Thermochemical studies on SrFe12O19(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₁₂O₁₉(s) powder sample and the compound was characterized by X-ray diffraction analysis. A solid-state electrochemical cell of the type: (−)Pt, O₂(g)/{CaO(s) + CaF₂(s)}//CaF₂(s)//{SrFe₁₂O₁₉(s) + SrF₂(s) + Fe₂O₃(s)}/O₂(g), Pt(+) was used for the measurement of emf as a function of temperature from 984 to 1151 K. The standard molar Gibbs energy of formation of SrFe₁₂O₁₉(s) was calculated as a function of temperature from the emf data and is given by: (SrFe₁₂O₁₉, s, T)/kJ mol⁻¹ (±1.3) = −5453.5 + 1.5267 × (T/K). Standard molar heat capacity of SrFe₁₂O₁₉(s) was determined in two different temperature ranges 130-325 K and 310-820 K using a heat flux type differential scanning calorimeter (DSC). A heat capacity anomaly was observed at 732 K, which has been attributed to the magnetic order-disorder transition from ferrimagnetic state to paramagnetic state. The standard molar enthalpy of formation, (298.15 K) and the standard molar entropy, (298.15 K) of SrFe₁₂O₁₉(s) were calculated by second law method and the values are −5545.2 kJ mol⁻¹ and 633.1 J K⁻¹ mol⁻¹, respectively.     

Decomposition of (Ti2xFe1−xSb1−x)O4 solid solutions below 1673 K

The pseudo-binary TiO₂-FeSbO₄ system was investigated by means of thermogravimetric analysis below 1673 K in O₂. Rutile-type solid solutions were synthesised at 1373 K in O₂ by means of a solid state reaction between the two pure end members TiO₂ (rutile) and FeSbO₄ mixed in stoichiometric amounts. Thermal stability of the (Ti_2xFe_1−xSb_1−x)O₄ solid solution increases with rutile content; equimolar (Ti_1.00Fe_0.50Sb_0.50)O₄ solid solutions decompose at about 1673 K forming a TiO₂-enriched solid solution and FeSbO₄, that subsequently decomposes into Fe₂O₃ (hematite) and a volatile Sb oxide, probably Sb₄O₆. For compositions characterised by higher Ti content the decomposition temperature is higher than 1673 K.     

Crystal and magnetic structure and cation distribution of Mn2−xV1+xO4 spinels (x = 0, 1/3 and 1)

We synthesized the spinel-type compounds belonging to the Mn_2−xV_1+xO₄ series with x = 0, 1/3 and 1 as polycrystalline powders. Crystal and magnetic structures were refined using synchrotron X-ray and neutron powder diffraction. At 300 K all members crystallize in the cubic system, space group , and show a structural transition at low temperature, changing to a tetragonal symmetry (space group I4₁/amd). Cations distributions between octahedral and tetrahedral sites were refined from neutrons diffraction (ND) data and explained based on crystal field stabilization energies (CFSE) and ionic radii. The magnetic unit cell is the same as the crystallographic one, having identical symmetry relations. The magnetic structure was refined as an arrangement of collinear spins, antiferromagnetically ordered, parallel to the c-axis of the unit cell. The refined site magnetic moments are smaller than those obtained from hysteresis cycles of the M vs. H measurements, indicating that some non-collinear disordered component coexists with the ordered component along the c-axis.     

