Pillaring of Layered Perovskites, K1−xLaxCa2−xNb3O10, with Nanosized Fe2O3 Particles

Nanosized Fe₂O₃ clusters are pillared in the interlayer spaces of layered perovskites, H_1−xLa_xCa_2−xNb₃O₁₀ (0≤x≤0.75) by a guest-exchange reaction using the trinuclear acetato-hydroxo iron cation, [Fe₃(OCOCH₃)₇ OH·2H₂O]⁺. The interlayer spaces of niobate layers are pre-expanded with n-butylammonium cations (n-C₄H₉NH⁺₃), which are subsequently replaced by bulky iron pillaring species to form Fe(III) complex intercalated layer niobates. Upon heating at 380°C, the interlayered acetato-hydroxo iron complexes are converted into Fe₂O₃ nanoclusters with a thickness of ca. 3.5 Å irrespective of the interlayer charge density (x). The band-gap energy of the Fe₂O₃ pillars (E_g2.25 eV) is slightly larger than that of bulk Fe₂O₃ (E_g2.20 eV) but is smaller than that expected for such a small-sized semiconductor, which can be assigned to the pancake-shaped Fe₂O₃ pillars of 3.5 Å in height with comparatively large lateral dimension. X-ray absorption spectroscopic measurements at the Fe K-edge are carried out in order to obtain structural information on the Fe₂O₃ pillars stabilized between the niobate layers. XANES analysis reveals that the interlayer FeO₆ octahedra have coordination environments similar to that of bulk α-Fe₂O₃, but noncentrosymmetric distortion of interlayered FeO₆ is enhanced due to the asymmetric electric potential exerted by the negatively charged niobate layers. Scanning electron microscopic observation and nitrogen adsorption–desorption isotherm measurement suggest that the pillared derivatives are nanoporous materials with the highest BET specific surface area of ca. 116 m²/g.     

A TEM, XRD, and crystal chemical investigation of oxygen/vacancy ordering in (Ba1−xLax)2In2O5+x, 0x0.6

A careful TEM and XRD study of the (Ba_1−xLa_x)₂In₂O_5+x, 0x0.6, ‘defect-perovskite’-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1×1×2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the regions of reciprocal space was found to be typical of all (Ba_1−xLa_x)₂In₂O_5+x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the -type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward.     

Pyrochlore phases in the system ZnOBi2O3Sb2O5: II. Crystal structures of Zn2Bi3.08Sb2.92O14+δ and Zn2+xBi2.96−(x−y)Sb3.04−yO14.04+δ

Rietveld refinement of combined X-ray and neutron diffraction data has, within errors, confirmed the stoichiometries of two, cubic pyrochlore phases in the ZnOBi₂O₃Sb₂O₅ system. Neither phase has the ‘ideal’ stoichiometry, Zn₂Bi₃Sb₃O₁₄. One phase, P₁, is a Zn-rich, Bi-deficient solid solution Zn_2+xBi_{2.96−(x−y)}Sb_3.04−yO_14.04+δ. The other, P₂, is a Bi-rich line phase, stoichiometry Zn₂Bi_3.08Sb_2.92O_14+δ. Both structures have a mixture of Bi, Zn on the A-sites and Zn, Sb on the B-sites. However, Zn is displaced off-centre in the A-sites to achieve lower co-ordination number with realistic ZnO bond lengths. Additional structural complexities arise from: displacement of O(2) atoms; partial occupancies of O(1) and O(2) sites; partial occupancy of a third, interstitial oxygen site, O(3). Since the multiplicities of the off-centre sites are much higher than those of the ideal positions, there is considerable possibility for correlated short range order throughout the structures.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.     

Synthesis, Crystal Structure, and Magnetic Properties of Quasi-One-Dimensional Oxides Ca3CuMnO6 and Ca3Co1+xMn1−xO6

Synthesis conditions, crystal structures, and magnetic properties of quasi-one-dimensional complex oxides Ca₃CuMnO₆ (space group P-1, z=4, triclinic cell) and Ca₃Co_1+xMn_1−xO₆ with x=0, 0.25, 1.0 (structural type K₄CdCl₆, space group R-3c, z=6) are presented. The crystal structures of Ca₃CoMnO₆ and Ca₃CuMnO₆ were refined using neutron and combined X-ray and neutron diffraction analysis, respectively. The interatomic distances in oxygen polyhedra were found. In contrast to ferromagnetic Ca₃Co₂O₆ (T_c=24 K), manganese-containing phases Ca₃Co_1+xMn_1−xO₆ are characterized by antiferromagnetic interactions with Neel temperatures 18 K (x=0.25) and 13 K (x=0). For Ca₃CuMnO₆T_N was established to be 6 K.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

