Mild hydrothermal synthesis and magnetic properties of the manganates Pr1−xCaxMnO3

The calcium-doped manganates, Pr_1−xCa_xMnO₃ (x=0.39, 0.46, 0.70, 0.76), were synthesized as cube-shaped crystalline phases under mild hydrothermal conditions for the first time. The crystals could be grown in one step from solutions of metal salts and potassium hydroxide at temperatures ∼240 °C, and found to adopt perovskite-like structure (space group Pbnm). Samples were characterized by powder X-ray diffraction, scanning electron microscopy, inductively coupled plasma analysis and variable temperature dc/ac magnetic susceptibility. The studies indicate that formation of the materials is dependent on the alkalinity and composition of the initial reaction mixtures. The magnetic properties show spin-glass-like behavior due to competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions in Pr_1−xCa_xMnO₃ with x=0.39, 0.46.     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, have the space group I2/m (tilt system a⁰b⁻b⁻) and Fm3¯m (tilt system a⁰a⁰a⁰), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a⁻a⁻a⁻) instead of the tetragonal I4/m phase (tilt system a⁰a⁰c⁻). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba₂MM′O₆-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d⁰-ion, determines the tetragonal symmetry in preference to the rhombohedral one.     

A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr1−xCaxTiO3 (x=0.04, 0.06, and 0.12)

Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr_1−xCa_xTiO₃ (SCT) are presented for x=0.04, 0.06 and 0.12 in the temperature range 8-473 K. It is proposed that the space group of SCT in the composition range 0.06?x?0.35 is Imma with a⁰b^-b^- tilt system and not I4/mcm with a⁰a⁰c^- tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I4/mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural E_g mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x=0.06 and x=0.12 but not of x=0.04 indicating the change in the local structure of the cubic phase of SCT for x?0.06. The presence of symmetry forbidden TO₂ mode in the Raman spectra of the cubic phase of SCT for x<0.06 and its absence for x?0.06 provides yet another characteristic feature distinguishing the I4/mcm and Imma space groups. The implications of the change in the tilt system from a⁰a⁰c^- to a⁰b^-b^- on the development of the polar order is also discussed.     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

Solvothermal synthesis and characterisation of La1−xAxMnO3 nanoparticles

The synthesis of La_1−xA_xMnO₃ (A=Ca, Sr, Ba) nanoparticles in nonaqueous solvents has been studied. For this solvothermal procedure benzyl alcohol and acetophenone were chosen as reaction media. After this treatment in mild conditions a precipitate was obtained, which, however, can be annealed to the crystalline perovskite structure. Our results show that acetophenone is more suited to obtain the clean pervoskite phase than benzyl alcohol.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Structural chemistry and magnetic properties of Pr3−xSr1+xCrNiO8

Polycrystalline samples of the n=1 Ruddlesden-Popper system Pr_3−xSr_1+xCrNiO₈ have been synthesized over the composition range 0.0<x?1.0 either by the ceramic method or from solution. They have been characterized by an appropriate combination of diffraction methods (X-ray, neutron and electron) and magnetometry (d.c. and a.c.). All compositions having x>0.1 adopt the tetragonal space group I4/mmm; Pr_2.9Sr_1.1CrNiO₈ adopts the orthorhombic space group Fmmm. There is no evidence of Cr/Ni cation ordering in any composition. A maximum in the zero-field cooled magnetic susceptibility is observed at a temperature T_f that decreases with increasing Sr content; 52?T_f (K)?13. The frequency dependence of T_f observed in a.c. susceptibility measurements, together with the analysis of neutron diffraction data, suggests that the atomic magnetic moments in these compositions adopt a spin-glass-like state below T_f.     

The nature of the orthorhombic to tetragonal phase transition in Sr1−xCaxMnO3

The orthorhombic-tetragonal phase transition in the perovskite series Sr_1−xCa_xMnO₃ 0.4?x?0.6 has been studied by synchrotron X-ray powder diffraction. At room temperature the Ca rich oxides x?0.45 have the orthorhombic Pbnm superstructure whereas Sr_0.6Ca_0.4MnO₃ is two phases with both tetragonal I4/mcm and orthorhombic Pbnm. Analysis of the octahedral tilts suggest the co-existence of these two phases is a consequence of a first-order I4/mcm to Pbnm transition. The evolution of the structure of Sr_0.5Ca_0.5MnO₃ with temperature is also described and this is found to evolve from orthorhombic to tetragonal and ultimately cubic.     

