Optical properties and structural phase transitions in Mg x Zn1−x O under hydrostatic pressure

This article reports on the optical properties under pressure of Mg _x Zn_1?x O (x?<?0.13?±?0.2) thin films deposited on mica and fluorite substrates by pulsed-laser deposition. The absorption edge of the semiconductor alloy is measured in both the ambient pressure wurtzite phase and the high-pressure rock-salt phase for several Mg contents. In wurtzite Mg _x Zn_1?x O, a larger value of the band-gap is clearly correlated to a larger pressure coefficient of the band-gap. This effect is shown to be consistent with the decreasing contribution of cation d-levels to the valence band maximum states as the Mg content increases. The wurtzite-to-rock-salt transition pressure is observed to decrease from 9.5?±?0.2 (pure ZnO) to 7.0?±?0.2?GPa (for x?=?0.13), with an almost linear dependence on the Mg content. The same linear dependence on x, with virtually the same slope, is also found for the rock-salt-to-wurtzite reverse transition in the pressure down-stroke. For x?>?0.13, the rock-salt phase is observed to be metastable at room pressure, after a pressure cycle up to 15?GPa.     

Dielectric study on Zn1−x Mg x O ceramic materials prepared by the solid-state route

The present work investigates the structural and dielectric properties of Zn_1?x Mg _x O composites prepared by the standard sintering method at 1200 °C during 24 h and doped with different weight percentages of MgO (x = 0–40 %). For this purpose, the scanning electron microscopy (SEM) was used to study the effect of the magnesium’s proportion on the morphology and crystallinity of the obtained samples. The SEM observations have shown rougher surfaces of the samples covered by grains having prismatic shapes and different sizes. The dielectric properties of the ceramics were investigated by spectroscopic impedance at different temperatures and frequencies, thus showing a frequency-dependent dispersion of the permittivity constants and dielectric losses. From these measurements, the relaxation processes were identified and their activation energies extracted. Dielectric responses were correlated with the microstructure and chemical composition of the ZnMgO composites. The mechanisms of ac conductivity are controlled by the polaron hopping and the electron tunneling models. Concerning the tunneling model, two types corresponding to the overlapping large polaron tunneling model for the composites Zn_0.9Mg_0.1O and Zn_0.8Mg_0.2O and the small polaron tunneling model for the composites Zn_0.64Mg_0.36O (in the frequency range 1.7 × 10⁴ Hz–1 MHz) and Zn_0.6Mg_0.4O were observed. Besides, one type of hopping model corresponding to the correlated barrier hopping for the composites ZnO and Zn_0.64Mg_0.36O (in the frequency range 6 × 10²–1.7 × 10⁴ Hz) was noted.     

First-principles calculations of structural,electronic and thermal properties of Zn1−x Mg x S ternary alloys

Structural, electronic and thermal properties of Zn_1?x Mg _x S ternary alloys are studied by using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). The Wu-Cohen generalized gradient approximation (WC-GGA) is used in this approach for the exchangecorrelation potential. Moreover, the modified Becke-Johnson approximation (mBJ) is adopted for band structure calculations. The dependence of the lattice constant, bulk modulus and band gap on the composition x showed that the first exhibits a small deviation from the Vegard’s law, whereas, a marginal deviation of the second from linear concentration dependence (LCD). The bowing of the fundamental gap versus composition predicted by our calculations agrees well with the available theoretical data. The microscopic origins of the gap bowing are explained by using the approach of Zunger and co-workers. Thermal effects on some macroscopic properties of Zn_1?x Mg _x S alloys are also investigated using the quasi-harmonic Debye model, in which the phononic effects are considered. As, this is the first quantitative theoretical prediction of the thermal properties of Zn_1?x Mg _x S alloys, no other calculated results and furthermore no experimental studies are available for comparison.     

Influence of Ba-substitution on the structural and ferroelectric properties of Pb1− x Ba x Zr0.40Ti0.60O3 ceramic materials

This work presents a comprehensive study about the influence of Ba-substitution on the structural and ferroelectric properties of Pb_1?x Ba _x Zr_0.40Ti_0.60O₃ (PBZT) ceramic system. Pb_1?x Ba _x Zr_0.40Ti_0.60O₃ ceramic samples were then prepared by solid state reaction method and characterized as a function of composition and temperature by X-ray diffraction (XRD) and impedance spectroscopy techniques. The dielectric measurements show that the substitution of Pb²⁺ for Ba²⁺ ions leads to a diffuse behavior of the dielectric permittivity curves for all samples and that only the x?=?0.50 sample presents a typical relaxor behavior. In good agreement with dielectric measurements, the structural phase transition study showed a phase transition from a tetragonal structure with P4mm space group to a cubic structure with Pm-3m space group for all samples, except for the x?=?0.50 sample were a cubic structure was observed in the complete temperature interval measured.     

Effect of Cd content on the structural characteristic and some physical properties of Zn1−x Cd x Se

The present work focuses on the structural, optical, and electrical properties of Zn_1?x Cd _x Se (0.1≤x≤0.25) compounds. The compounds were synthesized by solid state reaction. X-ray diffraction (XRD) patterns confirm that the samples have cubic single phase (zinc-blende) crystal structure with space group F-43m. The crystal structural parameters were refined by the Rietveld method using the FullProf program. It was found that the lattice parameters increase linearly with increasing the Cd content and obeys Vegard’s law. The refined values of the crystallite size and the bond lengths increase with increasing the Cd content. The energy band gap of the samples has been calculated and it was found that it decreased as Cd increased. The conductivity of the samples increases with increasing both of composition parameter x and temperature, and showing semiconducting behavior.     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.     

