Sol-gel preparation and characterization of non-substituted and Sr-substituted lanthanum cobaltates

This paper reports on the results concerning the sol-gel preparation and characterization of Sr-substituted perovskite lanthanum cobaltates La_1−xSr_xCoO_3−δ (x = 0.0, 0.25, 0.5 and 0.75). The metal ions, generated by dissolving starting materials in diluted acetic acid were complexed by 1,2-ethanediol to obtain the precursors for the non-substituted and Sr-substituted LaCoO₃. The influence of the synthesis temperature, heating time and the amount of substituent on the phase purity of La_1−xSr_xCoO_3−δ were investigated. The phase transformations, composition and micro-structural features in the gels and polycrystalline samples were studied by thermal analysis (TG/DTA), infrared spectroscopy (IR), powder X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM).      

Crystal structure of cobalt molybdate hydrate CoMoO4·nH2O

We have determined the crystal structure of the title compound, which has a triclinic cell with cell parameters of , , , α=76.617°, β=84.188°, γ=74.510° and space group . The crystal structure suggests the chemical formula CoMoO₄·3/4H₂O. The structure consists of MoO₄ tetrahedra and CoO₆ octahedra, confirming the earlier X-ray absorption near-edge spectroscopic (XANES) investigation on the hydrate. The comparison of the crystal structures of the hydrate and the α-,β-, and hp-phases shows that the hydrate exhibits metal cation coordinations similar to those of the β-phase, but had arrangements of CoO₆ and MoO_n polyhedra similar to those of the hp-phase.     

Magnetic and transport properties of high-pressure synthesized perovskite cobalt oxide (Sr1-xCax)CoO3 (0?x?0.8)

Polycrystalline perovskite cobalt oxides (Sr_1-xCa_x)CoO₃ (0?x?0.8) were synthesized under high pressure (6 GPa) and high temperature (1200-1300  °C) conditions. Powder X-ray diffraction data confirmed that they have cubic perovskite structures. The end member of the solid solution, SrCoO₃, exhibited a ferromagnetic transition with Curie temperature of and metallic electric conductivity. With increasing the Ca content x, T_c increased slightly to 286 K at x=0.2 then decreased rapidly to 148 K for x=0.8. The origin of the ferromagnetism is discussed in relation to the spin state of the Co⁴⁺ ions. The present system showed a relatively large (∼5.5%) negative magnetoresistance for x=0 or 0.2 in the vicinity of T_c.     

LiLa6I12Os, a substitutional derivative of rhombohedral La(La6I12Os)

The series of quaternary rare-earth-metal halide cluster compounds ALa₆I₁₂Z with transition metal interstitials Z and alkali or alkaline-earth metal cations A has been expanded to include A=Li. The compounds synthesized by high-temperature solid-state techniques for Z=Os, Ir, Pt, Ru are isotypic with rhombohedral R₇X₁₂Z (, Z=3). The refined single X-ray crystal structure of (Li_0.967La_0.033)La₆I₁₂Os is reported, along with supportive results from a Rietveld analysis of neutron powder diffraction from a different sample, ⁷Li MAS-NMR, and electronic resistivity and magnetic susceptibility measurements. The samples show continuous Li_1−xLa_x cation compositions and are generally semiconductors, but their complex paramagnetic properties are not those of simple spin-only systems.     

Crystal structure and magnetic properties of high-oxygen pressure annealed Sr1−xLaxCo0.5Fe0.5O3−δ (0?x?0.5)

Structural and magnetic studies are presented for the perovskite type Sr_1−xLa_xCo_0.5Fe_0.5O_3−δ (0?x?0.5) materials annealed under moderately high-oxygen pressures of ∼200 atm. A detailed analysis of the room temperature neutron time-of-flight diffraction data reveals that the crystal structure of the sample SrCo_0.5Fe_0.5O_2.89(1), previously described as vacancy-disordered cubic, is similar to the formerly reported, oxygen-vacancy ordered Sr₈Fe₈O₂₃ compound, i.e. Sr₈Co₄Fe₄O₂₃ is tetragonal with the I4/mmm symmetry. With an increase of the La content the studied materials become nearly oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). Low-temperature structural and magnetic measurements show a ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2.     

