Effect of sintering condition on properties of Cr-doped Pb<Subscript>0·95</Subscript>Sr<Subscript>0·05</Subscript>(Zr<Subscript>0·53</Subscript>Ti<Subscript>0·47</Subscript>)O<Subscript>3</Subscript> ceramics

The effects of sintering temperature (T _sinter) and sintering time (t _sinter) were investigated for Pb_0·95Sr_0·05(Zr_0·53Ti_0·47)O₃ + x%wtCr₂O₃ system to determine the optimum regimen of sintering. The study was performed by determining the following parameters: relative density (ρ _rel) and sintering rate $ (\dot \varepsilon _\rho ) $ (\dot \varepsilon _\rho ) . The grain size was diminished when small amounts of Cr₂O₃ were added to system and was enhanced when the sintering temperature and time was increased. The dielectric constant becomes maximum and the electric loss becomes minimum with the sintering temperature of 1290°C. The optimum regimen of sintering is T _sinter = 1290°C during 70 min. The piezoelectric properties for the optimum regimen of sintering were studied.     

Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

The high temperature ceramic oxide superconductor YBa₂Cu₃O_7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF₂ as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 10⁵ Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O₂, YBa₂Cu₃O_7−x , Y₂BaCuO₅, CuO, BaF₂/CaF₂/BaF₂, BaZrO₃, ZrO₂, O₂, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO₃ and Y₂BaCuO₅ from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol⁻¹.     

Phase relations in the system Cu-La-O and thermodynamic properties of CuLaO2 and CuLa2O4

Phase relations in the system Cu-La-O at 1200 K have been determined by equilibrating samples of different average composition at 1200 K, and phase analysis of quenched samples using optical microscopy, XRD, SEM and EDX. The equilibration experiments were conducted in evacuated ampoules, and under flowing inert gas and pure oxygen. There is only one stable binary oxide La₂O₃ along the binary La-O, and two oxides Cu₂O and CuO along the binary Cu-O. The Cu-La alloys were found to be in equilibrium with La₂O₃. Two ternary oxides CuLaO₂ and CuLa₂O₄₊ were found to be stable. The value of varies from close to zero at the dissociation partial pressure of oxygen to 0.12 at 0.1 MPa. The ternary oxide CuLaO₂, with copper in monovalent state, coexisted with Cu, Cu₂O, La₂O₃, and/or CuLa₂O₄₊ in different phase fields. The compound CuLa₂O₄₊, with copper in divalent state, equilibrated with Cu₂O, CuO, CuLaO₂, La₂O₃, and/or O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 K to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields, Cu + La₂O₃ + CuLaO₂, Cu₂O + CuLaO₂ + CuLa₂O₄ and La₂O₃ + CuLaO₂ + CuLa₂O₄. Although measurements on two cells were sufficient for deriving thermodynamic properties of the two ternary oxides, the third cell was used for independent verification of the derived data. The Gibbs energy of formation of the ternary oxides from their component binary oxides can be represented as a function of temperature by the equations:

Growth and applications of superconducting and nonsuperconducting oxide epitaxial films

Growth and characterization of high-temperature-superconducting YBa₂Cu₃O₇ and several metallic-oxide thin films by pulsed laser deposition is described here. An overview of substrates employed for epitaxial growth of perovskite-related oxides is presented. Ag-doped YBa₂Cu₃O₇ films grown on bare sapphire are shown to giveT _c=90 K, critical current >10⁶ A/cm² at 77 K and surface resistance =450μΩ. Application of epitaxial metallic LaNiO₃ thin films as an electrode for ferroelectric oxide and as a normal metal layer barrier in the superconductor-normal metal-superconductor (SNS) Josephson junction is presented. Observation of giant magnetoresistance (GMR) in the metallic La_0·6Pb_0·4MnO₃ thin films up to 50% is highlighted.     

Thermodynamic properties and dissociation of SrLaGaO4

The solution enthalpy of SrLaGaO₄ in the 2PbO · B₂O₃ melt at 973 K was measured using a hightemperature Calvet microcalorimeter. The enthalpies of formation of SrLaGaO₄ from the constituent oxides and constituent elements were determined to be -96.8 ±5.9 and -2128.7 ±6.1 kJ/mol, respectively. The partial pressures of the vapor species over molten SrLaGaO₄ at T_m = 1793 K were evaluated. The equilibrium oxygen pressure at 1793 K was found to be 4.13 x 10^-2 Pa.     

