High pressure synthesis, crystal, magnetic structure and magnetotransport of SrFe0.5Co0.5O3−δ

A polycrystalline sample with SrFe_0.5Co_0.5O_3−δ stoichiometry has been prepared under moderate pressures of 2 GPa in the presence of KClO₄ as oxidizing agent. The crystallographic and magnetic structures have been studied from neutron powder diffraction (NPD), complemented with magnetotransport and magnetic susceptibility data. The study of the crystallographic structure confirms that this compound, with the actual stoichiometry SrFe_0.5Co_0.5O_2.88(3), is a simple cubic perovskite at 2 and 295 K, defined in the Pm-3m space group, where Fe and Co atoms are distributed at random over the B positions. The magnetic measurements show that SrFe_0.5Co_0.5O_2.88 is a ferromagnet with , which is also confirmed by neutron diffraction: the magnetic structure is collinear, characterized by a propagation vector k=0; the ordered magnetic moment values for the (Fe⁴⁺,Co⁴⁺) cations are 1.54(9) and 0.65(15) μ_B at T=2 and 295 K, respectively. Transport measurements show a semiconducting behaviour, and a negative magnetoresistance (MR) of −6.5% is observed at for .     

Synthesis, crystal chemistry and physical properties of the Ruddlesden-Popper phases Sr3Fe2−xNixO7−δ (0?x?1.0)

The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr₃Fe_2−xNi_xO_7−δ with 0?x?1.0 have been studied. X-ray diffraction and selected area electron diffraction (ED) data indicate that the samples have a tetragonal (Space group I4/mmm) structure. ED patterns and high-resolution images reveal the presence of a regular stacking along the c-axis for the x=1.0 sample. The lattice parameters, oxygen content, and average oxidation state of iron and nickel decrease with increasing Ni content. The electronic structure of the x=1.0 sample was studied by M 2p X-ray photoelectron spectroscopy (XPS). An analysis of the spectra using the cluster model indicates that this material is in the negative charge-transfer regime and the ground state is dominated by the 3dⁿ⁺¹L configuration with 2p holes (L) in the oxygen band. The insulator states are stabilized due to a p-p type band gap that arises because the p-d transfer integral T_σ dominates over the O 2p bandwith. Although magnetic measurements reveal the presence of ferromagnetic interactions that lead to magnetic frustration at , no long-range magnetic order was observed for the samples with x?0.3. The electrical resistivity decreases with increasing Ni content as the p-p band gap tend to close due to the reduction of the T_σ value. Negative magnetoresistance (∼−24% for x=0.6 and −7% for x=1.0 at 20 K and 9 T) was observed for the Ni containing samples.     

Synthesis and characterization of the K2NiF4 phases La1+xSr1−xCo0.5Fe0.5O4−δ (x=0, 0.2)

The K₂NiF₄ phases LaSrCo_0.5Fe_0.5O₄ and La_1.2Sr_0.8Co_0.5Fe_0.5O₄, and their reduced forms LaSrCo_0.5Fe_0.5O_3.75 and La_1.2Sr_0.8Co_0.5Fe_0.5O_3.85, have been successfully prepared by solid-state reactions, followed by reduction in 10% H₂/N₂ in order to produce oxygen-deficient materials. All materials crystallize in a tetragonal K₂NiF₄ structure (space group I4/mmm) with Co and Fe randomly distributed over the B-sites of the structure. Mössbauer spectra have confirmed the trivalent state of Fe in these materials. In the reduced materials, oxide ion vacancies are confined to the equatorial planes of the K₂NiF₄ structure and the Co is present almost entirely as Co²⁺ ions; low-temperature neutron powder diffraction data reveal that these reduced phases are antiferromagnetically ordered with a tetragonal noncollinear arrangement of the moments. The Co³⁺ ions, present in stoichiometric LaSrCo_0.5Fe_0.5O₄ and La_1.2Sr_0.8Co_0.5Fe_0.5O₄, inhibit magnetic order and are assumed to be in the low-spin state.     

