Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

Structure and magnetism in Ho1−xSrxCoO3−δ

We have magnetically and structurally characterized the Ho_1−xSr_xCoO_3−δ family of materials where 0.67≤x≤0.95. The solid solution range and evolution of the structure as a function of x is established and correlated with the broad range of magnetic behavior observed. The structure is shown to be tetragonal I4/mmm although is possibly cubic when x=0.95. For 0.67≤x≤0.9 the material shows antiferromagnetic long range order and ferromagnetic clusters. At x=0.95 the magnetic transition is at 120 K and the imaginary susceptibility becomes non-zero and the temperature of the cusp in the ac susceptibility shows a frequency dependence indicative of glassiness.     

Electrical conductivity and structural stability of SrCo1−xFexO3−δ

Perovskite compounds in the system of SrCo_1−xFe_xO_3−δ (x=0.2, 0.4 and 0.6) were synthesized by solid state reaction. SrCo_1−xFe_xO_3−δ shows the p-type small polaron conduction behavior. Electrical conductivity and oxygen vacancy content decrease with increase in Fe content. The incorporation of Fe increases the structural stability of SrCo_1−xFe_xO_3−δ at low temperatures, while decreasing the structural stability at high temperatures. Oxygen partial pressure has a strong influence on electrical conductivity. At low oxygen partial pressure, SrCo_0.8Fe_0.2O_3−δ will transform from cubic to orthorhombic structure. This structure can remain in 5%H₂/Ar only for a short time and then dissociates into Sr₃Fe₂O_6.64 and Co due to the reduction of B-site elements.     

Structural and electronic properties of zinc blende BxAl1−xNyP1−y quaternary alloys via ?rst-principle calculations

The structural and electronic properties of cubic zinc blende BN, BP, AlN and AlP compounds and their B_xAl_1−xN_yP_1−y quaternary alloys, have been calculated using the non relativistic full-potential linearized-augmented plane wave FP-LAPW method. The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW) as well as the generalized gradient approximation (GGA) of Perdew-Burke and Ernzerhof (GGA-PBE). The calculated structural properties of BN, BP, AlN and AlP compounds are in good agreement with the available experimental and theoretical data. A nonlinear variation of compositions x and y with the lattice constants, bulk modulus, direct and indirect band gaps is found. The calculated bowing of the fundamental band gaps is in good agreement with the available experimental and theoretical value. To our knowledge this is the first quantitative theoretical investigation on B_xAl_1−xN_yP_1−y quaternary alloy and still awaits experimental confirmations.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La1−xSrxnO3−δ thin films

The paper presents the fabrication and characterization of La_0.65Sr_0.35MnO_3−δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

Excitons in cylindrical GaAs–Ga1−xAlxAs quantum dots under applied electric field

The exciton binding energy and photoluminescence energy transition in a GaAs-Ga_1−xAl_xAs cylindrical quantum dot are studied with the use of the effective mass approximation and a variational calculation procedure. The influence of these properties on the application of an electric field along the growth direction of the cylinder is particularly considered. It is shown that for zero applied field the binding energy and the photoluminescence energy transition are decreasing functions of the quantum dot radius and height. Given a fixed geometric configuration, both quantities then become decreasing functions of the electric field strength as well.     

High temperature transport and thermoelectric properties of Ca3−xErxCo4O9+δ

Ca_3−xEr_xCo₄O_9+δ (x=0, 0.05, 0.15, 0.3 and 0.5) samples were prepared using a sol-gel method followed by hot-pressing sintering technique. Powder X-ray diffraction analysis showed the single-phases of Ca_3−xEr_xCo₄O_9+δ were obtained up to x=0.3 and the crystallinity of the samples decreased with increasing Er-doping amount. The high-temperature (323-1073 K) thermoelectric properties of the samples were investigated. The substitution of Er³⁺ for Ca²⁺ resulted in the decrease of electrical conductivity, thermal conductivity and the increase of Seebeck coefficient for all the samples except the x=0.05 one. The dimensionless figure of merit ZT reached 0.28 at 1073 K for Ca_2.7Er_0.3Co₄O_9+δ, indicating the thermoelectric properties of CaCo₄O_9+δ can be enhanced by doping Er in the system.     

