Ionic conductivity properties and 19F NMR investigation of Pb1−xCrxF2+x solid solutions with fluorite-type structure

The F⁻ ion transport and diffusion properties inside the solid solution Pb_1−xCr_xF_2+x (0≤x≤0.12) with fluorite-type structure have been determined by impedance spectroscopy and ¹⁹F NMR investigation. The various electrical parameters of Pb_1−xCr_xF_2+x are compared with those of Pb_1−xAl_xF_2+x (0≤x≤0.12) whose structure derives also from the fluorite-type. The Cr³⁺ substitutional cation is of a slightly larger size than the Al³⁺ cation but both cations offer the same usual coordination (CN=6) in ternary fluorides. Comparison of ¹⁹F NMR spectra of some compositions Pb_1−xM_x′F_2+x (M′=Al, Cr) at different temperatures has been realized in order to discover whether the clustering process proposed in Pb_1−xAl_xF_2+x can be extended to Pb_1−xCr_xF_2+x.     

Electronic structure of ternary semiconductor alloys: CPA calculations using sp3s1 bandstructures

Coherent potential approximation (CPA) calculations of the electronic structure of the III-V ternary alloys GaAs_1−x P_x, Ga_x In_1−x As, Ga_x Al_1−xAs, Ga_x In_1−x P, and GaSb_1−xP_x are reported. Results are presented for the CPA self-energies, state densities, and band-bowing functions for these materials. The calculations utilize the sp³s¹ bandstructures as input into the Chen and Sher CPA formalism. The present work is the first utilization of these bandstructures in CPA calculations for ternary alloys and the first CPA treatment ofGaSb_1−xP_x. In qualitative agreement with previous studies, the results of these calculations show that alloy disorder effects are small for GaAs_1−xP_x, of moderate importance for Ga_xIn_1−xAs and Ga_xAl_1−xAs, and relatively large for Ga_xIn_1−xP. However, such effects are found to be significant for GaSb_1−xP_x, with this alloy predicted to be in the strong scattering regime.     

Mechanisms controlling magnetic properties of pseudobinary compounds TbNi5−xMx (M=Cu or Al)

Magnetic properties and heat capacity of the pseudobinary TbNi_5−xCu_x and TbNi_5−xAl_x alloys have been studied for x⩽2.5 and x⩽1.5, respectively. The substitution of Cu or Al for Ni decreases sharply the spontaneous magnetic moment, increases the magnetic anisotropy in the “easy” basal plane, and leads to strong magnetic domain wall pinning. The Curie temperature T_C in TbNi_5−xCu_x depends on x non-monotonously and has a maximum value at x=1, while T_C in TbNi_5−xAl_x does not depend on composition up to x=1 and sharply decreases at x>1. These results are explained by the effects of random local crystal fields, band magnetism and heterogeneous polarization of the mixed 3d band.     

Electronic conductivity in La1−xBaxF3−x crystals

The electronic conductivities of solid solutions La_1?xBa_xF_3?x (0 ? x ? 0.0952) were investigated up to 533 K using the Hebb-Wagner dc polarization technique. The electrochemical cell (-)La|La_1?xBa_xF_3?x|Pt(+) has been utilized with Pt as the ion-blocking electrode. Under steady-state conditions the La_1?xBa_xF_3?x solid solutions exhibit electronic conductivity. The electronic conductivity vanishes in pure LaF₃. Together with ac conductivity measurements it appears that the ionic transference number for La_1?xBa_xF_3?x (0 ? x ? 0.0952) is essentially unity over the temperature range studied.     

Long period helimagnetism in the cubic B20 FexCo1−xSi and CoxMn1−x Si alloys

We report magnetization, resistivity and small angle neutron scattering experiments on cubic B20 Fe_xCo_1?x SI alloys (0.3 ? x ? 0.9) and Co_xMn_1?xSi allpys (x = 0.02, 0.04 and 0.06). The magnetically ordered Fe_xCo_1?xSi alloys and the Co_xMn_1?xSi alloys with x = 0.02 and 0.04 are found to be helimagnetic with very long period. The low temperature resistivity of the magnetically ordered Fe_xCo_1?xSi alloys exhibits an anomalous positive magnetic contribution. The occurence of the Dzyalosyinsky-Moriya spin-orbit type interaction in these non-centrosymmetric alloys is given as a possible explanation of the helimagnetic order.     