Crystal structure and magnetic properties of complex oxides Mg4−xNixNb2O9, 0?x?4

In the Mg_4−xNi_xNb₂O₉ (0?x?4) system two ranges of solid solution have been found. One of the solid solutions has a corundum-related structure type (space group ); the second one adopts the II-Ni₄Nb₂O₉ structure type (space group Pbcn). The unit cell constants and atomic positions have been determined and refined using neutron powder diffraction data. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) from MgNi₃Nb₂O₉ crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg₃NiNb₂O₉, measured in the temperature range T=2-300 K, shows no indications of magnetic ordering at low temperatures, while for MgNi₃Nb₂O₉ there is a magnetic ordering at temperatures below 45.5 K.     

Structure and magnetic properties of Ca2Fe1−xMnxAlO5+δ

Ca₂Fe_1−xMn_xAlO₅ (0?x?1) compounds were prepared by a self-combustion method under air (x=0, 0.1, 0.2 and 0.3) and nitrogen (x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Mössbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x?0.3 and increases in the case of the x?0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments’ orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Mössbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn³⁺ over Fe³⁺, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions.     

Ca3−xLaxCo4O9+δ (x=0, 0.3): New cobaltite materials as cathodes for proton conducting solid oxide fuel cell

Misfit-type Ca_3−xLa_xCo₄O_9+δ (x=0, 0.3) oxides were synthesised to be evaluated as possible cathode materials for proton conducting fuel cells (PCFCs) based on BaCe_0.9Y_0.1O_3−δ (BCY10) dense ceramic electrolyte. The electrical conductivity value of Ca_2.7La_0.3Co₄O_9+δ (σ≈53 S cm^-1 at 600 °C) is in the range of usually required value for a cathode application (about 50-100 S cm^-1). In order to test the performance of each compound as cathode material, impedance measurements were carried out on Ca_3−xLa_xCo₄O_9+δ/BaCe_0.9Y_0.1O_3−δ/Ca_3−xLa_xCo₄O_9+δ symmetrical half cells over the temperature range 400-800 °C under wet air. A promising electrocatalytic activity has been observed with both compounds Ca₃Co₄O_9+δ and Ca_2.7La_0.3Co₄O_9+δ. Factually, the area specific resistance obtained was about 2.2 Ω cm² at 600 °C.     

Structure, microstructure and magnetic properties of Sr1−xCaxCrO3 (0?x?1)

The effect of the calcium concentration on the structural, microstructural and magnetic properties of Sr_(1−x)Ca_xCrO₃ with 0?x?1 has been studied. The compounds were prepared using high pressure and high temperature synthesis. X-ray diffraction shows that the samples evolve from the cubic Pm-3m space group for x=0-0.2 to tetragonal I4/mcm for x=0.4-0.5, then to the orthorhombic Pbnm space group for x=0.6, 0.8 and 1.0. Electron diffraction and high-resolution transmission electron microscopy confirmed the respective cells for the end compositions: a_p×a_p×a_p (Pm-3m) for SrCrO₃; and the (Pbnm) for CaCrO₃. For intermediate compositions some extra spots appear in the electron diffraction patterns while the electron micrographs indicate the presence of microdomains. Magnetic measurements show Curie-Weiss behaviour at high temperature for all the samples. A sharp antiferromagnetic (AFM) transition at about 91.5 K appears for x=0.8-1 together with a weak ferromagnetic ordering below T_N.     

Sol-gel synthesis, structural and superconducting properties of (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ

A new series of Sr-based Hg-1212 superconducting cuprate (Hg_1−ySe_y)Sr₂(Y_1−xCa_x)Cu₂O_6+δ (y=0.25; 0.0?x?0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr₂(Y_1−xCa_x)Cu₂O_z prepared by the citrate sol-gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg_1−ySe_y)Sr₂YCu₂O_6+δ; y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg_0.75Se_0.25)Sr₂(Y_1−xCa_x)Cu₂O_6+δ samples. Superconductivity is observed only for samples with x?0.3 and T_c increases with increasing Ca content as well as O₂-annealing. A maximum T_c(onset) of 85 K is found in the (Hg_0.75Se_0.25)Sr₂(Y_0.3Ca_0.7)Cu₂O_6.84 sample annealed in an oxygen atmosphere. The structure of O₂-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them.     