Optical properties of Hg1−xCdxTe

Optical properties of Hg_1−xCd_xTe are summarized in this study. Based on Penn-like models, the Moss relation and the Wemple and DiDomenico approach, calculations of energy gap, plasmon energy, Fermi energy, oscillator strength and electronic polarizability have been made. Comparisons are made with the data available in the literature. Details of the dependency of the properties on composition are presented.     

Magnetic and electrical properties of Mg1−x CuxO solid solutions and magnetic shielding in Cu-Mg1−x CuxO structures

Mg_1−x Cu_xO solid solutions having an NaCl structure with 0⩽x⩽0.20 are synthesized and Cu-Mg_1−x Cu_xO structures are prepared for superconductivity studies. The magnetic susceptibility χ, electron paramagnetic resonance (EPR), and electrical conductivity of the solid solutions are studied at temperatures of 5–550 K. It is shown that χ ⁻¹(T) obeys the Curie-Weiss law with a paramagnetic Curie temperature Θ close to zero and an effective magnetic moment μ _eff=1.9 μ _B, close to the 1.73 μ _B of a Cu²⁺ ion with spin S=1/2. The width ΔH of the EPR line depends weakly on temperature and increases as x is raised. The volume narrowing of the EPR linewidth ΔH is used to estimate the exchange interaction parameter, 3×10⁻⁴ eV. The g-factor is close to 2 and is temperature independent. The electrical conductivity of Mg_1−x Cu_xO at T=300 K is ≈10⁻¹¹–10⁻¹² Ω⁻¹ cm⁻¹ for x=0 and increases to 10⁻⁵–10⁻⁶ Ω⁻¹ cm⁻¹ for x=0.15–0.20. The conductivity is p-type. Magnetic shielding is observed in Cu-Mg_1−x Cu_xO structures with x=0.15 and 0.20. The possible connection of this phenomenon with interference superconductivity in the contact layer of the structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 293–296 (February 1999)     

High-Pressure Synthesis of SeCu 1− x Zn x O 3 Perovskites

The SeCu 1 m x Zn x O 3 solid solution, with a distorted perovskite-type structure, has been synthesized under high pressures and temperatures. X-ray diffraction analysis indicates that the zinc ions occupy the copper sites, a solid solution being formed. It seems that high-pressure stabilises a small cation such as Se 4+ in the A site of the perovskite structure ABO 3 although the material is better described as formed by selenite anions \rm{SeO}_{3}^{-} and Cu 2+ /Zn 2+ cations.     

Magnetic properties and structure of Fe2 − x Mg x CrO4 chromites

The structural and magnetic properties of Fe_{2 ? x} Mg _x CrO₄ chromite synthesized by the ceramic method to receive analogs of natural minerals and to solve the rock magnetism problems are investigated. The dependences of cubic-lattice parameters and magnetic characteristics on composition, with slight deviation from linearity, are obtained. It is established in the course of experiments related to the partial thermal remanent magnetization in weak fields that some compositions are characterized by self-reversal under sample heating to 600°C in air. It is suggested that the self-reversal is caused by a nonuniform distribution of cations in chromite structure and phase transformation under oxidation.     

Sc doping of MgB2: the structural and electronic properties of Mg1−xScxB2

We have investigated the effect of electron doping on the superconducting properties of MgB₂. For the purpose we have synthesized several samples along the Mg_1−xSc_xB₂ section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron σ bands. Radio frequency surface resistivity measurements has been used to obtain the variation of T_c with Sc doping. Increasing the Sc content, the experimental T_c diverges from the T_c predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance.     

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs_1?x N _x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77?K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs_1?x N _x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2? metastable state, was strongly influenced by the optical exposure and thermal history of the sample.     

Optical and electrical properties of flash-evaporated Pb1−xHgxSe films

Results of optical and electrical properties of flash-evaporated Pb_1−xHg_xSe films, in the composition range 0.07 ⩽ x ⩽ 0.93, show that the alloy films are formed by alloying of PbSe and HgSe. The optical band gaps of films grown on substrates at 25 and 100°C decrease linearly from 0.20 to 0.09 eV and from 0.15 to 0.065 eV, respectively, as the Hg concentration in the films is increased from 0.07 to 0.93. Activation energy values, as obtained from conductivity measurements, suggest that between 225 and 400 K the conduction results mainly from thermally-generated charge carriers, whereas at the lower temperatures (<225 K) impurity conduction dominates. Irreversible changes in the properties of the films are observed at > 400 K.     

Structural and electronic properties of GaN x As1−x alloys

The structural and electronic properties of cubic GaN _x As_1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN_0.25As_0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Magnetic and electrical properties of weakly doped manganese-deficient La1 − x Ca x Mn1 − z O3 manganites

Samples of La_{1 ? x} Ca _x Mn_{1 ? z} O_{3 + δ} (x = 0.05?0.15) with deficient manganese and excess oxygen δ do not pass into a metallic state and have low spin ordering temperatures T _C at acceptor Mn⁴⁺ concentrations near the percolation threshold. These results are explained by carrier localization in clusters near cation vacancies. A break in the carrier transport chain Mn-O-Mn in the form of absent manganese favors cluster formation and decreases the double exchange energy and T _C of the samples. Closeness to the percolation threshold results in strong (more than four orders of magnitude) changes in the electrical resistivity in a magnetic field. The changes in the cluster sizes with the temperature and the magnetic field that are determined from the magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a dielectric paramagnetic and an antiferromagnetic matrices.