Phase analysis of the system La1−x Ba x FeO3−y (0≦x≦0.70) by means of x—ray diffraction and Mössbauer spectroscopy

X-ray diffraction and Mössbauer measurements were performed on novelly synthesized La_1?x Ba _x FeO_3?y (0≤x≤0.70). Two phases were found in the system. La_1?x Ba _x FeO_3?y for 0≤x≤0.10 is an orthorhombic perovskite. La_1?x Ba _x FeO_3?y for 0.54≤x≤0.70 is a cubic perovskite. La_1?x Ba _x FeO_3?y for 0.10≤x≤0.54 consists of these two phases.     

Commensurability oscillations by runaway and pinned electrons

We report on an investigation of commensurability oscillations in antidot square-lattices, and show that the commensurate peaks in resistivity ρ_xxderive from two kinds of electron: runaway and quasipinned electrons. The runaway electrons, which skip away along the antidot arrays, increase the value of conductivity σ_xx; and the quasipinned electrons, which orbit some antidots for a long time, decrease the value of σ_xx. Therefore, by competition between the two different types of electron, the conductivity σ_xxis determined. The conductivity σ_yxin the antidot lattice always has dips at the peaks in ρ_xx. As a result, by combining the of values of σ_xxand σ_yx, the resistivity is determined through the orthodox relation of ρ_xx =  σ_xx/ (σ_xx²  +   σ_yx²).     

Evolution des proprietes de transport des solutions solides M1−xM′xF2+x (M=Sr,Pb; M′=Sb,Bi) ET M1−xM″xF2+2x (M″=Zr,Th) pour les faibles taux de substitution

A detailed study of the transport properties of the solid solutions M_1?xM′_xF_2+x (M=Sr, Pb; M′=Sb, Bi) and M_1?xM″_xF_2+2x (M″=Zr, Th) has been carried out for x<0.10. Contrary to solid solutions of strontium, solid solutions of lead show for a very small value of x (x = 0.10 forPb_1?xM′_xF_2+x, x = 0.005 for Pb_1?xM″'_xF_2+2x) a minimum of conductivity associated with a maximum of activation energy. Hypotheses are proposed to explain the results.     

The magnetic properties of oxide spinel Li0.5Fe2.5−2xAlxCrxO4 solid solutions

The exchange interactions (J_BB and J_AB are the intra and the inter-sublattice exchange interactions between neighbouring spins, respectively) are obtained by using the general expressions of canting angle and critical temperature obtained by mean field theory of Li_0.5Fe_2.5−2xAl_xCr_xO₄. The expression of magnetic energy of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is obtained for different spin configurations and dilution x. The saturation magnetisation of Li_0.5Fe_2.5-2xAl_xCr_xO₄ is obtained with different values of dilution x. The magnetic phase diagram of Li_0.5Fe_2.5-2xAl_xCr_xO₄ materials is obtained by high temperature series expansions (HTSEs). The critical exponent associated with the magnetic susceptibility of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is deduced.     

Spin resonant optical four wave mixing in Pb1−xSnxTTe epitaxial layers and in Pb1−xSnx/PbTe superlattices

Spin resonances of the third-order non-linear susceptibility of epitaxial layers of n- and p-type Pb_1?xSn_xTe and Pb_1?xSn_xTe/PbTe superlatttices have been observed by four photon mixing of the radiation of two CO₂-lasers. Precise data on the effective g-values of the electrons and holes of Pb_1?xSn_xTe were obtained. These data and the results of magneto-optical interband absorption measurements are used to obtain k·p parameters within the Mitchell and Wallis band model. In the Pb_1?xSn_xTe/PbTe superlattices, the same g-values for electrons as in the Pb_1?xSn_xTe films with the same composition x are found. Consequently, the electrons in the superlattices are confined within the Pb_1?xSn_xTe layers. Therefore the Pb_1?xSn_xTe/PbTe system forms a type I and not a staggered superlattice.     