Non-universal dielectric relaxation in SrFeO3 − δ

SrFeO_3 − δ compound is prepared by the thermal decomposition method followed by ball milling. Analysis of Mössbauer spectrum and X-ray diffraction study proves the presence of multi-phase nature, i.e., Sr₈Fe₈O₂₃ and Sr₄Fe₄O₁₁ phases at room temperature. Furthermore, the Mössbauer spectrum at room temperature evidenced the presence of major Fe^3.5+ which is the resultant of equal contributions of Fe⁴⁺ and Fe³⁺. The Nyquist plot at all measured temperatures (80–230 K) suggests that the dielectric response is well associated with single relaxation time (exponential parameter, n∼1$n\sim 1$) i.e., the Debye-type. Modulus analysis exhibits the non-universal dielectric behaviour (stretched exponential parameter, β>1$\beta >1$) below 230 K and the Debye-type responds (β∼1$\beta \sim 1$) at and above 230 K. The Debye-type behaviour exhibited by SrFeO_2.81 at around room temperature in its defect state offers a new opening for this material for multifunctional applications.     

Effects of Zn substitution on the magnetic and transport properties of La0.6Sr0.4Mn1−yZnyO3−δ (0≤y≤0.3)

Phase-singular solid solutions of La_0.6Sr_0.4Mn_1−yZn_yO_3−δ(0≤y≤0.3) [LSMZO] perovskite of rhombohedral symmetry (space group: ) with y up to 30 at.% could be synthesized notwithstanding the differences in ionic radii of Mn_{V I}³⁺ (i.r.=0.645 Å) and Zn_{V I}²⁺ (i.r.=0.74 Å). The LSMZO≤02 compositions are ferromagnetic metallic (FMM) at room temperature whereas LSMZO-02-08 are ferromagnetic insulators (FMI) and LSMZO>08 are paramagnetic insulators (PMI). Total obliteration of the FM transition is unique to Zn-doping at leading to PMI even at low temperatures, measured up to 8 K (presently). The FM to PM transition (T_c) and the peak (T_p) in resistivity-temperature curves decreases with the Zn-content. The charge-transport in p-type LSMZO is predictable by variable range hopping (VRH), which changes to nearest-neighbor hopping of small polarons (NNHP) at T>T_p. Non-stoichiometry (0.005≤δ≤0.21) evaluated chemically from redox titrations indicated the prevalence of excess oxygen vacancy rather than charge compensatively predictable values which, in turn, indicates the diminishing Mn⁴⁺ content in LSMZO. The ’s act as electron donors in p-LSMZO and this increases the resistivity (ρ_RT) associated with the shift in T_c to low temperatures. Increased ρ_RT on annealing in low is a clear evidence on the role of in LSMZO.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

Effects of hydrostatic pressure on the Coulomb-bound states in GaAs–Ga1−xAlxAs semiconductor superlattices

The effects of hydrostatic pressure on the Coulomb-bound states in GaAs–Ga_1−xAl_xAs and GaAs–AlAs semiconductor superlattices are theoretically studied. Calculations of the impurity binding energies for different configurations of the system and for various values of the hydrostatic pressure are performed in the framework of the parabolic-band and effective-mass schemes, and within the variational procedure. The hydrostatic-pressure dependence on the exciton energy is also obtained, and theoretical results are compared and found in good agreement with available experimental measurements.     

Formation of α-Si1−xCx:H and nc-SiC films grown by HWCVD under different process pressure

In this work, hydrogenated amorphous silicon carbide (α-Si_1−xC_x:H) and nanocrystalline SiC (nc-SiC) thin films were deposited by hot wire CVD (HWCVD) using SiH₄/C₂H₂/H₂ gas mixtures. It was found that the films prepared under low gas pressure were α-Si_1−xC_x:H and those prepared under high gas pressure were nc-3C-SiC. The α-Si_1−xC_x:H films showed enhanced density of C-H_n and Si-C bonds with increasing C₂H₂ fraction, which induced an increase in optical gap from 1.8 to 3.0 eV. For the deposition process of nc-SiC, the E_g opt of the deposited films varied from 1.9 eV to 2.5 eV as the filament temperature increased from 1700 to 2100 °C. The deposition rate decreased rapidly from 5.74 nm/min to 0.8 nm/min with increasing T_F.     

Multi-band nature of superconducting anisotropy in PrFeAsO1−δ single crystal

We investigated the magnetic torque of superconducting PrFeAsO_1−δ single crystal using the torque magnetometer at temperatures from 2 K to 200 K in magnetic field of 30 kG. The torque curve tends to have a singular shape compared to other superconductors. The analyses of the PrFeAsO_1−δ torque curves by the single-band and the multi-band Kogan model yield 3 < γ_λ < 5 in 30 kG. With the aid of the multi-band Kogan model, however, we obtain γ_ξ = 0.79 ± 0.01 and γ_λ = 19 ± 3 in 30 kG at 20.5 K. The smallness of γ_ξ(<1) thus obtained is due to the multi-band nature of the iron pnictide.     