Chemical and electronic characterization of cobalt in a lanthanum perovskite. Effects of strontium substitution

Two different cobaltites, LaCoO₃ and La_0.5Sr_0.5CoO_3−δ, have been prepared and characterized by means of high energy Co K-edge and low energy O K-edge X-ray absorption spectroscopy (XAS). Even though half of the La(III) is substituted by Sr(II), little or no changes can be detected in the formal oxidation state of cobalt atoms. The presence of strontium cations induces two main effects in the chemical and electronic state of the perovskite. The charge balance with Sr(II) species is reached by the formation of oxygen vacancies throughout the network, which explains the well-known increase in the reactivity of this substituted perovskite. O K-edge XAS experiments show that the Sr(II) species induce the transitions of d electrons of cobalt cations from low to high spin configuration. We propose that this change in spin multiplicity is induced by two cooperative effects: the oxygen vacancies, creating five coordinated cobalt atoms, and the bigger size of Sr(II) cations, aligning the Co-O-Co atoms, and favoring the overlapping of π-symmetry cobalt and oxygen orbitals, reducing the splitting energy of e_g and t_2g levels.     

Synthesis and crystal structure of a new oxychalcogenide La5Ti∼3.25Zr∼0.25S5O9.25

A new phase in the quinary system La/Ti/Zr/S/O was obtained from a mixture of La₂O₃, La₂S₃, ZrO₂, and TiO₂ by a solid-state reaction at 1273 K in a sealed fused-silica tube. The structure of this new phase, La₅Ti_∼3.25Zr_∼0.25S₅O_9.25, was solved by single-crystal X-ray diffraction, with R_(obs)=3.37% for 2764 reflections (I>3σ(I)) and 125 variables. This compound crystallizes with four formula units in the monoclinic space group C2/m with lattice constants , , , and β=106.100(8)°. The structure can be viewed as a 2D building constituted from two-atom-thick slabs of rock salt type (=sulfide part) which are interleaved with double-octahedral chains centered on titanium/zirconium atoms (mixed Ti/Zr sites) and drawing a zigzag arrangement (=oxide part). In addition, EDXS analyses show that a solid solution Ti/Zr exists with a general formulation La₅Ti_3.5−xZr_xS₅O_9.25 (where 0.1?x?0.5).     

High-pressure synthesis, crystal structures, and characterization of CdVO3−δ and solid solutions CdVO3-NaVO3

CdVO_3−δ and solid solutions of Cd_1−xNa_xVO₃ with the GdFeO₃-type perovskite structure were prepared using a high-pressure (6 GPa) and high-temperature technique. No significant oxygen and cation deficiency was found in CdVO₃. Cd_1−xNa_xVO₃ are formed in the compositional range of 0?x?0.2. CdVO₃ and Cd_1−xNa_xVO₃ demonstrate metallic conductivity and Pauli paramagnetism between 2 and 300 K. A large electronic contribution to the specific heat (γ=13.4 and ) for CdVO₃ and Cd_0.8Na_0.2VO₃, respectively) was observed at low temperatures due to the strongly correlated electrons. Crystal structures of CdVO₃ and Cd_0.8Na_0.2VO₃ were refined by X-ray powder diffraction: space group Pnma; Z=4; , , and for CdVO₃ and , , and for Cd_0.8Na_0.2VO₃.     

High temperature phase transition in SOFC anodes based on Sr2MgMoO6−δ

The double perovskite Sr₂MgMoO_6−δ has been recently reported as an efficient anode material for solid oxide fuel cells (SOFCs). In the present work, this material have been investigated by high temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC) and impedance spectroscopy to further characterise its properties as SOFC anode. DSC and XRD measurements indicate that Sr₂MgMoO_6−δ exhibits a reversible phase transition around 275 °C from triclinic () with an octahedral tilting distortion to cubic () without octahedral distortion. This phase transition is continuous with increasing temperature without any sudden cell volume change during the phase transformation. The main effect of the phase transformation is observed in the electrical conductivity with a change in the activation energy at low temperature. La³⁺ and Fe-substituted Sr₂MgMoO_6−δ phases were also investigated, however these materials are unstable under oxidising conditions due to phase segregations above 600 °C.     