Lattice dynamics and debye temperature of AlCu,AlSi and AlGe alloy systems

The lattice dynamics and specific heat at constant volume for AlCu alloy system is studied using our previous treatment based on the microscopic electronic theory. Considering the volume and electron density effect on the dynamical matrix of the pure constituent, we obtain the band and local mode frequencies at the temperature-dependent specific heat in the Al_1−x Cu_x solid solution. Then, using the data about the mean elastic wave and the specific heat extrapolated to the absolute zero temperature, the Debye temperature is presented for the Al_1−xCu_x, Al_1−xSi_x and Al_1−xGe_x alloy systems. The Debye temperature decreases as a function of the Cu atomic fraction x for Al_1−xCu_x alloy, and remarkably for Al_1−xSi_x and Al_1−xGe_x solid solutions.     

Ab-initio investigations of structural, electronic, magnetic and optical properties of ferromagnetic Cd1-xMnxTe

We report ab-initio calculations of the structural, electronic, magnetic and optical properties of the alloy Cd_1-xMn_xTe as a function of the Mn concentration ‘x’. Ab-initio calculations are based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice constants of the Cd_1-xMn_xTe alloys exhibit Vegard's law downward bowing parameter. For the minority spin channel the Fermi level shifts toward higher energy with the value of ‘x’ in Cd_1-xMn_xTe. The spin-exchange splitting energy Δ_x(d) increases with increasing ‘x’ in Cd_1-xMn_xTe and the values of p-d exchange splitting energy Δ_x(pd) of Cd_1-xMn_xTe show that the effective potential for the minority spin is more attractive than that for the majority spin. The values of exchange constants N₀α and N₀β obtained for Cd_1-xMn_xTe are in agreement with the reported data. The magnetic moment per Mn atom reduces from its free space charge value of 5μ_B to around 4μ_B due to p-d hybridization and this results into an appearance of small local magnetic moments on the non-magnetic Cd and Te sites. The absorption threshold shifts toward higher energy and the static refractive index decreases with the increasing value of ‘x’ in Cd_1-xMn_xTe.     

Preparation and magnetic properties of barium ferrites substituted with manganese, cobalt, and tin

Barium ferrites substituted by Mn-Sn, Co-Sn, and Mn-Co-Sn with general formulae BaFe_12−2xMn_xSn_xO₁₉ (x=0.2-1.0), BaFe_12−2xCo_xSn_xO₁₉ (x=0.2-0.8), and BaFe_12−2xCo_x/2Mn_x/2Sn_xO₁₉ (x=0.1-0.6), respectively, have been prepared by a previously reported co-precipitation method. The efficiency of the method was refined by lowering the reaction temperature and shortening the required reaction time, due to which crystallinity improved and the value of saturated magnetization increased as well. Low coercivity temperature coefficients, which are adjustable by doping, were achieved by Mn-Sn and Mn-Co-Sn doping. Synthesis efficiency and the effect of doping are discussed taking into account accumulated data concerning the synthesis and crystal structure of ferrites.     

New ferrimagnetic spinel compositions in the system MCr2S4−xSex where M=Fe,Co, Mn

Phase relationship between ferrimagnetic semiconducting spinels of the type MCr₂S₄ and defect NiAs compositions of the type MCr₂Se₄ (where M=Mn, Fe, Co) are established. It was found that the amount of Se which can be substituted, with retention of the spinel structure, decreases from MnCr₂S_4?xSe_x (x=0 to x=2) to FeCr₂S_4?xSe_x (x=0 to x=1·25) to CoCr₂S_4?xSe_x (x=0 to x=1). This phenomena is accounted for on the basis of sulfide spinel stability and unit cell size. The Curie temperatures of the spinel compositions decrease slowly with increasing Se content. This is presumably caused by weakening of the spinel A-B superexchange interaction.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

Structure and magnetic properties of RFe13−xSix (R = Pr,Nd or Gd,x = 2.5–5)