Oxygen vacancy ordering in strontium doped rare earth cobaltate perovskites Ln1−xSrxCoO3−δ (Ln = La, Pr and Nd; x > 0.60)

A family of Sr-doped perovskite compounds Ln_1−xSr_xCoO_3−δ (Ln = La³⁺, Pr³⁺ and Nd³⁺; x > 0.60), were prepared by sol-gel chemistry and reaction at 1100 °C under 1 atm of oxygen. This structural family has been shown to be present only for rare earth ions larger than Sm³⁺ and an upper limit of Sr²⁺ solubility in these phases was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m) perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite unit along the crystallographic c-axis. No additional Ln³⁺/Sr²⁺ cation ordering was observed.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

The effect of Cu content on the structure of Ni1−xCuxFe2O4 spinels

A series of Ni_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.5) spinels were synthesized employing sol-gel combustion method at 400 °C. The decomposition process was monitored by thermal analysis, and the synthesized nanocrystallites were characterized by X-ray diffraction, transmission electron microscopy, infra-red and X-ray photoelectron spectroscopy. The decomposition process and ferritization occur simultaneously over the temperature range from 280 °C to 350 °C. TEM indicates the increase of lattice parameter and particle size with the increase of copper content in accordance with the XRD analysis. Cu²⁺ can enter the cubic spinel phase and occupy preferentially the B-sites within x = 0.3, and redundant copper forms CuO phase separately. A broadening of the O 1s region increases with the increment of copper content compared to pure NiFe₂O₄, showing different surface oxygen species from the spinel and CuO. Cu²⁺ substitution favors the occupancy of A-sites by Fe³⁺.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

Synthesis and characterization of a new organic dihydrogenomonophosphate [3,5-(CH3O)2C6H3NH3]2(H2PO4)2

Chemical preparation, crystal structure, calorimetric studies and spectroscopic investigation are given for a new organic cation dihydrogenomonophosphate [3,5-(CH₃O)₂C₆H₃NH₃]₂(H₂PO₄)₂. This compound is triclinic with the following unit cell parameters: a=9.030(6) Å, b=16.124(5) Å, c=8.868(3) Å, α=75.04(3)°, β=110.71(4)°, γ=104.61(1)°, Z=4, V=1148.0(1) Å³, Z=2 and ρ_cal.=1.454 g cm⁻³. Crystal structure was solved and refined to R=0.04, 2752 independent reflections. The atomic arrangement can be described as inorganic layers of H₂PO₄⁻ anions parallel to planes, between which are located the organic groups. Solid-state and MAS-NMR spectroscopies are in agreement with the X-ray structure. Ab initio calculations allow the attribution of the phosphorous and carbon signals to the independent crystallographic sites and to the various atoms of the organic groups.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

Spectroscopic and glass transition investigations on Nd-doped NaF-Na2O-B2O3 glasses

New developments in photonic technology need new materials for various applications. In the present report, Nd³⁺-doped NaF-Na₂O-B₂O₃ glasses were prepared and the spectroscopic and glass transition properties were analysed. The Fourier transform infrared spectral studies reveal that the glass contains BO₃ and BO₄ units as the local structures and the Na⁺ ions as the network modifiers. The absorption studies were carried out by using Judd-Ofelt theory, the experimental and theoretical oscillator strengths were also calculated. The emission spectral study was done for the 1 mol% Nd-doped glass and the spontaneous emission probability and stimulated emission cross-sections for the , transitions were calculated using the J-O parameters.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Synthesis and optical properties of (Gd1−x,Eux)2O2SO4 nano-phosphors by a novel co-precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphors were synthesized by a novel co-precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and NaOH starting materials. Composition of the precursor is greatly influenced by the molar ratio of NaOH to (Gd_1−x,Eu_x)₂(SO₄)₃ (the m value), and the optimal m value was found to be 4. Fourier transform infrared spectrum (FT-IR) and thermal analysis show that the precursor (m = 4) can be transformed into pure (Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphor by calcining at 900 °C for 2 h in air. Transmission electron microscope (TEM) observation shows that the Gd₂O₂SO₄ phosphor particles (m = 4) are quasi-spherical in shape and well dispersed, with a mean particle size of about 30-50 nm. Photoluminescence (PL) spectroscopy reveals that the strongest emission peak is located at 617 nm under 271 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 10 mol% and the concentration quenching mechanism is exchange interaction among the Eu³⁺ ions. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions has a single exponential decay behavior.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.