Influence of ceramic process and Eu content on the composite multiferroic properties of the Ba6−2xLn2xFe1+xNb9−xO30 TTB system

The present work reports the optimisation of the composition and elaboration process of Ba_6−2xLn_2xFe_1+xNb_9−xO₃₀ (0.6 < x < 1.0), a composite multiferroic below 440 K. Both firing and sintering conditions have been investigated in order to lower the two spurious phases content and to improve the ceramic quality. The Eu content has also been varied, with subsequent adjustment of cationic content to maintain charge compensation and site occupancies. The evolution of phase content, dielectric and magnetic properties has been evaluated with respect to these parameters. It was found that optimal processing parameters allow a significant improvement of dielectric properties but leave magnetic properties unaffected. Eu content modification impacts both the dielectric and magnetic properties of the TTB samples and changes their dielectric behaviour from ferroelectric to relaxor.     

Synthesis, structure and oxygen nonstoichiometry of La0.4Sr0.6Co1−yFeyO3−δ

The samples of La_0.4Sr_0.6Co_1−yFe_yO_3−δ (y = 0.2 and 0.4) were prepared using both conventional ceramic technique and nitrate–citrate precursors technique. The phase identification was made by X-ray diffraction method. The refinement of structural parameters from the XRD and neutron diffraction measurements was performed by full profile Rietveld analysis. Neutron diffraction showed that both samples possess distorted perovskite-type structure. Oxygen nonstoichiometry was measured by chemical analysis and thermogravimetry (TG) analysis in the range 20 ≤ T/°C ≤ 900 and 2E-5 ≤ pO₂/atm ≤ 4E-1. TG-experiments indicate a relatively fast and reversible oxygen exchange at pO₂ > 1E-2 atm. Mass saturation occurs at T < 300 °C upon cooling. The absolute value of oxygen nonstoichiometry was determined by iodometric titration measurements. It was found that both samples have practically stoichiometric composition at 300 °C in air and δ increases with increasing temperature and decreasing oxygen partial pressure.     

Preparation and properties of La2−xAxCuO4−y, where A = Pb, Cd

The substitution of lead and cadmium for lanthanum in La₂CuO₄ was studied and the limits of solubility were established. Single-phase compositions were characterized and their physical properties were correlated with the substitution of bismuth and are interpreted on the basis of a model proposed by Goodenough.     

Charge–orbital ordering above room temperature in the 2D Pr1−xCa1+xMnO4 manganites

The investigation of the n=1 member of the Ruddlesden Popper family, Pr_1−xCa_1+xMnO₄, using electron microscopy, transport and magnetic measurements shows that these 2D manganites exhibit long-range charge–orbital ordering over a wide composition range (0.50x0.80). These oxides show a remarkably high T_CO temperature depending on the x value, up to 330 K, the highest that has been observed to date in 2D manganites. They are characterized by the appearance of a smooth structural transition from P- to C-type inside the charge-ordered state. The high-resolution electron microscopy images of Pr_0.5Ca_1.5MnO₄ registered at room temperature evidence a system of double stripes similar to those observed for Bi_0.5Sr_0.5MnO₃, suggesting that double stripes of one sort of manganese alternate with double or quadruple stripes of a second sort of manganese.     

X-ray study of the PbCl2−xIx and PbBr2−xIx systems

Compounds from the systems PbCl₂/PbI₂ and PbBr₂/PbI₂ were examined by x-ray diffraction. The lattice parameters of these phases are presented and the refined crystal structures of the intermediate compounds PbClI and PbBr_1.2I_0.8 are reported. Both structures have Pbnm symmetry, are isostructural with PbCl₂, and have the different halogens ordered in the two Cl sites. Phase studies showed that PbCl₂ and PbClI have practically no mutual solubility, while PbBr₂ and PbBr_1.2I_0.8 have appreciable solubility ranges, particularly for PbBr₂-rich concentrations. At least 17% Br is present in the I site of PbBr_1.2I_0.8. Nevertheless, it is a distinct phase with miscibility gaps toward PbBr₂ and PbI₂. This behavior is explained by the size disparity between the halogens. The intermediate phases do not form solid solutions with hexagonal PbI₂.     

