A comparative study of Sr-doped LaMnO3 synthesised via solid-state reaction and sol–gel methods

ABSTRACT

This work deals with the synthesis of hole-doped La_1-xSr_xMnO₃ (x?=?0.25 and 0.33) perovskite via two different ways, namely a solid-state reaction method and a sol–gel process. Various properties of the samples have been investigated and compared by means of different techniques, namely X-ray diffraction (XRD), temperature-dependent resistivity measurements in the temperature range 25–300?K using a closed cycle refrigerator and vibrating sample magnetometry (VSM). All samples have orthorhombic crystal symmetry and an irregular variation in unit-cell volume with increase in Sr content. The average crystallite size was determined from the XRD data and found to lie in the range 19–22?nm. Metal–insulator transitions (MIT) were observed in all the samples via low-temperature electrical characterisations. The magnetic measurements confirm that the magnetisation does not saturate for the 25% Sr doped samples up to the available field of 10?kOe. The magnetic response also supports the interpretation of XRD and resistivity data. The obtained results are explained on the basis of double-exchange and super-exchange mechanisms.     

Study on the damping behaviour of eutectic high-entropy alloys with lamellar structures

ABSTRACT

Damping is the ability to reduce the energy produced by vibrations in a system. In this research, the eutectic high-entropy alloys (EHEAs), AlCo_xCrFeNi_2.1 (x?=?0.7, 1.0 and 1.5), are firstly studied for their damping behaviour. EHEA is a promising concept of high-entropy alloys. By combining the advantages of eutectic phases and HEAs, the EHEAs show better liquidity, castability and compositional homogeneity than conventional HEAs. Meanwhile, it can strike a great balance in mechanical strength and tensile ductility owing to the lamellar structures. The microstructures and crystal structure of two near-EHEAs are also studied. AlCo_0.7CrFeNi_2.1 is a hypo-eutectic HEA with primary body-centred cubic phases. AlCo_1.5CrFeNi_2.1 is a hyper-eutectic HEA with primary face-centred cubic phases. The results showed that these eutectic and near-eutectic alloys gave rise to greater tan δ values than conventional dual-phase HEAs, which might be attributed to their lamellar structures.     

Microwave-dielectric and magnetic properties of Ta-doped BiFeO3 nanopowders

BiFe_{1? x} Ta _x O₃ (x = 0, 0.05, 0.1) nanopowders have been fabricated by a simple sol–gel method. Dielectric measurements at microwave frequencies (2–18 GHz) were made using a vector network analyser. Without tantalum, the BiFeO₃ nanopowder presents a relaxation-like response with a characteristic frequency of 15 GHz, which can be associated with an overdamped process. The Ta-doped nanopowders, however, show resonant behaviour with resonant frequencies of 12.5 and 14.6 GHz. The intensity of the resonant peak near 14.6 GHz decreases with increasing Ta addition. This behaviour is associated with a damped resonance process. Room-temperature magnetic measurements indicate that the addition of Ta influences the magnetic properties of the BiFeO₃ nanopowders, with BiFe_0.95Ta_0.05O₃ having the strongest ferromagnetism with a saturation magnetisation M _s of about 0.05 µ_B/Fe. The origin of the enhanced ferromagnetism is possibly associated with the distortion of the oxygen octahedral by the Ta substitution or/and the statistical distribution of Fe³⁺ and Fe²⁺.     

Influence of Zn incorporation on the microstructural and magnetic properties of La0.67Sr0.33Mn1−xZnxO3 nanoparticles synthesised by the sol–gel method

Perovskite manganite nanoparticles with the chemical formula La_0.67Sr_0.33Mn_1?xZn_xO₃ (x = 0, 0.1 and 0.2) were prepared by the sol–gel process and the influence of Zn substitution on their microstructural and magnetic properties studied in detail. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for crystal structure determination and microstructural analysis. The results of XRD analysis confirmed the polycrystalline nature and rhombohedral crystal structure with space group R(-3c) for all compositions. SEM micrographs of the samples revealed an agglomeration/well-connected appearance of the particles. The composition and uniform distribution of all the elements were established by energy-dispersive spectroscopy. TEM images clearly confirmed an almost spherical shape of the nanoparticles with an average particle size of about 11, 14 and 13 nm for the 0, 10 and 20% Zn-doped samples, respectively. The magnetic behaviour of the nanoparticles was studied at room temperature with a vibrating sample magnetometer (VSM) up to a field of 1.8 Tesla. These measurements established a superparamagnetic nature with a variation in the saturation magnetization (M_s) with Zn content.     

