O.G. D'yachenko

Synthesis and characterization of reduced niobates CaLnNb2O7, Ln = Y, Nd with a pyrochlore structure

The first known reduced niobates with a pyrochlore structure, CaLnNb(2)O(7) Ln = Y, Nd, have been synthesized from Ca2Nb2O7, Ln(2)O(3), and NbO2 in sealed Nb ampoules heated at 1400 degrees C for 6 ...

Synthesis and investigation of novel Mn-based oxyfluoride Sr2Mn2O5–xF1+x

A new oxyfluoride Sr 2Mn2O5–xF1+x was synthesized by low temperature fluorination of Sr 2Mn2O5 with XeF2 and studied using X-ray powder diffraction, electron diffraction and high resolution electron microscopy. The oxyfluoride has a tetragonal perovskite-like structure with cell parameters a = 3.8069(8) A, c = 3.989(2) A (S.G. P4/mmm). No additional superstructure or lattice distortions were detected by electron microscopy. The Mn oxidation state was evaluated using bond valence sum calculations as close to VMn =+ 3.5. The tetragonal distortion of the perovskite structure arises from the Jahn–Teller deformation of the Mn(O,F) 6 octahedra.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Synthesis, structure, and resistivity properties of K1 − xBaxNbO3 (0.2 ≤ x ≤ 0.5) and K0.5Sr0.5NbO3

The K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=}x{<=}0.5), BaNbO{sub 3} and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} compounds have been synthesized by high-pressure high-temperature technique in a Belt-type apparatus. The synthesis of the K-containing phases carried out in evacuated sealed silica tubes led to the preparation of polyphasic samples. The phases obtained have an ideal perovskite-like structure. The a parameter of Ba-containing compounds increases concomitantly with increasing Ba concentration. Studies of the K-containing phases by X-ray powder diffraction did not reveal any distortion from cubic symmetry. Electron Dispersive Spectroscopy (EDS)analysis confirmed the cation nominal stoichiometry. Resistivity measurements showed semiconducting-like type of conductivity for the K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=} x {<=} 0.5) and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} samples, but the relative increase of {rho} with the decrease of temperature became smaller for the phases with lower Nb valence, and moreover K{sub 0.5}Sr{sub 0.5}NbO{sub 3} exhibited a slight decrease of the resistivity in the 100--4.2 K temperature range. On the contrary, BaNbO{sub 3} after high-pressure treatment showed a metallic-like behavior down to 4.2 K.

The structures of Nd1+xBa2−xCu3O7−δ (x = 0.05, 0.22, 0.42) phases refined from X-ray single-crystal data

Abstract Crystal structures of Nd 1+ x Ba 2− x Cu 3 O 7−δ phase ( x =0.05, 0.22 and 0.42) were refined from X-ray single-crystal data (tetragonal system, space group P4/mmm, Z =1, R =0.027, 0.027, 0.025 respectively). The main differences between these similar structures deal with the cation and oxygen sites in the (Ba/NdO) layer. Joint occupation of the same site by Ba and Nd ions with essentially different ionic radii results in the splitting of that site along the c -axis and the shift of the oxygen in this layer from the ideal position at the four-fold axis to achieve appropriate coordination for the smaller rare earth cation. These changes may be the structural reason for the suppression of superconductivity in R 1+ x Ba 2− x Cu 3 O 7−δ (R is rare earth) phases.

Synthesis and characterisation of reduced niobates [Sr1−xLnxNb2O6, Ln = La, Nd]

Sr[sub 1-x]Ln[sub x]Nb[sub 2]O[sub 6], Ln = La (x [le] 0.4), Nd (x [le] 0.3) solid solutions with a CaTa[sub 2]O[sub 6]-type structure have been studied. No superconductivity was found in the prepared samples down to 5K. The CaTa[sub 2]O[sub 6] type crystal structure of Sr[sub 0.6]La[sub 0.4]Nb[sub 2]O[sub 6] was refined from X-ray powder diffraction data: R[sub i] = 0.054, R[sub p] = 0.117. The oxidation process of the lanthanum-containing compounds was studied. A drastic decrease of the lanthanum content, accompanied by the formation of the cation vacancies at the Sr/La site occurs during oxidation. Electrical and magnetic properties were studied.

Structure and properties of Ba6 − xLnxNb10O30, Ln = La, Ce and Nd compounds

Ba{sub 6{minus}x}Ln{sub x}Nb{sub 10}O{sub 30} compounds, Ln = La, Ce and Nd, with a tetragonal tungsten bronze-type structure have been synthesized and were characterized by X-ray powder diffraction and EDS analysis. The homogeneity regions of the obtained compounds were found to be 0 {le} x {le} 2 for La, and 0 {le} x {le} 1.5 for Ce and Nd. The crystal structures of Ba{sub 5}LaNb{sub 10}O{sub 30} and Ba{sub 4}La{sub 2}Nb{sub 10}O{sub 30} were refined using X-ray powder diffraction data. Refinement indicated the presence of vacancies in the Ba/La positions. This observation was supported by oxidation studies of the La-substituted compounds. The temperature dependence of the normalized resistivity of the compounds showed a nonmetallic behavior. The increase in the R/R{sub 273} ratio with decreasing temperature is less distinct for the phases with the higher concentration of rare-earth cation.

New strontium and rare earth-based mixed niobium misfit sulfides

New niobium-based misfit sulfides [(Ln1/3Sr2/3S)1.5]1.15NbS2 (Ln=Sm, Tb) have been synthesized from a mixture of SrS, Ln2S3, Nb(m), and S in evacuated and sealed silica tubes by annealing at 100 ...

Crystal structure of (Mo,W)9O25, homologue of the Mo4O11 (orthorhombic)-type structure

Abstract The structure of Mo 7.6 W 1.4 O 25 has been determined from single crystal X-ray data. The symmetry is monoclinic with lattice parameters a = 5.448(1), b = 27.639(8), c = 6.739 (1) A, β = 90.180(9)°, and space group P 2 1 / n . The refinement led to R = 0.046 for 2060 observed unique reflections. The Mo : W ratio was confirmed by microanalysis. The (Mo, W) 9 O 25 structure is built up of corner-sharing distorted M O 6 octahedra in slabs of ReO 3 -type, cut parallel to {211} and seven octahedra wide along two subcell axes. The slabs appear alternatively in mirrored orientations. The slabs are mutually linked by corner sharing of fairly regular MoO 4 tetrahedra so that five-sided tunnels are formed. The (Mo, W) 9 O 25 structure is related to the orthorhombic modification of Mo 4 O 11 and can be considered as the second member ( m = 7) of the homologous series with the general formula M m +2 O 3 m +4 . High-resolution electron microscopy images showed well-ordered crystal fragments.

New Sr-Substituted Niobium-Based Misfit Layer Sulfides

Two series of [(Ln1/3Sr2/3S)1.5]1.15NbS2 and (Ln1-xSrxS)1+yNbS2 misfit layer sulfides have been synthesized and studied by X-ray powder and single crystal diffraction, electron diffraction, and EDX-analysis. The [(Sm1/3Sr2/3S)1.5]1.15NbS2 compound belongs to the 1.5Q/1H homologue type where the Sr-cations are mainly located at the exterior of the Q-slab whereas Sm preferably occupy the positions at its center. The (Pr0.55Sr0.45S)1.15NbS2 sulfide is a member of the 1Q/1H homologue series where almost half of the Pr-atoms in the Q-slab are replaced by Sr ones with a statistical distribution of Pr and Sr over the cation positions.