Antiferroelectric phase transition in pyrochlore-like (Dy1− xCax)2Ti2O7 − δ (x = 0, 0.1) high temperature conductors

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy_1 − xCa_x)₂Ti₂O_7 − δ (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy₂Ti₂O₇ leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.     

Synthesis and magnetic properties of Co1−xZnxFe2O4+γ nanoparticles as materials for magnetic fluid hyperthermia

Nanoparticles of the single spinel phase Co_1−xZn_xFe₂O_4+γ of mean size 3-23 nm, as determined by X-ray diffraction analysis, were synthesized by the co-precipitation method followed by a temperature treatment. Magnetic studies carried out in the range of 4.5-550 K revealed gradual transition from ferrimagnetic to superparamagnetic to paramagnetic behaviour depending on the composition and particle size. The observed behaviour indicates a broad distribution of volume sizes of the nanoparticles. Particular importance can be ascribed to the composition of x=0.6 where the observed transition temperature to the paramagnetic state at 310-334 K suggests applicability of this material for magnetic fluid hyperthermia in a self-controlled regime.     

Study on the structure and optical property of Zn1−xCuxO sol–gel thin films on quartz substrate

Zn_1−xCu_xO thin films (x=0, 1.0, 3.0, 5.0%) are prepared on quartz substrate by sol–gel method. The structure and morphology of the samples are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results show that Cu ions were effectively penetrated into the ZnO crystal lattices with substitutional and interstitial impurities to form stable solid solutions without changing the polycrystalline wurtzite structure. Two peaks at 420 nm (2.95 eV, violet), 485 nm (2.56 eV, blue) have been observed from the photoluminescence (PL) spectra of the samples. It is concluded that the violet peak may correspond to the exciton emission; the blue emission corresponds to the electron transition from the bottom of the conduction band to the acceptor level of zinc vacancy. The optical test shows that the optical band gap E_g is decreased with the increase amount of Cu doping in ZnO. The band gap decrease from 3.40 eV to 3.25 eV gradually. It is also found that the transmission rate is increased rapidly with the increase of Cu ions concentration.     

Stability, oxygen permeability and chemical expansion of Sr(Fe,Al)O3 − δ- and Sr(Co,Fe)O3 − δ-based membranes

Perovskite-type SrFe_0.7Al_0.3O_3 − δ and SrCo_0.8Fe_0.2O_3 − δ, and two related dual-phase composites with nominal compositions (SrFeO_3 − δ)_0.7(SrAl₂O₄)_0.3 and (SrCo_0.8Fe_0.2O_3 − δ)_0.7(SrAl₂O₄)_0.3, were comparatively studied employing controlled-atmosphere dilatometry, thermogravimetry, Mössbauer spectroscopy, and measurements of steady-state oxygen permeation fluxes through dense ceramic membranes. The composite materials display lower thermal and chemical expansion compared to the parent single-phase perovskites. The thermal expansion coefficients at 1023-1223 K are however still high, (20-23) × 10^− 6 K^− 1 at atmospheric oxygen pressure and (17-18) × 10^− 6 K^− 1 at p(O₂) = 10 Pa, thus limiting the range of possible membrane reactor configurations. Sr(Co,Fe)O_3 − δ-based materials exhibit extensive vacancy-ordering processes in inert atmospheres, resulting in a slow relaxation of the oxygen nonstoichiometry, chemical expansion and oxygen permeation fluxes. In comparison to Sr(Fe,Al)O_3 − δ, the stability of cobalt-containing ceramics in CO₂ is also poor, which leads to a partial blocking of the membrane surface by decomposition products and degradation of the oxygen transport. Thermogravimetric analysis showed that the interaction with carbon dioxide occurs even at elevated temperatures, up to 1223 K. Under high oxygen chemical potential gradients such as air/(H₂-H₂O), the composite membranes showed kinetically stable operation without bulk decomposition at 1073 K. The kinetic stabilization associated with surface-limited oxygen permeation was confirmed by the conversion-electron Mössbauer spectroscopy analysis of one (SrFeO_3 − δ)_0.7(SrAl₂O₄)_0.3 membrane exposed to dry CH₄ at 1173 K, where no traces of Fe²⁺ and metallic iron were detected in the reduced surface layer.