Electrical conductivity of Nd and Ce co-doped Gd2Zr2O7 ceramics

In this study, (Gd_{1? x} Nd _x )₂(Zr_{1? x} Ce _x )₂O₇ (0 ≤ x ≤ 0.5) ceramics have been prepared by pressureless sintering at 1973 K to investigate the influence of Nd and Ce co-doping on their electrical conductivity. The electrical conductivity of the ceramics was investigated by impedance spectroscopy measurements from 723 to 1173 K over the frequency range of 20 Hz to 2 MHz in air. The measured values obey the Arrhenius relation. For each composition, the grain conductivity gradually increases with increasing temperature from 723 to 1173 K. At a given temperature, it gradually decreases with increasing neodymium and cerium contents from x = 0 to 0.3; thereafter, the grain conductivity exhibits a slight increase with further increasing neodymium and cerium contents up to x = 0.5.     

On the glass transition temperature and the elastic properties in Zr-based bulk metallic glasses

The temperature dependence of the elastic moduli was estimated from ultrasound time of flight measurements performed on bulk metallic glasses of composition Zr_{63? x} Cu₂₄Al _x Ni₁₀Co₃. Using the corresponding values at the glass transition temperature, the local atomic strain was determined. The obtained values for the critical atomic strain calculated for 8 at%?<?x?<?15 at% are close to the predicted universal criterion derived from a topological model, but may also reflect the difference in the chemical interaction that are not accounted by a topological approach.     

A study of the spin-Hamiltonian parameters for Cr5+ at the rhombically-distorted tetrahedral P5+ site of Ca2PO4Cl crystal using a two-mechanism model

The complete high-order perturbation formulae of spin-Hamiltonian (SH) parameters (g factors g_i and hyperfine structure constants A_i , where i = x, y, z) containing contributions from both the crystal-field (CF) and charge-transfer (CT) mechanisms (the latter mechanism is neglected in the widely-used CF theory) are established for d¹ ions in rhombic tetrahedra. From these formulae, the SH parameters of Cr⁵⁺ ion at the rhombically-distorted tetrahedral P⁵⁺ site of Ca₂PO₄Cl crystal are calculated. The CF and CT energy levels used in the calculation are obtained from the optical spectra of the studied Ca₂PO₄Cl : Cr⁵⁺ crystal. The calculated results showed reasonable agreement with the experimental values. The signs of the hyperfine structure constants A_i and the relative importance of the CT mechanism to SH parameters are acquired from the calculations.     

Determination of glass-forming ability of Ge1− x Sn x Se2.5 (0 ≤ x ≤ 0.5) alloys using differential scanning calorimetry

The glass-forming ability of Ge_{1? x} Sn _x Se_2.5 (0 ≤ x ≤ 0.5) alloys was studied using differential scanning calorimetry. Samples were scanned at different heating rates under non-isothermal conditions. Various simple quantitative methods were employed to assess the stability of the glassy materials in the above-mentioned system. All of these methods are based on characteristic temperatures, such as the glass-transition temperature, T _g, the onset-of-crystallization temperature, T _c, and the peak crystallization temperature, T _p. A crystallization rate factor, K, has also been used as a measure of the thermal stability of the glasses. It was found that Ge_0.7Sn_0.3Se_2.5 was the least stable among all the samples.     