Inherent instabilities of some W5Si3-type binary phases with large electropositive metal atoms: Six examples in the Ba-Pb, Ca-Sn-Mg,Cu,Zn, and La-Ga-Al,Zn systems

A particular pathology of certain W₅Si₃-type A₅B₃ structures (I4/mcm) appears to arise because of unduly close approaches of the A1-type atoms on the cell faces at , 0, () that occur with the larger and more electropositive A and/or in the presence of smaller B atoms. A structure refinement of binary Ba_4.81Pb₃ indicates such a marginal stability in that the Ba atoms in the facial Ba_0.81 chains exhibit an extreme displacement ellipsoid along . Although Ca₅Sn₃ and La₅Ga₃ binaries are unknown in this structure type, five stable ternary derivatives of these have been synthesized via substitution reactions and characterized by single crystal X-ray diffraction means: Ca₄Sn_3.223(4)Mg_0.777, Ca₄Sn₃Cu_1.30(4), Ca_4.66(6)Sn₃Zn_0.704(4), La_4.81(1)Ga_1.38(2)Al_1.62, and La_4.762(5)Ga_1.5(1)Zn_1.5. Only the Ca-Sn-Zn phase exhibits lower symmetry, P4/mbm. The problematic A1 sites exhibit diverse changes in these, whereas the surrounding B2 tetrahedra are largely unaltered. The Ca-Sn results are, respectively: direct Mg/Sn substitution at the Ca1 site; mixed fractional distribution of the smaller Cu at two sites around the A1 position with an unresolved disorder; a pair of apparently independent modes, fractional Ca in the normal position and fractional Zn rectangles thereabout. The two La-Sn phases contain normal Ga,Al (Ga,Zn) tetrahedral chains with pairs of fractional disordered La atoms along , 0, z. Each can be rationalized in terms of a reasonable incommensurate structure. Electronic effects may also be operable.     

Neutron diffraction study of the crystal structure and structural phase transition of La0.7Ca0.3−xSrxCrO3 (0≤x≤0.3): The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La_0.7Ca_0.3−xSr_xCrO₃ series, including the compositional and temperature dependence of the structural parameters, has been studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO₆] octahedron and [(La/Ca/Sr)O₁₂] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic (Pnma) structure to the rhombohedral one, the volume of the [CrO₆] octahedron decreases whereas that of the [(La/Ca/Sr)O₁₂] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.     

Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

A comparative study on the oxidation and charge compensation in the A_xCoO_2−δ systems, A=Na (x=0.75, 0.47, 0.36, 0.12) and Li (x=1, 0.49, 0.05), using X-ray absorption spectroscopy at O 1s and Co 2p edges is reported. Both the O 1s and Co 2p XANES results show that upon removal of alkali metal from A_xCoO_2−δ the valence of cobalt increases more in Li_xCoO_2−δ than in Na_xCoO_2−δ. In addition, the data of O 1s XANES indicate that charge compensation by oxygen is more pronounced in Na_xCoO_2−δ than in Li_xCoO_2−δ.     

Structural characterisation of the perovskite series SrxCa1−x−yNdyMnO3: Influence of the Jahn-Teller effect

The crystal structures of the perovskite manganites Sr_xCa_1−x−yNd_yMnO₃ with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from depending on the cation distribution, the different structures exhibiting different tilts of the MnO₆ octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr_0.4Ca_0.5Nd_0.1MnO₃, this high temperature tetragonal structure does not exhibit orbital ordering.     

Independent structural and valence state transitions in the cation-ordered double perovskites Ba2−xSrxTbIrO6

Synchrotron X-ray and neutron powder diffraction were used to investigate the formation, structure and bonding in the double perovskite Ba_2−xSr_xTbIrO₆ solid solutions. The results showed that these oxides all exhibit ordering of the Tb and Ir cations in a double perovskite-type structure. Three distinct structural types differing in symmetry and/or valence states were formed depending on the precise Ba:Sr ratio on the perovskite A site; x?0.3 cubic () with Tb⁴⁺ and Ir⁴⁺; 0.4?x?1.0 cubic () with Tb³⁺ and Ir⁵⁺ and x?1.2 monoclinic (P2₁/n) with Tb³⁺ and Ir⁵⁺. The transitions between these appear to be first order in nature.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.     