Structural features, nonstoichiometry and high-temperature transport in SrFe1−xMoxO3−δ

The oxide solid solutions SrFe_1−xMo_xO_3−δ, where x=0.05, 0.1 and 0.2, are studied in this work. It is shown that substitution of iron for molybdenum results in stabilization of a cubic quasi-perovskite locally inhomogeneous structure, which is evidenced by HREM and Mössbauer spectroscopy. The coulometric titration is used in order to determine changes of oxygen nonstoichiometry in the obtained solutions with temperature and ambient oxygen partial pressure. Partial molar thermodynamic functions of the labile oxygen are calculated from the measured data. The variations of partial molar entropy with oxygen content follow the ideal gas model reasonably well thus demonstrating approximately random distribution of oxygen vacancies in the doped ferrite at high temperatures. The partial molar enthalpy is found to increase with doping, which is indicative of a progressive decrease in average values of the bonding energy of labile oxygen ions. The measurements of total conductivity are used in order to determine partial contributions of charge carriers. The oxygen ion component is shown to increase at small level of doping, x=0.05 while further increase in molybdenum content is accompanied with the decline in the ion conductivity. The electron contribution in reducing conditions tends to increase with molybdenum content, which is interpreted as a manifestation of involvement of Mo⁵⁺ cations in electron transport. Concentration and mobility of electron carriers are calculated. Some increase in mobility of electron holes at small doping is explained as related to the filling of oxygen vacancies.     

Magnetic and dielectric properties of YbFe2−xMnxO4 (0?x?1)

We have investigated the magnetic and dielectric properties of YbFe_2−xMn_xO₄ (0?x?1), which is an Fe-site-substituted system of new multiferroic oxides RFe₂O₄ (R=Y, Ho-Lu). X-ray diffraction measurements show that a solid solution is formed between YbFe₂O₄ (x=0) and YbFeMnO₄ (x=1) for 0?x?1, whereas only compounds with x=1 (i.e., RM₁M₂O₄; M₁ and M₂=trivalent and divalent cations, respectively) have been known for the Fe-site substitution in RFe₂O₄. The valence of the Mn ion is determined to be ∼2+, consistently with the suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature (T_N) and the dielectric constant (ε′) decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe₂O₄ are governed by the electron transfer between Fe-site ions, unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed as well.     

Influence of iron addition on the oxygen-deficient Sr0.85Bi0.15Co1−xFexO3−δ (0.0?x?1.0) perovskites

A series of oxygen-deficient Sr_0.85Bi_0.15Co_1−xFe_xO_3−δ (0.0?x?1.0) perovskite phases were prepared using solid-state reaction. Results of neutron powder diffraction analyses show that the introduction of Fe onto the B-site severely effects the long range coherence of the oxygen vacancy ordered, I4/mmm supercell, observed for the x=0.0 sample. For x=0.1 a smaller, a=b≈a_p, c≈2a_p, P4/mmm supercell gives the best agreement to the diffraction data, whilst phases in the range 0.2?x?0.6 adopt disordered cubic perovskite structures. Pseudo-cubic, a=b≈a_p, c≈a_p, structures are found for x?0.8. Evidence of weak superstructures, reflecting local oxygen ordering, is also obtained from electron diffraction. For all oxygen-annealed phases the average structure reverts to cubic Pm3¯m. The as-prepared samples show G-type antiferromagnetic order at room temperature. The oxygen annealed x=0.10, 0.25 and 1.0 samples display low-temperature spin-glass transitions.     

Synthesis and electrochemical properties of nonstoichiometric LiAlxMn2−xO4−δ as cathode materials for rechargeable lithium ion battery

Nonstoichiometric spinel oxides, LiAl_xMn_2−xO_4−δ (x=0.1,0.2), were synthesized under controlled partial pressure of oxygen, and their elecrochemical performances were investigated. As an Al content increases, solubility limit of the oxygen nonstoichiometry, δ, increased, while partial molar enthalpy of the formation of oxygen nonstoichiometry decreased.Cycle performance of LiAl_xMn_2−xO₄ showed significant improvement comparing with that of LiMn₂O₄ cathode. However, the decrease of theoretical capacity was accompanied with Al doping. Nonstoichiometric LiAl_xMn_2−xO_4−δ showed the increase in capacity with keeping good cycle performances as well as stoichiometric LiAl_xMn_2−xO₄. Although the introduction of oxygen nonstoichiometry leads to the increase of Mn³⁺ which is known as Jahn-Teller ion, DSC curves for LiAl_xMn_2−xO_4−δ showed no exothermic peak due to phase transition arising from Jahn-Teller distortion around room temperature.     