Synthesis and characterization of Ti1−2xNbxNixO2−x/2 solid solutions

Doped-rutile has been traditionally used in ceramic pigments for its intense optical properties. In this paper, we compare the classical ceramic synthesis of Ti_1−2xNb_xNi_xO_2−x/2 system with the sol-gel methodology, which allows a reduction of the anatase-rutile transformation temperature. The composition was optimised in order to obtain a unique rutile phase with the minimum amount of pollutant Ni(II) and enhanced chromatic coordinates. Incorporation of the doping ions in the rutile structure was corroborated by XRD and Rietveld refinements. The species responsible for the colour mechanism were studied by different techniques. UV-VIS spectroscopy showed the characteristic features of Ni²⁺ ions, whose existence was corroborated by EPR and magnetic measurements. From these results, (Ni,Nb)doped-TiO₂ powder samples can be now shaped as thin films, monoliths, etc. by using sol-gel methodology without modifying their properties. This study introduces new possibilities of coloured TiO₂-based solid solutions in new combined advanced applications (colouring agent and photocatalyst, etc.).     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Structural and electrical properties evolution in Ba1−xSrxRuO3 synthesized under high pressure

The 6H and 6M Ba_1−xSr_xRuO₃ at x?0.6 with the normal and distorted hexagonal BaTiO₃ structures were synthesized by using high-pressure and high-temperature method. It is found that the unit cell volume deviates from Vegard's law between 0.3 and 0.4 for the solid solutions due to the increasing distortion degree of crystal structure. With the increasing x, the electrical resistivity at the same temperature is increasing. With the substitution of Sr for Ba ion, the 6H BaRuO₃ transforms to a Fermi-liquid metal at x=0.25 from the primal non-Fermi-liquid metal, and then becomes a semiconductor at low temperature when x is larger than 0.4.     

Hydrothermal synthesis, characterization and composition-dependent magnetic properties of LaFe1−xCrxO3 system (0≤x≤1)

Hydrothermal synthesis, characterization and magnetic properties of a series of ABO₃-perovskites LaFe_1−xCr_xO₃ (0≤x≤1) are reported. The alkalinity in initial reaction mixtures plays a critical role in controlling the designed stoichiometry of the final compositions. Their magnetic properties are strongly dependent on the compositions and a maximum magnetic moment is found for the sample at x=0.5. Weak ferromagnetic interaction observed for the samples from x=0 to 0.9 arises from the presence of Fe-O-Fe antisymmetric exchange and Fe-O-Cr superexchange interaction. The weak ferromagnetism as well as the linear variation of the lattice parameters implies the possible random distribution of Fe and Cr ions in B sites of the perovskites. The evolution of magnetic ordering transition temperatures has a close relationship with substituent ratios, for the competition of antiferromagnetism and ferromagnetism. The saturated magnetic moment shows a great improvement compared with that for the samples synthesized by solid state method.     

Surface properties and performance for VOCs combustion of LaFe1−yNiyO3 perovskite oxides

LaFeO₃, LaNiO₃ and substituted LaFe_1−yNi_yO₃ (y=0.1, 0.2 and 0.3) perovskites were synthesized by the citrate method and used in the catalytic combustion of ethanol and acetyl acetate. Chemical composition was determined by atomic absorption spectrometry (AAS) and specific areas from nitrogen adsorption isotherms. Structural details and surface properties were evaluated by temperature-programmed reduction (TPR), infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), temperature-programmed desorption of oxygen (O₂-TPD) and photoelectron spectroscopy (XPS). Characterization data revealed that total insertion of nickel in the LaFeO₃ takes place for substitution y=0.1. However, NiO segregation occurs to some extent, specifically at higher substitutions (y>0.1). The catalytic performance of these perovskites was evaluated in the combustion of acetyl acetate and ethanol. Among these molecules, ethanol exhibited the lowest ignition temperature, and the catalytic activity expressed as intrinsic activity (mol m⁻² h⁻¹) was found to increase substantially with the nickel substitution. These results can be explained in terms of the cooperative effect of a LaFe_1−yNi_yO₃ and NiO phases, whose relative concentration determines the oxygen activation capability and hence their reactivity.     