A new anatase-type phase in the system Mg-Ta-O-N

Magnesium doped tantalum oxynitrides were prepared by ammonolysis of amorphous mixed oxides. An orange colored anatase-type phase with the composition Mg_0.05Ta_0.95O_1.15N_0.85 was found. It is metastable and undergoes a phase transformation to a baddeleyite-type phase between 900 and 1000 °C. X-ray diffraction measurements indicate spacegroup I4₁/amd with lattice parameters and . A possible anion ordering was examined by theoretical methods and neutron diffraction experiments. In addition, anosovite-type (Ti₃O₅) phases Mg_xTa_3−xO_3xN_5−3x; 0?x?0.3 were obtained. The electronic spectra of all phases were investigated by UV/vis spectroscopy.     

Structural aspects of Pr1−xSrxFeO3−w

Structural aspects of the distorted perovskite ABO₃ phase Pr_1−xSr_xFeO_3−w,x=0.00-0.80,w=0.000-0.332, were studied by powder X-ray diffraction, powder neutron diffraction, Mössbauer spectroscopy, and Fe K-, Sr K-, and Pr L_III-edge EXAFS techniques. The diffraction data revealed no indications for ordering of Pr and Sr at the A site, nor for oxygen vacancy ordering at O sites for heavily reduced samples. Mössbauer spectroscopy showed octahedral, square pyramidal, and tetrahedral Fe coordinations with relative amounts closely following the predictions for a binomial distribution of oxygen vacancies. In addition to Fe³⁺ and Fe⁴⁺, also Fe⁵⁺ appears at 77 K for (G-type) antiferromagnetic samples with high average Fe valence. This suggests dynamic 2 Fe⁴⁺↔Fe³⁺+Fe⁵⁺ fluctuations. At 296 K, a mixed valence Fe⁽³⁺ⁿ⁾⁺ component significantly improved the fit of Mössbauer spectra for the most oxidized paramagnetic samples. The qualitative EXAFS study shows that the local environments for Fe, Pr, and Sr strongly depend on x and w. The local Pr- and Sr-site geometries differ significantly from the cubic average structure for Pr_0.50Sr_0.50FeO_2.746.     

Synthesis, Mo-valence state, thermal stability and thermoelectric properties of SrMoO3−xNx (x>1) oxynitride perovskites

Oxynitrides of the general composition SrMoO_3−xN_x (x>1) were synthesized by thermal ammonolysis of crystalline SrMoO₄. According to neutron and X-ray diffraction experiments, the materials crystallize in the cubic perovskite structure (space group Pm3¯m). X-ray absorption spectroscopy (XAS) shows evidence of local distortions of the Mo(O,N)₆ octahedra. The oxidation states of Mo determined by X-ray absorption near edge structure (XANES) spectroscopy are slightly lower than that calculated from the oxygen/nitrogen(O/N) content. The disagreement arises from the higher covalence of the Mo-N bonding when compared to the Mo-O bonding (“chemical shift”). The electrical transport properties of the samples are strongly different from SrMoO₃. It was found that the conductivity of the samples decreases with increasing nitrogen content. The Seebeck coefficient values are up to three times higher than those of SrMoO₃.     

Large magnetoresistance in a ferromagnetic cobaltite: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba_0.9CoO_2.6 crystallizes in the 12H structure [P6₃/mmc, a=5.6612 (1) Å and c=28.4627(8) Å]. Interestingly, this compound is a ferromagnet with a Curie temperature T_C=50 K and a saturation magnetization . This value is smaller than expected from the effective paramagnetic moment, μ_eff=3.7μ_B/Co, corresponding to an average spin per Co, from which one would expect . This suggests either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoO_n polyhedra. A clear transition in the electrical resistivity is found at T_C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spin-charge coupling is evidenced by the large negative magnetoresistance effect optimum near T_C=50 K, with .