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.     

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.     

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.     

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.     

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.     

The crystal structure of Hf1.5+δNb1.5−δAs and structure-composition relations in the section Hf3As-Nb3As

The title compound Hf_1.5+δNb_1.5−δAs was characterized by means of single crystal X-ray diffraction. It represents a new structure type of intermetallic compounds (space group Pnma; lattice parameters a=7.142(2) Å, b=3.583(2) Å, c=11.640(2) Å) and shows a small homogeneity range corresponding to (0.1<δ<0.25) at 1400 °C. The crystal structure may be visualized by a combination of As-centred trigonal prisms of the metal atoms and bcc-like fragments formed by metal atoms. Structural relations with various binary arsenides are discussed. The structure of Hf_1.5+δNb_1.5−δAs shows significant preferred site occupation of Hf and Nb at the three independent metal positions (differential fractional site occupancy). Structure-composition relations in the section Hf₃As-Nb₃As which also contains the new phase Hf_2+δNb_1−δAs with Ti₃P-type structure (space group P4₂/n) are discussed. Ground state energies of various ordered compounds with Hf_1.5+δNb_1.5−δAs-, Ti₃P- and Ta₃As-type structures were calculated from ab initio density functional theory. These energies were used for thermodynamic calculations employing the compound energy formalism (CEF) with the aim to model the experimentally observed site fraction data for both ternary compounds as well as Gibbs energies at the temperature of equilibration (1400 °C).     

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.     

Conditions for superconductivity in the electron-doped copper-oxide system, (Nd1−xCex)2CuO4+δ

We report systematic studies on the relations among the Ce^IV-for-Nd^III substitution level (x), oxygen-partial pressure (P_O2), oxygen content (4+δ), lattice parameters (a, c) and superconductivity characteristics (T_c, volume fraction) in the (Nd_1−xCe_x)₂Cu_1−yO_4+δ system which includes electron-doped superconductors. Independent of the Ce-doping level x, samples synthesized in air are found oxygen deficient, i.e. δ<0. Nevertheless, reductive annealing is needed to induce superconductivity in the air-synthesized samples. At the same time, the amount of oxygen removed upon the annealing is found very small (e.g. 0.004 oxygen atoms per formula unit at x=0.075), and consequently the effect of the annealing on the valence of copper (and thereby also on the electron doping level) is insignificant. Rather, the main function of the reductive annealing is likely to repair the Cu vacancies believed to exist in tiny concentrations (y) in the air-synthesized samples.     

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.     

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.     

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.     

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.     

Incommensurately modulated LT″-Ni1+δSn (δ=0.60, 0.63): Rietveld refinement, line-broadening analysis and structural relation with LT- and LT′-Ni1+δSn

X-ray powder diffraction data of NiAs/Ni₂In-type Ni_1.60Sn and Ni_1.63Sn alloys annealed at or below about 573 K reveal the development of an incommensurately ordered phase called LT″. In this phase Ni(2) atoms occupy partially the trigonal-bipyramidal interstices formed by five Sn within an NiAs-type arrangement Ni(1)Sn. The modulated occupational ordering of Ni(2) in the LT″ phase can be described in the superspace group Cmcm(α00)0 s 0, and the parameters describing this occupational modulation were refined together with atomic displacement modulations using the Rietveld method. The structure parameters revealed close structural analogies of the LT″ phase with the previously reported commensurate LT-Ni_1+δSn and incommensurate LT′-Ni_1+δSn phases (A. Leineweber, J. Solid State Chem. 177 (2004) 1197-1212), which both occur for lower Ni contents than the LT″ phase. The 1st-order satellite reflections visible in the powder-diffraction patterns exhibit, with respect to the fundamental reflections, a considerable diffraction-line broadening, caused by a small size of the particularly ordered domains. This small-domain-size broadening was successfully described by a recently developed reflection-index (hklm) dependent (anisotropic) line-broadening model (A. Leineweber, V. Petricek, J. Appl. Crystallogr. 40 (2007) 1027-1034) designed to consider the effect of fluctuations of the lattice metrics on the peak widths in powder diffraction patterns of incommensurately modulated crystal structures. The small domain sizes encountered for the LT″ phase indicate that domain coarsening is much more difficult than for the LT and LT′ phases. This special feature of the LT″ phase goes along with a compared to the LT and LT′ phases absent orthorhombic distortion and the low ordering temperature, which are discussed as a consequence of the ordering patterns due to the Ni(2) atoms.     

Pr1－xSrxAlO3－δ的合成及其作为固体氧化物燃料电池阴极的性能研究

采用固相反应法合成了一系列样品Pr_1－xSr_xAlO_3－δ (x＝0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5). XRD结果表明, Sr在A位上的固溶度约为20%; 利用交流阻抗法测量了样品的电导率, 结果表明, 电导率随着Sr掺入量的增加而增大, Pr_0.8Sr_0.2AlO_3－δ的电导率最大, 在850 ℃达到0.02 S/cm; 离子迁移数的测试结果说明, Pr_1－xSr_xAlO_3－δ为离子电子混合导体; 考察了它们作为固体氧化物燃料电池(SOFCs)阴极的性能, 极化曲线和阻抗的测试结果表明, 阴极性能随着Sr掺入量的增加而提高; 阴极稳定性测试结果表明, 在测量时间范围内, 阴极过电位随时间缓慢下降.     

Ce1－xNdxO2－δ固溶体的电子顺磁共振谱和阻抗谱的研究

利用溶胶-凝胶方法在800 ℃焙烧10 h后, 合成了固溶体Ce_1－xNd_xO_2－δ (x＝0.05～0.55), X射线衍射(XRD)测试表明固溶体已经形成立方萤石结构; 电子顺磁共振谱(EPR)研究表明在固溶体Ce_1－xNd_xO_2－δ中随着掺杂量x的增大, Ce^3＋离子含量减少, 说明掺杂Nd^3＋离子可以抑制Ce^4＋的还原; 交流阻抗谱的测量表明固溶体Ce_0.9Nd_0.1O_2－d 具有离子导电特性, 600和700 ℃时的电导率分别为4.25×10^－3和1.12×10^－2 S•cm^－1, 活化能为0.68 eV.     

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 and spectroscopic characterization of Mo1−xWxO3−δ mixed oxides

Mo_1−xW_xO₃ oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO₃ and WO₃ were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo_1−xW_xO₃ in oxidizing atmosphere to Mo_1−xW_xO_3−δ in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperature are peculiar of each mixed oxide.     

载铂Sr(Zr1－xYx)O3－δ-TiO2异质结光催化剂模拟太阳光催化产氢

利用光化学还原法制备载铂Sr(Zr_1－xY_x)O_3－δ-TiO₂ (Pt-SZYT)异质结光催化剂, 采用XRD, SEM, UV-Vis, PL对催化剂的形貌及性能进行表征. 以工业有机污染物草酸为电子给体, 模拟太阳光下光催化产氢为探针, 评价了催化剂的活性. 研究了载铂量、草酸浓度对催化剂产氢活性的影响, 并探讨了Pt-SZYT催化剂产氢活性与光致发光(PL)性能的关系, 以及催化剂的连续使用效果. 结果表明: 模拟太阳光条件下, Pt-SZYT催化剂具有良好稳定的光催化产氢活性, 催化剂的最佳载铂量为0.90 wt%. PL分析表明Pt-SZYT的光催化产氢活性越高, 其荧光强度越弱. 草酸50 mmol•L^－1, 催化剂用量1.0 g•L^－1时, 催化剂(w＝0.90%) Pt-SZYT-70的平均产氢速率为1.68 mmol•h^－1.     

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.     

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.