Structure of PbMnO2.75 determined by electron crystallography

The PbMnO_{3– x} system has a great number of phases with large unit cells for different values of x. In the present work, we study a PbMnO_2.75 phase that has proven too complex to be solved by X-ray powder diffraction. Using electron crystallography, we have determined the positions of all metal atoms without the necessity of using a model as a starting point. The results correspond, within experimental uncertainties, to those obtained by Bougerol et al. (J. Solid State Chem. 169 131 (2002)).     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

The crystal structure of Al20.6Ta22.4 and its relation to Frank–Kasper phases

The monoclinic phase in the Al–Ta system has been identified by means of X-ray single-crystal structure analysis as Al_{19+ x} Ta_{24? x} (x?=?1.6), Pearson symbol mP86. Al_20.58Ta_22.42 crystallizes in the space group P2₁/n, with a?=?987.86?pm, b?=?990.12?pm, c?=?1489.45?pm and β?=?99.958°. Twinned crystals of the title compound can be obtained by iodine-promoted reactions of the elements. In the experiments described here, the reactions took place in sealed tantalum ampoules at 1400°C. Single-phase samples were obtained by a metallurgical powder method, which minimizes aluminium loss by incongruent vapourization. The composition was found to span the range 0.51?≤?x _Ta?≤?0.53. Al_{19+ x} Ta_{24? x} decomposes in the solid state at 1450°C into Al₆₉Ta₃₉ and σ-AlTa₂. The crystal structure is analyzed in relation to tetrahedral close-packed structures. The kinship with Cr₃Si-type and H-type structures is accentuated.     

Investigations of the optical spectra and EPR g-factors for the tetragonal Ce3+ centers in YPO4 and LuPO4 crystals

The optical spectral band positions and EPR g-factors (g_‖ , g _⊥) for the tetragonal Ce³⁺ centers in YPO₄ and LuPO₄ crystals with the zircon-structure are calculated using a complete diagonalization (of the energy matrix) method (CDM) related to 4f¹ ions in tetragonal symmetry. In this method, the Zeeman interaction term are added to the Hamiltonian in the conventional CDM and so no perturbation calculations are required to obtain the g factors. The crystal-field parameters used in the calculations are obtained from the superposition model in which the local lattice relaxation related to the bonding lengths is considered. The calculated results are in reasonable agreement with the experimental values. It is found that the four observed optical bands for both the systems can be attributed to Ce³⁺ ions in a tetragonal crystal field.     

Synthesis and evolution of crystalline garnet phases in Y3Sc5– x Ga x O12

Mixed-metal oxides with the composition Y₃Sc _x Ga_{5? x} O₁₂ (x?=?2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, and 3.0) have been prepared by an aqueous sol–gel method. The effects of scandium substitution on the garnet phase formation were studied by IR spectroscopy and X-ray powder diffraction (XRD). The XRD data indicate that single-phase Y₃Sc _x Ga_{5? x} O₁₂ ceramic samples were obtained for x?=?2.0, 2.1, 2.2, 2.25, 2.3, and 2.4. The results also show that the formation of Y₃Sc _x Ga_{5? x} O₁₂ garnets depends on the molar ratio of scandium and gallium in the investigated composition, and consequently on the mean cationic radius at the Al³⁺ sites. The variation of lattice parameters for the Y₃Sc _x Ga_{5? x} O₁₂ phases with different x is reported.     

Synthesis,transformation and superconductivity of dual phase In–Sn alloy nanoparticles embedded in an Al matrix

We report the synthesis of nanoembedded biphasic alloy particles of In–Sn near eutectic alloy compositions embedded in an aluminium matrix by rapid solidification processing. Detailed transmission electron microscopy indicates that the two phases present at room temperature in as-synthesized samples are β and γ phases with tetragonal and hexagonal crystal structures, respectively. These co-exist with a small amount of single phase In or Sn particles with sizes less than 10?nm. Low temperature magnetization measurements indicate a superconducting transition temperature of 5?K, suggesting complete decomposition of the β-phase at small sizes and at low temperature. The small particles show type II behavior with a critical field Hc₁≈44?G and two values for Hc₂ of 250 and 1000?G, respectively. These values are considerably lower than those observed in bulk In–Sn alloys.     