Neutron diffraction and magnetic study of the Nd0.7Pb0.3Mn1−xFexO3 (0?x?0.1) perovskites

The effect of Fe doping on the ferromagnetic Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075, 0.1) phases has been studied in order to analyze the double-exchange interaction. The structural and magnetic study has been carried out by neutron powder diffraction and susceptibility measurements between 1.7 and 300 K. The substitution of Fe at the Mn site results in reductions in both the Curie temperature T_c and the magnetic moment per Mn ion without appreciable differences in the crystal structures. All the compounds crystallize in Pnma space group. The thermal evolution of the lattice parameters of the Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0.025, 0.05, 0.075) compounds shows discontinuities in volume and lattice parameters close to the magnetic transition temperature. Increasing amounts of Fe³⁺ reduces the double exchange interactions and no magnetic contribution for x=0.1 is observed. The magnetic structures of Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075) compounds show that the Nd and Mn ions are ferromagnetically ordered.     

Crystal structures and magnetic properties of two-dimensional antiferromagnets Co1−xZnxTeMoO6

Crystal structures and magnetic properties of metal telluromolybdates Co_1−xZn_xTeMoO₆ (x=0.0, 0.1,…,0.9) are reported. All the compounds have an orthorhombic structure with space group P2₁2₁2 and a charge configuration of M²⁺Te⁴⁺Mo⁶⁺O₆. In this structure, M ions form a pseudo-two-dimensional lattice in the ab plane. Their magnetic susceptibility measurements have been performed in the temperature range between 1.8 and 300 K. The end member CoTeMoO₆ shows a magnetic transition at 24.4 K. The transition temperature for solid solutions rapidly decreases with increasing x and this transition disappears between x=0.4 and 0.5, which is corresponding to the percolation limit for the square-planer lattice. From the magnetization, specific heat, and powder neutron diffraction measurements, it is found that the magnetic transition observed in the CoTeMoO₆ is a canted antiferromagnetic ordering of Co²⁺ ions. The antiferromagnetic component of the ordered magnetic moment (3.12(3)μ_B at 10 K) is along the b-axis. In addition, there exists a small ferromagnetic component (0.28(3)μ_B) along the a-axis.     

Structural and physical properties evolution of BaIr1−xMnxO3 solid solutions synthesized by high-pressure sintering

The BaIr_1−xMn_xO₃ (0.0≤x≤1.0) solid solutions were synthesized by using the solid-state chemical method and high pressure sintering in the pressure range 0-5 GPa. According to the pressure-composition “phase diagram” at 1000 °C, the 9M BaIr_1−xMn_xO₃ transforms to the 6M form at 5 GPa and x≤1/6. In the x range 0.5-1.0, it transforms to the 9R form in a large pressure range. For the 9M BaIrO₃, the Mn ions substitution for Ir ions enhances the semiconducting property, and reduces the weak ferromagnetism. When x is larger than 1/3, the 9M/9R BaIr_1−xMn_xO₃ behave spin-glass-like state at low temperature, with the glass transition temperature T_g about 60 K. For the 6M BaIrO₃, the Mn ions doping results in that it transforms to insulator and spin-glass-like magnetism from the initial paramagnetic metal.     

Phase separation, cation ordering and nano-structural complexities in Nd2/3−xLi3xTiO3 with x=0.14

Transmission electron microscopy (TEM) investigations on Nd_2/3−xLi_3xTiO₃ with x=0.14 reveal a rich variety of structural features in the samples prepared under different conditions, such as superstructures, anti-phase domains, and nano-chessboard structures. Our careful analysis shows that these structural phenomena can be fundamentally understood as the combination of structural effects of (Nd, Li)-ordering along the 〈001〉_p direction and the spinodal decomposition along the 〈100〉_p/〈010〉_p direction. The coexistence of phase separation and cation ordering can lead to visible nano-structural complexities in many crystals, as the typical results, the regular lamella structure, nano-chessboard structures and anti-phase boundaries have been extensively studied.     

Synthesis of silicated hydroxyapatite Ca10(PO4)6−x(SiO4)x(OH)2−x

The preparation of silicated hydroxyapatite Ca₁₀(PO₄)_6−x(SiO₄)_x(OH)_2−x (SiHA) with 0?x?2 was investigated using a wet precipitation method followed by a heat treatment. X-ray diffraction and Rietveld refinement, Fourier transformed IR (FTIR) spectroscopy, elemental analyses, transmission electron microscopy and thermal analyses were used to characterize the samples. The raw materials were composed of a partially silicated and carbonated apatite and a secondary minor phase containing the excess silicon. Single phase silicated hydroxyapatites, with 0?x?1, could be synthesized after a thermal treatment of the raw powders above 700 °C. The presence of carbonate groups in the raw apatite played an important role in the incorporation of silicates during heating. From the different results, the mechanisms of formation of SiHA are discussed.