solid solution

Abstract We studied the low and upper stability boundaries and the structure evolution of the Nd 1+ x Ba 2− x Cu 3 O z solid solution along its single-phase field. It was found that in oxygen the limit of the solid solution extends up to Nd 1.9 Ba 1.1 Cu 3 O z at 950–1050°C while at lower and higher temperatures it becomes narrower. In nitrogen atmosphere the substitution range is much smaller and x does not exceed 0.3 at 800°C. The replacement of Ba 2+ by Nd 3+ is accompanied by decreasing both a and c lattice constants of quenched samples. At the same time in oxygen at x ≥ 0.6 ordering phenomena occur leading to the orthorhombic distortion of the tetragonal subcell. A model of the solid solution crystal structure with a B-centered unit cell and lattice parameters a = 2 a sub , b = b sub and c = 2 c sub was suggested and refined.

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Abstract Bulk superconductivity with Tc up to 94 K has been induced by fluorination of non-superconducting YBa2Cu3O6.11 using XeF2 as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 A which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa2Cu3O6F2 phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.

Fluorinated Heterometallic β-Diketonates as Volatile Single-Source Precursors for the Synthesis of Low-Valent Mixed-Metal Fluorides

Hexafluoroacetylacetonates that contain lead and divalent first-row transition metals, PbM(hfac)(4) (M = Ni (1), Co (2), Mn (3), Fe (4), and Zn (5)), have been synthesized. Their heterometallic structures are held together by strong Lewis acid-base interactions between metal atoms and diketonate ligands acting in chelating-bridging fashion. Compounds 1-5 are highly volatile and decompose below 350 °C. Fluorinated heterometallic β-diketonates have been used for the first time as volatile single-source precursors for the preparation of mixed-metal fluorides. Complex fluorides of composition Pb(2)MF(6) have been obtained by decomposition of 1-5 in a two-zone furnace under low-pressure nitrogen flow. Lead-transition metal fluorides conform to orthorhombically distorted Aurivillius-type structure with layers of corner-sharing [MF(6)] octahedra separated by α-PbO-type (Pb(2)F(2)) blocks. Pb(2)NiF(6) and Pb(2)CoF(6) were found to exhibit magnetic ordering below 80 and 43 K, respectively. The ordering is antiferromagnetic, with a weak, uncompensated moment due to the canting of spins. The Pb(2)MF(6) fluorides represent a new class of prospective magnetoelectric materials combining transition metals and lone-pair main-group cations.

A new way of synthesis and characterization of superconducting oxyfluoride Sr2Cu(O, F)4 + δ

Abstract Superconducting Cu mixed oxyfluoride, Sr2Cu(O, F)4 + δ, was obtained via fluorination of Sr2CuO3 by XeF2 in the 100–250°C temperature range in a closed Ni container. The prepared samples exhibited a lower Tc in comparison with earlier reported values. Different samples in the SrCuOF system were prepared by a solid-state reaction at 220–400°C, but no formation of the oxyfluoride was detected even with an addition of xenon difluoride. These data allow one to draw conclusions on the metastability of the oxyfluoride under the conditions used. Electron microscopy and X-ray powder diffraction studies revealed large amounts of badly crystallized SrF2; this could be a reason for the small superconducting volume fraction as well as for the inhomogeneous distribution of the anions.

Frustrated spin-1/2 square lattice in the layered perovskite PbVO3

We report on the magnetic properties of the layered perovskite PbVO(3). The results of magnetic susceptibility and specific heat measurements as well as band structure calculations consistently suggest that the S=1/2 square lattice of vanadium atoms in PbVO(3) is strongly frustrated due to next-nearest-neighbor antiferromagnetic interactions. The ratio of next-nearest-neighbor (J(2)) to nearest-neighbor (J(1)) exchange integrals is estimated to be J(2)/J(1)\approx 0.2-0.4. Thus, PbVO(3) is within or close to the critical region of the J(1)-J(2) frustrated square lattice. Supporting this, no sign of long-range magnetic ordering was found down to 1.8 K.

Synthesis and properties of niobium bronzes R1 + xNb3O9 (R = La,Ce,Nd)

Abstract The phases with general formula R 1 + x Nb 3 O 9 (R = La, Ce, Nd, x = 0–0.15) were synthesized and studied by X-ray powder diffraction, resistivity and magnetic susceptibility measurements. The homogeneity range of solid solutions increases concomitantly with the increase of the rare-earth cation radius, and it was found to be 0.05 for Nd, 0.1 for Ce and 0.15 for La at 1200 °C. The crystal structure of La 1.15 Nb 3 O 9 was refined by X-ray powder data. It was found that all La cations occupy only one of two possible cation positions. Resistivity measurements revealed drastic changes of R(T) for increasing x. No diamagnetic effects were detected by susceptibility measurements down to 12K.

Sr2V3O9 and Ba2V3O9: Quasi-one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behavior of the low-dimensional vanadium oxides

Synthesis and Structure of the Double Oxide Pb6Re6O19

{\mathrm{Sr}}_{2}{\mathrm{V}}_{3}{\mathrm{O}}_{9}

The phase transitions and crystal structures of Ba3RM2O7.5 complex oxides (R = rare-earth elements, M = Al, Ga).

and

Crystal structure of Ba5In2Al2ZrO13

{\mathrm{Ba}}_{2}{\mathrm{V}}_{3}{\mathrm{O}}_{9}