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.     

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.     

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.     

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.     

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.     

Stability and oxygen-storage characteristics of Al-substituted YBaCo4O7+δ

A series of Al-substituted YBa(Co_1−xAl_x)₄O_7+δ samples was synthesized and characterized with respect to the capability to store large amounts of oxygen at low temperatures (at 200-400 °C) and the phase decomposition upon heating under oxidizing conditions at higher temperatures (above 550 °C). It was revealed that increasing the Al-substitution level up to x≈0.10 boosts up the phase-decomposition temperature from ∼550 to ∼700 °C, while the unique oxygen absorption/desorption characteristics remain nearly the same as those of the pristine YBaCo₄O_7+δ phase. The maximum amount of excess oxygen absorbed by the Al-substituted YBa(Co_1−xAl_x)₄O_7+δ samples was determined to be as large as δ≈1.45 for x=0.10 (in 100 atm O₂ at 320 °C). Isothermal annealing experiments carried out for the same x=0.10 phase at 300 °C revealed that it could be reversibly charged and discharged with 1.2 oxygen atoms per formula unit by switching the gas flow from N₂ to O₂ and vice versa.     

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.     

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 .     

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.     

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.     

Synthesis and tunable thermal expansion property of Al2−δScδW3O12

A series of solid solutions of Al_2−δSc_δW₃O₁₂ (δ = 0-2) were successfully synthesized by the solid-state reaction with aluminum oxide, scandium oxide and tungsten trioxide as raw materials. The phase composition and structure of the products were analyzed by X-ray powder diffraction and scanning electron microscopy, while the linear thermal expansion coefficients were measured by thermo dilatometer. The results indicate that Al_2−δSc_δW₃O₁₂ with high purity can be successfully prepared by the solid-state method. All samples of different δ values crystallize in the same orthorhombic structure with space group of Pnca. The lattice constants and cell volume increase linearly with increasing Sc content. The average linear thermal expansion coefficients of Al_2−δSc_δW₃O₁₂ measured by thermo dilatometer indicate that the thermal expansion coefficients of the solid solutions could be adjusted to the desired values, such as positive, near zero or negative by simply changing the δ value.     

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.     

Thermodynamic quantities and defect equilibrium in La2−xSrxNiO4+δ

In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La_2−xSr_xNiO_4+δ, statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ-P(O₂)-T relation by using Gibbs-Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La_2−xSr_xNiO_4+δ. Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry 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 Ba2LnSnxSb1−xO6−δ (Ln=Pr and Tb)

Compounds in the double perovskites series Ba₂LnSn_xSb_1−xO_6−δ (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba₂PrSn_xSb_1−xO_6−δ series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr³⁺ to Pr⁴⁺ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba₂TbSbO₆ and Ba₂TbSnO_6−δ are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb³⁺ to Tb⁴⁺. Multiple linear regression analysis of the Pr and Tb L_III-edge XANES indicates that the rate of Ln³⁺ transforming to Ln⁴⁺ is such that there are no oxygen vacancies in Ba₂PrSn_xSb_1−xO_6−δ but in Ba₂TbSn_xSb_1−xO_6−δ there is a small amount of oxygen vacancies, with a maximum of δ≈0.05 present.     

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.     

Ni7−δSnTe2: Modulated crystal structure refinement, electronic structure and anisotropy of electroconductivity

Single crystals of Ni_7−δSnTe₂ were grown during re-crystallization of the presynthesized powder in a two zone furnace. The modulated structure was solved and refined in the (3+2)-dimensional superspace group I4/mmm(0-α0, α00)0.ss.mm with lattice parameters a=3.759(1) and c=19.410(2) Å (measured at 153 K) and Z=2. Satellite reflections observed in the diffraction images can be assigned to the incommensurate modulation vectors q₁=da* and q₂=db* with d=0.410(1). The composition resulting from X-ray structure refinement is Ni_5.81SnTe₂. The structure model has been also developed in the orthorhombic (3+1)-dimensional superspace group Immm(α00)00s assuming twinning according to [110], giving thus the composition Ni_5.79SnTe₂. The origin of the modulation can be attributed to a variation of the occupancy of the Ni(3) site in Ni/Te slabs of the structure. Band structure calculations on a commensurate approximant and single crystal electrical resistivity measurements reveal anisotropic metallic conductivity for this compound.     

The Sr2.75Ce0.25Co2O7−δ oxide, n=2 member of the Ruddlesden-Popper series: Structural and magnetic evolution depending on oxygen stoichiometry

The second member of the Ruddlesden-Popper series, n=2 in Sr_n+1Co_nO_3n+1, has been stabilized by substituting cerium for strontium leading to the pure compound Sr_2.75Ce_0.25Co₂O_7−δ. The oxygen vacancies of this phase can be partially filled by a post-annealing oxidizing treatment with δ decreasing from 1.1 to 0.3 for the as-prepared and oxidized phases, respectively. As the samples are oxidized from δ≈1.1 to 0.3, the a and b unit cell parameters decrease from 3.836 to 3.815 Å and from 20.453 to 20.047 Å, respectively. Despite the average value of the cobalt valence state, V_Co≈+3.5, obtained in the oxidized Sr_2.75Ce⁺⁴_0.25Co₂O_6.7 phase, a clear ferromagnetic state wit T_C=175 K and M_S=0.8 μB/Co is reached.     

Experimental and theoretical studies of Sn3−δPbδBi2Se6 (δ=0.0-0.7)

New ternary and quaternary chalcogenides, Sn_3−δPb_δBi₂Se₆ (δ=0.0-0.7), were synthesized from pure elements using the solid-state method. Their crystal structures, determined using single crystal X-ray diffraction, belong to the orthorhombic space group Pnma (No. 62). The structure is related to Pb₃Bi₂S₆, which contains NaCl [311] layer units and zigzag arrays of metal atoms along the c-axis. A correlation between the Pb composition and the shifted position of a metal site was observed. Band structure calculations confirmed that the structure is stabilized when the position of the M5 site is farther from the mirror plane. Thermopower and conductivity measurements indicated that all of the compounds are n-type semiconductors with small band gaps.     

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.