Preparation and characterization of Ca1 −x La x Ti1 −x Co x O3 (0·00<x⩽0·50) system

The possibility of the formation of solid solution in the system Ca_{1 −x} La _x Ti_{1 −x} Co _x O₃ forx⩽0·5 has been investigated. X-ray diffraction studies show that compositions withx=0·05, 0·1, 0·2, 0·3 and 0·5 prepared by the ceramic method are single-phase materials. All the compositions have a structure similar to CaTiO₃ with a pseudo-cubic unit cell. Preliminary studies show that interfacial polarization contributes significantly to their dielectric constant.     

Thermodynamic properties and dissociation of SrLaAlO4

The enthalpy of solution of SrLaA1O₄ in the 2PbO · B₂O₃ melt at 973 K was measured using a hightemperature Cal vet microcalorimeter. The enthalpies of formation of SrLaA1O₄ from the constituent oxides and constituent elements at 298.15 K were determined to be -97.7 ±5.5 and -2422.9 ±5.6 kJ/mol, respectively. The partial pressures of the vapor species over molten SrLaA1O₄ at T_m = 1923 K were evaluated. The equilibrium oxygen pressure at 1923 K was found to be 5.05 × 10^-3 Pa.     

Effect of calcium substitution on superconductivity and hole concentration in La1·5Ba1·5Cu3O z

The superconducting properties of single phase La_1·5−x Ca _x+y Ba_1·5−y Cu₃O _z , 0·0≤x≤0·60 (LC) and 0·0≤y≤0·70 (CB), compounds with tetragonal triple-perovskite structure are studied, using X-ray diffraction for their resistivity, a.c. susceptibility, and oxygen-content. La_1·5−x Ca _x Ba_1·5Cu₃O _z (LC) samples, 0·15≤x≤0·60, are superconducting withT _c ^R=0 between 40 and 74 K. With the increase inx, the oxygen content, hole concentration in the CuO₂ layers as well as theT _c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LaCa_0·5+y Ba_1·5−y Cu₃O _z (CB) compounds increase with the increase iny, theT _c ^R=0 remains nearly constant around 74 K fory=0·0−0·70. A correlation exists between theT _c and the hole concentration for LC and CB compounds.     

Investigation on the stability of Li5La3Ta2O12 lithium ionic conductors in humid environment

In this paper, the stability in humid air of Li₅La₃Ta₂O₁₂ lithium ionic conductors synthesized by conventional solid-state reaction was investigated by internal friction, conductivity, weight variation, X-ray diffraction, and thermogravimetric analysis methods. It was found that when the Li₅La₃Ta₂O₁₂ samples were aged in open air at room temperature, the internal friction peaks associated with the short-distance diffusion of lithium vacancies gradually shift toward higher temperature and increase in height, while the weight of the sample increases and impurity phases of LiOH·H₂O appear. These results reveal that the Li₅La₃Ta₂O₁₂ compounds are unstable against moisture in open air at room temperature. It was suggested that the protons from the moisture substitute the lithium ions in Li₅La₃Ta₂O₁₂ samples to form Li₂O and new protonic derivatives, Li_5−x La₃Ta₂O_12−x (OH) _x (0<x<2.15), and the resultant Li₂O may react further with water to form LiOH·H₂O.     

Preparation of acicular NiZn-ferrite powders

Acicular NiZn-ferrite powder particles have been prepared from goethite-derived acicular Fe₂O₃ and spherical constituent oxides in the presence of molten chloride or sulphate. The morphology of NiZn-ferrite particles has been studied with reference to the effect of chemical species of molten salts, ferrite composition and particle size of constituent divalent oxides. Hence the best preparation conditions for acicular NiZn-ferrite powder with homogeneous composition have been determined; a mixture of acicular Fe₂O₃ and submicron constituent oxides is heated at 900° C in an amount of potassium chloride with four times the weight of ferrite. Electron diffraction analysis shows that the particle axes of acicular Fe₂O₃ and NiZn-ferrite are parallel to [¯1 1 0 0]_haematite and [¯1 0 1]_ferrite and that a topotactic relation is retained.     

Thermodynamic properties of copper thallium tellurides

The Cu-Tl-Te system has been studied in the composition region Cu-CuTe-Cu₂TlTe₂ using emf measurements on reversible concentration cells relative to a thallium electrode. The partial subsolidus phase diagram inferred from the emf data includes the ternary compounds Cu₉TlTe₅, Cu₃TlTe₂, and Cu₂TlTe₂. The emf data were used to evaluate the relative partial thermodynamic functions ($ \Delta \bar G $ \Delta \bar G , $ \Delta \bar H $ \Delta \bar H and $ \Delta \bar S $ \Delta \bar S ) of the thallium in the alloys. From the subsolidus phase diagram, we identified the potential-determining reactions responsible for these functions and calculated the standard thermodynamic functions of formation and standard entropies of the ternary compounds Cu₉TlTe₅ (−Δ_f G ⁰(298 K) = 244.0 ± 2.4 kJ/mol, −Δ_f H ⁰(298 K) = 241 ± 14 kJ/mol, S ⁰(298 K) = 621 ± 7 J/(mol K)), Cu₃TlTe₂ (117.1 ± 1.2, 117 ± 5, 264 ± 4), and Cu₂TlTe₂ (94.8 ± 0.9, 92 ± 7, 237 ± 3).     