Energetics of La1−xAxCrO3−δ perovskites (A=Ca or Sr)

A series of perovskites with the general formula La_1−xA_xCrO_3−δ (A=Ca or Sr) have been synthesized in the solid solution range 0.0<x?0.3 and 0.0?δ?0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B₂O₃ at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from −620±260 to −280±80 kJ/mol O₂ when x=0.1-0.3 for Ca-doped samples, and from −440±150 to −290±50 kJ/mol O₂ for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of ) for the reaction (A=Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO₃ is more stable than its Ca-doped counterpart, which is consistent with basicity arguments.     

Gas-sensing properties of Fe2−xTixO3+γ (x = 0–1.4)

As part of a systematic study of mechanisms of response of semiconducting oxides as trace gas sensors, we have explored the behaviour of iron–titanium oxide solid solutions Fe_2−xTi_xO₃ (x = 0.1–1.4). The materials were single-phase for x = 0.1 with increasing proportions of a pseudobrookite second phase at higher degree of substitution. Unmodified, pure iron oxide does not show sensitivity to CO. A significant signal was developed for x = 0.1, that then diminished with increasing x and was lost for x = 1.4. Three effects have been deduced important for the gas response: significant surface segregation of Ti at low Ti content; grain growth inhibition and agglomeration into more massive, non-porous lumps as Ti content increased; and the appearance of a band-gap state associated with Fe(II) at higher Ti content. The effects of microstructure change have been analyzed by fitting the data to a simple 2-resistor model of gas-insensitive ‘grains’ in series with gas-sensitive ‘grain boundaries’. A Mars–van Krevelen type model for the response is presented, based on reactions at surface-segregated defect clusters, to develop and remove electrically-active surface trap states.     

Oxygen nonstoichiometry and chemical stability of Nd2−xSrxNiO4+δ

Nonstoichiometric variation of oxygen content in Nd_2−xSr_xNiO_4+δ (x=0, 0.2, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O₂) range from 10⁻²⁰ to 1 bar. Nd_2−xSr_xNiO_4+δ shows the oxygen excess and the oxygen deficient composition depending on P(O₂), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La_2−xSr_xNiO_4+δ: Nd_2−xSr_xNiO_4+δ show more oxygen excess than La_2−xSr_xNiO_4+δ in the higher P(O₂) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd_2−xSr_xNiO_4+δ in the oxygen excess region is much smaller than that of La_2−xSr_xNiO_4+δ. The oxygen nonstoichiometric behavior of Nd_2−xSr_xNiO_4+δ is more ideal-solution-like than that of La_2−xSr_xNiO_4+δ.     

Compositional control of electrical transport properties in the new series of defect thiospinels, Ga1−xGexV4S8−δ (0≤x≤1)

A new series of non-stoichiometric sulfides Ga_1−xGe_xV₄S_8−δ (0≤x≤1; δ≤0.23) has been synthesized at high temperatures by heating stoichiometric mixtures of the elements in sealed quartz tubes. The samples have been characterized by powder X-ray diffraction, SQUID magnetometry and electrical transport-property measurements. Structural analysis reveals that a solid solution is formed throughout this composition range, whilst thermogravimetric data reveal sulfur deficiency of up to 2.9% in the quaternary phases. Magnetic measurements suggest that the ferromagnetic behavior of the end-member phase GaV₄S₈ is retained at x≤0.7; samples in this composition range showing a marked increase in magnetization at low temperatures. By contrast Ga_0.25Ge_0.75V₄S_8−δ appears to undergo antiferromagnetic ordering at ca. 15 K. All materials with x≠1 are n-type semiconductors whose resistivity falls by almost six orders of magnitude with decreasing Ga content, whilst the end-member phase GeV₄S_8−δ is a p-type semiconductor. The results demonstrate that the physical properties are determined principally by the degree of electron filling of narrow-band states arising from intracluster V-V interactions.     