Solid-solution strengthening in ordered In x Ga1 − x P alloys

Ordered In _x Ga_{1??? x} P alloys have been grown on to GaAs(001) substrates by metal–organic vapour-phase epitaxy. Lattice-matched compositions of alloys (In _x Ga_{1??? x} P with x?≈?0.5) were used in order to produce epitaxial layers free of structural defects (threading dislocations). Growth temperature and substrate orientation were adjusted to control the degree η, of order, of the alloys in the range 0.3–0.5. Nanoindentation tests were carried out to measure the mechanical response of the heteroepitaxial layers. Transmission electron microscopy was used to characterize the structure as well as the plastic zones generated into the specimen by nanoindentation. Comparison of the alloys with binary references (InP and GaP bare substrates) showed strengthening, with the hardness and flow-stress values that are much larger than those of InP and of the same order of magnitude as those of GaP.     

Electronegativity difference as a factor for evaluating the thermal stability of Al-rich metallic glasses

This article proposes a factor, the critical electronegativity difference Δx _cri, to correlate alloy composition with thermal stability and glass-forming ability of Al–Ni–RE (RE: Rare Earth element) ternary metallic glasses. The Al-rich metallic glasses with Δx > Δx _cri exhibit glassy behavior, whereas alloys with Δx < Δx _cri are nanocrystalline. Nanoglassy alloys occur when Δx ≈ Δx _cri. The best glass formers are located near Δx _cri. Furthermore, an equation has been deduced to calculate Δx _cri with varying RE covalent atomic radius.     

Conduction-electron mediated 1H nuclear spin-lattice relaxation in Ti45Zr38Ni17H x icosahedral quasicrystals

Nuclear magnetic resonance spin-lattice relaxation rates for ¹H in quasicrystalline Ti₄₅Zr₃₈Ni₁₇H _x are presented as a function of temperature and hydrogen concentration x. The temperature dependence demonstrates that the relaxation is via interaction with conduction electrons. The relaxation rate is extremely sensitive to hydrogen content, with the rate changing by a factor of as much as two for samples that differ in x     

Structural,elastic, electronic,phonon and thermal properties of Ir3Ta and Rh3Ta alloys

We have studied the structural, elastic, electronic, phonon and thermodynamic properties of Ir₃Ta and Rh₃Ta alloys, using ab initio calculations. For the L1₂ phase, we report the calculated lattice constants, bulk modulus and elastic constants, and these values are compared with previously published values. We also derive the elastic constants from the values of the slopes of the acoustic branches in the phonon dispersion curves. The band structures show that both materials are metallic. The phonon dispersion curves, and their corresponding total and projected densities of states, are obtained using a linear response in the framework of the density functional perturbation theory. The specific heat capacity at constant volume and different temperatures is calculated, and this aspect is discussed using the quasi-harmonic approximation.     

Crystal growth velocity in deeply undercooled Ni–Si alloys

The crystal growth velocity of Ni₉₅Si₅ and Ni₉₀Si₁₀ alloys as a function of undercooling is investigated using molecular dynamics simulations. The modified imbedded atom method potential yields the equilibrium liquidus temperatures T _L?≈?1505 and 1387?K for Ni₉₅Si₅ and Ni₉₀Si₁₀ alloys, respectively. From the liquidus temperatures down to the deeply undercooled region, the crystal growth velocities of both the alloys rise to the maximum with increasing undercooling and then drop slowly, whereas the athermal growth process presented in elemental Ni is not observed in Ni–Si alloys. Instead, the undercooling dependence of the growth velocity can be well-described by the diffusion-limited model, furthermore, the activation energy associated with the diffusion from melt to interface increases as the concentration increases from 5 to 10?at.% Si, resulting in the remarkable decrease of growth velocity.     

The evaluation of linear forms

This presentation deals with the evaluation and transformation of linear forms. Especial emphasis is given to implicit methods in which it is not necessary to find the explicit values,x _i . The relation of the Aitken triple product matrixCA ^–1 B to the result of a linear transformation of linear forms is noted, and the numerical computation of this triple product matrix is indicated with the use of the simple Abbreviated Doolittle solution. Application is also made to the evaluation ofA ^–1 and ofA ^–1 C.