Enthalpy of Formation of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> · 0.793Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nonstoichiometric Spinel at 1900 K from High-Temperature Mass Spectrometry Data

The composition and partial pressures of vapor species over the pseudobinary system Al₂O₃–MgO have been determined by high-temperature mass spectrometry. The data obtained have been used to evaluate the standard enthalpy of formation of nonstoichiometric spinel with the composition MgAl₂O₄ · 0.793Al₂O₃ from its constituent oxides by third-law calculations. The entropy of MgAl₂O₄ · 0.793Al₂O₃ has been evaluated using a regular-solution model with allowance for cation inversion.     

Kinetics of lanthanum oxide sulfurization in ammonium rhodanide vapor

We have studied the kinetics of phase formation in the La-O-S system during La₂O₃ sulfurization in ammonium rhodanide vapor and identified the sequence of transformations of the La₂O₂S₂, La₂O₂S, and La₂S₃ phases in the temperature range 600–900°C. The kinetic curves for the reactions involved were fitted by the Jander equation with a correlation coefficient R ² = 0.99. We have calculated the reaction rate constants, pre-exponential factors, and effective activation energies for the formation of the above compounds: E _a(La₂O₂S₂) = 22.35 ± 3.32 kJ/mol, E _a(La₂O₂S) = 57.67 ± 0.91 kJ/mol, and E _a(La₂S₃) = 45.88 ± 1.16 kJ/mol.     

Nitrates-melt synthesized LiNi0·8Co0·2O2 and its performance as cathode in Li-ion cells

Layered LiNi_0·8Co_0·2O₂ crystallizing in R $\bar 3$ m space group is synthesized by decomposing the constituent metal-nitrate precursors. Oxidizing nature of metal nitrates stabilizes nickel in +3 oxidation state, enabling a high degree of cation ordering in the layered LiNi_0·8Co_0·2O₂. The powder sample characterized by XRD Rietveld refinement reveals < 2% Li-Ni site exchange in the layers. Scanning electron microscopic studies on the as-synthesized LiNi_0·8Co_0·2O₂ sample reflect well defined particles of cubic morphology with particle size ranging between 200 and 250 nm. Cyclic voltammograms suggest that LiNi_0·8Co_0·2O₂ undergoes phase transformation on first charge with resultant phase being completely reversible in subsequent cycles. The first-charge-cycle phase transition is further supported by impedance spectroscopy that shows substantial reduction in resistance during initial de-intercalation. Galvanostatic charge-discharge cycles reflect a first-discharge capacity of 184 mAh g^?1 which is stabilized at 170 mAh g^?1 over 50 cycles.     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Synthesis of Molybdenum- and Vanadium-Containing Mixed Oxides in Polymer–Salt Systems

Rare-earth alkaline-earth mixed oxides containing transition metals (Mo, V) were prepared via pyrolysis in polymer–salt systems. The products were characterized by thermal analysis, resistance measurements, dilatometry, optical microscopy, and x-ray diffraction. The introduction of polyvinyl alcohol into the system containing lanthanum or strontium nitrate and ammonium molybdate was found to have a significant effect on the thermal decomposition process, testifying to changes in the bonding configurations of the constituent components in the systems studied, capable of forming stable gels, which are then used as precursors to synthesize oxide materials. The temperatures of different stages of dehydration were shown to be lower in the polymer-containing systems. The effect of solution acidity was assessed by examining thermal decomposition in systems containing a polymer and Mo or W salts and acidified with nitric acid. The reaction of nitrates (oxidants) with the polymer was accompanied by an exotherm at 170°C, corresponding to the melting of ammonium nitrate, resulting from an exchange reaction. The exothermic reaction was found to reduce the decomposition temperatures of the salts involved. The use of polymer–salt systems allowed the mixed oxides SrMoO₄ and La₂(MoO₄)₃ to be synthesized at lower temperatures in comparison with the coprecipitation of poorly soluble compounds. The method was also shown to be suitable for preparing perovskite oxides in the La_{1 – x} Sr _x Co_{1 – z} M _z O_{3 ± y} (M = Mo, V) systems.     