RFe_13−xSi_x (R = Pr, Nd and Gd) alloys with x = 2.5–5 have been synthesized and characterized in a magnetic field up to 17 kOe and in a temperature range 10–1173 K to ascertain whether they might be useful as high temperature, high-energy permanent magnet materials. It was found that a body-centered tetragonal (BCT) Ce₂Ni₁₇Si₉-type structure forms in PrFe_13−xSi_x alloys when x ⩾ 4. The Curie temperatures T_c of this BCT phase are in a range 50–90 K, higher than that of the corresponding PrCo_13−xSi_x BCT phase (∼ 20 K). The PrFe_13−xSi_x alloys with x⩾ 4 show spin reorientation at cryogenic temperatures (15–47 K) and exhibit significant coercivity in loose powder samples below their spin-reorientation temperature. Using the information about the magnetization of LaFe_13−xSi_x BCT alloys, one can estimate the moment of Pr ion in PrFe_13∼-xSi_x alloys. It is found to be very close to that in PrCo_13−xSi_x BCT alloys, around 2 μ_2/atom. For RNd or Gd, the RFe_13−xSi_x alloys occur only as mixtures of RFe₂Si₂, R(Fe, Si)₁₁ and FeSi. The Ce₂Ni₁₇Si₉-type structure cannot be formed in these alloys even when x = 5. The low T_c for the PrFe_13−xSix alloys precludes their use as a permanent magnet material, except perhaps at low temperature.     

The electron trap associated with an anion vacancy in ZnSe and ZnSxSe1−x

A dominant electron trap in ZnSe and ZnS_xSe_1?x has been studied by DLTS. The trap depth of about 0.3 eV is independent of the crystal growth, doped impurities and S mole fraction x in ZnS_xSe_1?x, while the electron-capture cross-section decreases with x. These properties indicate that the electron trap is due to the anion vacancy in ZnSe and ZnS_xSe_1?x.     

On the characteristics of AlGaN films grown on (111) and (001) Si substrates

High Al-content Al_xGa_1−xN films were deposited on (001) and (111) Si substrates at 1000 °C using high temperature AlN buffer layers. Experimental results show that Al_xGa_1−xN films grown on (111) Si substrates exhibit better crystalline quality than that in the films deposited on (001) Si substrates. Cracks were found in the high Al-content Al_xGa_1−xN/(111) Si samples but they were not observed in the Al_xGa_1−xN films grown on (001) Si substrates having the same film thicknesses and Al compositions. Based upon the results of X-ray diffraction (XRD) and transmission electron microscopy (TEM), it appears that mono-crystalline Al_xGa_1−xN films were achieved on (111) Si substrates while columnar structure was observed in the Al_xGa_1−xN/(001) Si samples. According to the depth profiles of Al_xGa_1−xN/Si samples using secondary ion mass spectroscopic (SIMS) analyses, enhanced Al inter-diffusion in the Al_xGa_1−xN/(001) Si samples was identified. Room temperature (RT) photoluminescence (PL) measurements of the Al_xGa_1−xN (x≦0.10)/(111) Si samples exhibit strong near band edge luminescence. The PL emission linewidth was found to decrease with the decrement of Al-content.     

Disorder induced core photoelectron linewidth broadening in AgPd alloys

High resolution core level photoelectron spectra for a series of Ag_xPd_1−x alloys have been measured and reveal a large “disorder broadening” effect. A comparison of measurements of this effect for three alloy systems, Ag_xPd_1−x, Cu_xPd_1−x and Cu_xZn_1−x, with results of ab initio calculations is presented. Poor agreement is obtained and possible reasons for this observation are discussed.     

Partial pair correlation functions of La1−x(AlGaAu)x metallic glasses from x-ray diffraction data

X-ray diffraction studies of La_1?xAl_x, La_1?x(Al_0.5Ga_0.5x, La_1?xGa_x and La_1?xAu_x metallic glasses have been performed for x = 0.20, 0.24 and 0.28. Similarities in the radial distribution functions for these materials as well as other data suggest that these alloys are isostructural. Experimental data from different alloys have therefore been used to separate the three different pair distribution functions, which are then compared to dense random packing models and data for other metallic glasses.