Phase and valence transitions in Ba2LnSnxNb1−xO6−δ

The structures of compounds in the perovskite series Ba₂LnSn_xNb_1−xO_6−δ (Ln=Pr and Tb and x=0, 0.1, 0.2, …, 1.0) have been examined using synchrotron X-ray and neutron diffraction. It was found that niobate members of both series feature full B-site cation ordering but that this order is lost with increasing x. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopies indicate that the oxidation state of the lanthanide cations gradually changes from Ln³⁺ to Ln⁴⁺ with increased Sn⁴⁺ doping. This is believed to be the cause of the loss of B-site ordering. Least squares analysis of the XANES spectra suggests that the rate of the transformation of Ln³⁺ cations to the tetravalent state is such that the Pr series contains no oxygen vacancies while the Tb series may contain a very small amount of vacancies, with δ≈0.02.     

Investigation on the structural and electrical properties of NdSrNi1−xCrxO4+δ (0.1≤x≤0.9) system

The phases NdSrNi_1−xCr_xO_4+δ (0.1≤x≤0.9) have been synthesized by modified sol-gel method and subsequent annealing at 1250 °C in 1 atm of flowing argon. X-ray diffraction (XRD) analysis and electrical resistivity have been measured at room temperature. Rietveld refinement shows that all compositions with x>0.1 were found to crystallize in the tetragonal K₂NiF₄ type structure in the space group I4/mmm, while for x=0.1, a mixture of two phases with the tetragonal space group I4/mmm and the orthorhombic space group Fmmm. Variations of a and c parameters show a complex behavior with increasing chromium content. It was established that compounds with chromium content less then x≤0.5 are oxygen-deficient, while for x>0.5 the sample are oxygen-overstoichiometric. The NdSrNi_0.5Cr_0.5O_4+δ compound exhibits semiconductive behavior and the electrical transport mechanism agrees with the non-adiabatic small polaron hopping model in the temperature ranges 298-493, 493-573 and 573-703 K separately.     

Structural characterisation of the Ce1−xLaxNbO4+δ solid solution series: In-situ high-temperature powder diffraction studies

High-resolution neutron powder diffraction has been utilised to investigate the effect of lanthanum substitution on the structure of cerium niobate, CeNbO_4+δ, as a function of temperature. Two members of the Ce_1−xLa_xNbO_4+δ solid solution series, Ce_0.8La_0.2NbO_4+δ and Ce_0.2La_0.8NbO_4+δ, were examined over a temperature range of 293-923 K under a positive pressure of O₂ (500 mbar). From this data it was found that on increasing lanthanum substitution there was an associated reduction in the temperature of the monoclinic-to-tetragonal phase transition. The data also suggested that increasing lanthanum substitution caused an associated decrease in the excess oxygen content. In addition, high-temperature X-ray powder diffraction data recorded in static air were also examined for four compositions of the Ce_1−xLa_xNbO_4+δ series (x=0.2, 0.4, 0.6 and 0.8). These data corroborated the results of the neutron diffraction experiments and also suggested that there was formation of an intermediate phase, analogous to the CeNbO_4.08 phase of the parent material, during the phase transitions of the x=0.8 and 0.6 compositions.     

Ba3Fe2WO9−δ: Effect of oxygen non-stoichiometry on structural and magnetic properties

The magnetic and structural properties of oxygen-deficient perovskites with composition Ba₃Fe₂WO_9−δ (BFWO) have been systematically studied for two different oxygen contents corresponding to δ=0.00 and 0.55 in the chemical formula in order to determine and correlate their chemical composition, structural and magnetic properties. The evolution of nuclear and magnetic structures with temperature has been investigated by neutron powder diffraction. It was shown that at room temperature the stoichiometric compound (δ=0.00) adopts a hexagonal 6H-perovskite structure (space group P6₃/mmc). This phase, when heated at high temperature under a stream of Ar gas, transforms to an oxygen-deficient phase δ=0.55), which is an ordered cubic perovskite structure (space group Fm-3m). The crystallographic and magnetic properties of the obtained phases are compared, and it is clear that the magnetic properties are significantly affected by oxygen non-stoichiometry. These changes of magnetic properties for such a slight decrease in oxygen content are interpreted as a result of structural transformations. Together with the experimental results based on neutron powder diffraction data a discussion of some aspects of the structural transformation () is presented.