Synthesis, structural and ferromagnetic properties of La1?xKxMnO3 (0·0 ≤ x ≤ 0·25) phases by solution combustion method

We describe the solution combustion synthesis and characterization of La_1–x K _x MnO₃ (0·0 ≤ x ≤ 0·25) perovskite phases, which is a low temperature initiated, rapid route to prepare metal oxides. As-synthesized compounds are amorphous in nature; crystallinity was observed on heating at 800°C for 5 min. Structural parameters were determined by the Rietveld refinement method using powder XRD data. Parent LaMnO₃ compound crystallizes in the orthorhombic structure (space group Pbnm, No. 62). Potassium substituted compounds were crystallized with rhombohedral symmetry (space group R-3c, No. 167). The ratio of the Mn³⁺/Mn⁴⁺ was determined by the iodometric titration. The Fourier transform infrared spectrum (FT-IR) shows two absorption bands for Mn-O stretching vibration (ν _s mode), Mn-O-Mn deformation vibration (ν _b mode) around 600 cm⁻¹ and 400 cm⁻¹ for the compositions, x = 0·0, 0·05 and 0·10. Four-probe electrical resistivity measurements reveal a composition controlled metal to insulator transition (T _M–I), the maximum T _M–I was observed for the composition La_0·85K_0·15MnO₃ at 287 K. Room temperature vibrating sample magnetometer data indicate that for the composition up to x = 0·10, the compounds are paramagnetic whereas composition with x = 0·15, 0·20 and 0·25 show magnetic moments of 27, 29 and 30 emu/g, respectively.     

Verres formes dans les systemes Ga2S3La2X3MnS (X = S,O)

Preparation of glasses. Glass forming domains in the ternary systems [Ga₂S₃, (La₂S₃, La₂O₂S or La₂O₃), MnS]. Thermal behaviour. U.V. and visible spectroscopy. E.P.R. spectra. Manganese has an octahedral environment of sulfur.     

Standard Gibbs' energy of formation of LaFeO3 and comparison of stability of LaMO3 (M = Mn,Fe, Co or Ni) compounds

The thermodynamic stability of LaFeO₃ at high temperatures is of interest in materials technology. For evaluating the thermodynamic data of LaFeO₃, the e.m.f. of the following galvanic cell was studied over the range 1094 to 1299 K: Pt, LaFeO₃, Fe, La₂O₃/15 mol % CaO stabilized ZrO₂/FeO, Fe, Pt. For the galvanic cell reaction 1/2 La₂O₃ + 3/2 FeO 1/2Fe + LaFeO₃, the standard Gibbs energy change, G _r ⁰ , was measured to be (G _r ⁰ ±0.34) (kJ)=–22.67–0.011 55T (K) for the passage of 3 F of electricity. From this, the stability of LaFeO₃ with respect to La₂O₃ and FeO as well as with La₂O₃ and Fe₂O₃ was computed and compared with other equilibrium measurements on LaFeO₃ reported in the literature. In addition, a comparison was made on the stabilities of LaMO₃ compounds (where M=Mn, Fe, Co or Ni) with respect to dissociation into La₂O₃ and MO making use of the published data on other compounds. Likewise a comparison of disproportionation of LaFeO₃ and LaMO₃ into their respective sesquioxides was also made.     

Effect of low level substitution of Sr–Ba on transport and magnetic behaviour of La<Subscript>0·67</Subscript>Ca<Subscript>0·33</Subscript>MnO<Subscript>3</Subscript>

In this paper we report the investigation of transition metal oxide compound, La_0·67Ca_0·25Sr_0·04Ba_0·04MnO₃ (LCSBMO), along with La_0·67Ca_0·33MnO₃ (LCMO), synthesized by sol–gel route under identical conditions. The effect of simultaneous low level substitution of large size ions such as Sr²⁺ and Ba^2 + for Ca^2 + ions on the electronic transport and magnetic susceptibility properties are analysed and compared apart from microstructure and lattice parameters. The temperature dependent electrical transport of the polycrystalline pellets of LCSBMO and LCMO when obeying the well studied law, r = r₀ + r₂   T²\rho = \rho_{0} + \rho_{2} \;T^{2} for T < T _MI , is observed to differ by more than 50% from the values of ρ ₀ and ρ ₂, with the former compound showing enhanced electrical conductivity than the latter. Similarly in fitting the adiabatic small polaron model for resistivity data of both the samples for T > T _MI , the polaron activation energy is found to differ by about 11%. In addition, the temperature dependent a.c. magnetic susceptibility study of the compounds shows a shift of about 6% in the paramagnetic to ferromagnetic transition temperature (285 K for LCSBMO and 270 K for LCMO).