Nowotny chimney ladder compounds in Nd1+EFe4−xCoxB4

Structural investigations on Nd_1+EFe_4−xCo_xB₄ show the presence of distinct new phases with multiplicative c axis extensions (ladder compounds) and limited homogeneous range. We also report on compoundsR_1+EFe₄B₄ (R = rare earth) for which several new representatives (e.g., with R = La and Lu) are described. The ladder compounds in both substitutional series appear related and a common sequence as a function of composition is suggested. As an example the first new ladder type Nd_1.06Fe_1.5Co_2.5B₄ to develop out of NdCo₄B₄ is similar to La_1.06Fe₄B₄ and the third namely Nd_1.09Fe₃CoB₄ resembles Pr_1.10Fe₄B₄.     

Structure and Magnetism of Pr1−xSrxFeO3−δ

Crystal structure, redox, and magnetic properties for the Pr_1−xSr_xFeO_3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO₃-type structure for 0≤x≤0.80, and probably in the Sr₈Fe₈O₂₃-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe³⁺) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe⁴⁺ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K.     

Excess properties of the (Ln2−2xCaxThx)(PO4)2 (Ln = La, Ce) solid solutions

X-ray powder diffraction at room temperature and drop calorimetry at T = 1005 K were performed on LnPO₄–CaTh(PO₄)₂ (Ln = La, Ce) solid solutions. The results show excess molar volume and excess enthalpy indicating deviations from ideal behaviour that have been interpreted in terms of lattice strains resulting from the ion size effects of substitution of the Ln³⁺ with (Ca²⁺ + Th⁴⁺).     

Efficient UV-emitting X-ray phosphors: octahedral Zr(PO4)6 luminescence centers in potassium hafnium–zirconium phosphates K2Hf1−xZrx(PO4)2 and KHf2(1−x)Zr2x(PO4)3

Potassium hafnium–zirconium phosphates, K₂Hf_1−xZr_x(PO₄)₂ and KHf_2(1−x)Zr_2x(PO₄)₃, are broad-band UV-emitting phosphors. At room temperature, they have emission peak maxima at approximately 322 and 305 nm, respectively, under 30 kV peak molybdenum X-ray excitation. Both phosphors demonstrate luminescence efficiencies that make them up to 60% as bright as commercially available CaWO₄ Hi-Plus. The solid-state and flux synthesis conditions, and X-ray excited UV luminescence of these two phosphors are discussed. Even though the two compounds have different atomic structures, they contain zirconium in the same active luminescence environment as that found in highly efficient UV-emitting BaHf_1−xZr_x(PO₄)₂. All the three materials have hafnium and zirconium in octahedral coordination via oxygen-atom corner sharing with six separate PO₄ tetrahedra. This octahedral Zr(PO₄)₆ moiety appears to be an important structural element for efficient X-ray excited luminescence, as are the edge-sharing octahedral TaO₆ chains for tantalate emission.     

Critical covalence and superconductivity in Ln2−xCexCuO4

The variations of superconductive properties with x of the n-type Ln_2−xCe_xCuO₄ (Ln = La_0.5Nd_0.5, Nd, or Gd) systems have been investigated. As the size of Ln³⁺ decreases, (i) the solubility limit x of Ce decreases, (ii) the value of x at which a transition from antiferromagnetic semiconductor to superconductor occurs increases, and (iii) the width Δx of the superconductive region decreases. The decreasing solubility of Ce with decreasing size of Ln³⁺ is due to decreasing tensile strain in the CuO₂ sheets. The progressive shift of the semiconductor to superconductor transition to higher x values with decreasing size of Ln³⁺ is explained on the basis of increasing electrostatic Madelung energy E_M caused by decreasing Cu---O bond length. A larger E_M means a larger charge transfer gap Δ and a smaller covalent-mixing parameter λ and bandwidth W; so a decreasing size of Ln³⁺ necessitates a higher level of Ce-doping in order to achieve a critical covalence essential for superconductivity to occur.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Sodium “stuffed” Ba2−xSrxFe4O8 ferrites: new cationic conductors

Sodium insertion in the tetrahedral layer structure of the ferrites Ba_2−xSr_xFe₄O₈ was performed by solid state reaction at 1220 K in air. Superstoichiometric oxides with the actual formula (Ba_2−xSr_x)_1−y/4Na_yFe₄O₈—y0.56; 0.60Ba/Sr1.67—were characterized by X-ray and neutron powder diffraction. The hexagonal unit-cell volume shows an increasing dependence on the sodium insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter is the likely signature of the location of the inserted sodium cations within the interlayer space. One-half of the sodium cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. ac conductivity measurements point to a cationic conductivity whose thermally activated regime—E_a 0.7 eV—evidenced from 570 K, is unsensitive to the sodium content. The bottleneck of the 2D sodium mobility regards the crossing of the oxygen triangular faces shared by the different polyhedra within the interlayer space.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.