Preparation and Characterization of (1 — x) Pb(Mg1/3Nb2/3) O3 — x PbTiO3 Electrocaloric Ceramics

Lead magnesium niobate Pb(Mg_1/3Nb_2/3)O₃ — lead titanate PbTiO₃ [abbr. as (1 — x) PMN — x PT] ferroelectric ceramics with different excesses of MgO and PbO were systematically studied under different processing conditions for ferroelectric refrigeration application. It was found that the excess amount of MgO and PbO, and the sintering temperature have great effect on the crystallographic properties of the ceramics. In our experiments, (1 — x) PMN —x PT (x = 0.08, x = 0.10, and x = 0.25 respectively) with the excesses of 2 mol% MgO and 2 mol% PbO ceramics sintered at 1250 °C/1 hour possess the desired perovskite structures and large electrocaloric temperature change (ΔT = 1 K and more) in the vicinity of room temperature under a dc electric field of 1.5 kV/mm. It is expected that (1 — x) PMN — x PT electrocaloric ceramics could be applied for cascade refrigeration near room temperature.     

The influence of the Mg content and the quenching conditions on the decomposition kinetics of Al?Zn?Mg alloys

By means of resistivity measurements, TEM and microhardness measurements the decomposition kinetics of A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 0.2; 0.5; 1.0; 1.5; 2.25; 3.0) were investigated. For T_a < T_{a, crit} < T_h (T_h: temperature of the rapid homogenous nucleation) in A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 1.0; 1.5; 2.25; 3.0) an inversion of the decomposition kinetics for T_q > T_inv: (T_inv: inversion temperature) was established. This result can be explained with reference to the binding of excess vacancies in dislocation loops. Proceeding from the discrepancy between the experimentally estimated concentrations of vacancies bound in dislocation loops and the concentrations of free vacancies at T_q, calculated by means of the Lomer equation, a model is proposed, which explains the high vacancy concentration of the loops. In terms of this model the values of the migration energy, obtained for A1-3 at.% Zn — 1.5 at.% Mg in the T_a-range 23 °C ≦ T_a ≦ 100 °C, and the shift of the inversion temperatures T_inv towards higher values with increasing T_a are interpreted. The same applies to the influence of the Mg content on the decomposition kinetics in A1-4.5 at.% Zn-x at.% Mg alloys.     

Magnetic Susceptibility in 2D Superconductor NaxHfNCl System

Abstract

Superconducting critical temperature, T_c and the shielding volume fraction, SVF, of layered nitride superconductor Na_xHfNCl have been studied as a function of x, i.e. Na concentration. Although T_c decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K² by its difference of magnetic susceptibility between HfNCl and Na_0.5HfNCl. The γ value is relatively small compared with the high T_c value, revealing double honeycomb lattice system as new potential higher T_c superconductor series by intercalation.     

The dependence of surface morphology of GaSb- and AlxGa1–xSb epitaxial layers on the conditions of LPE growth

The present work shows the dependence of surface morphology of the GaSb and Al_xGa_1–xSb epitaxial layers on the conditions of LPE growth. It is found that for LPE growth at 500 °C a supersaturation of 5—10 °C and a cooling rate of 0.24—0.4°C/min for GaSb epitaxial layers and 0.8—1.2 °C/min for Al_xGa_1–xSb epitaxial layers is necessary to obtain a flat and smooth surface.     

Die Konzentrationsabhängigkeit des Brechungsindex von Ga(PAs)- und (GaIn)P-Mischkristallen

In the two systems of mixed crystals Ga_xPAs_1–x and Ga_xIn_1–xP the refraction index — using the wavelength of the green Hg line (λ = 5461 Å) — is determined ellipsometrically in dependence on the mole fraction x. The nonlinearity parameter c_n of this dependence is compared with that estimated theoretically by the use of PENNS model.     

Concentration Magnetic Phase Transitions in the (La1−xCaxMnO3 System

Comprehensive investigations into crystal structures, electrical and magnetic properties of the (La_1−xCa_x)MnO₃ (x = 0 ÷ 0.6) solid solutions are performed. Two concentration magnetic phase transitions are found: antiferromagnetic-ferromagnetic with x = 0.14 and ferromagnetic (T_c = 150 K) - ferromagnetic (T_c = 260 K) at f = 0.2. It is shown that the first of them is due to the crystal structure O′ O orthorhombic transition, the second - to the increase of ferromagnetic interaction at the expense of additional interaction through conduction electrons. A diagram of the magnetic states for (La_1−xCa_x)MnO₃ at x = 0 / 0.6 is given.     

Magnetic and electric properties of the ternary alloys AB

The results of experimental studies of the electrophysical and magnetic properties of solid solutions based on the AB compounds with anion (Zn₃(P_xAs_1−x)₂ and cation (Zn_xCd_1−x)₃ P₂ substitution are reported. The Zn₃(As_xP_1−x)₂ solid solutions are characterized by p-type conductivity (∼ 10¹⁵  10¹⁷ cm⁻³), low Hall mobility and conductivity of ∼ 10⁻² — 10⁻³ ohm⁻¹ cm⁻¹. In the (Zn_xCd_1−x)₃P₂ solid solutions the samples rich in zinc are p-type, those rich in cadmium — n-type, the carrier concentration is ∼ 10¹⁶ — 10¹⁸ cm⁻³. From the measurements of the magnetic properties of the above mentioned alloys it is shown that in the investigated solid solutions a transition from the structure with direct optical transition to those with indirect optical transition occurs.     

Nematic-Isotropic Transition in MBBA and its Mixtures with 4-Methoxybenzaldehyde: Applications to the Purity Control of Liquid Crystals

The nematic-isotropic transition temperatures of MBBA(1) + 4-methoxybenzaldehyde(2) mixtures (where MBBA=N(4-methoxybenylidene)-4-n-butylaniline) for 0  x₂  0.06 and additionally the transition enthalpy of pure MBBA were measured by a calorimetric method. Straight lines were found for the T¹(x₂) and Tⁿ(x₂) curves respectively where T¹ and Tⁿ are the upper and the lower limiting temperatures of the transition range for a given x₂. The results are compared with literature data on similar systems and confirm the wellknown rule that the decrease of the nematic-isotropic transition temperature with increasing x₂ depends more strongly on the size and the shape than on the polarity of component 2. From a plot of δT [tbnd] T¹ - Tⁿ against x₂ the mole fraction of impurity in the original sample can be extrapolated.     

Review: Different types of ordering

Homogeneous structures A_xB_y with identical A–A or B–B self‐coordination numbers T₁, T₂, T₃, of nearest, next‐nearest and third neighbors are selected. The maximum T_i values are 2 2 2 (1D row), 6 6 6 (2D hexagonal net) and 12 6 24 (3D cubic close‐packing). Reduced T₁ or T₂ values for A_xB_y can be related with attractive or repulsive A–A (or B–B) interactions. A single set of T₁ T₂ T₃; y/x values was obtained for y/x = 1–4 and 6. The y/x = 2 structures of three equivalent positions A, B and C are unfavourable for A = ⊕ and B = ⊖ charge or direction of magnetic moments. The spatial distribution of positive and negative potential near A⁺ and B⁻ positions gives rise to neutral C positions. Frustrated magnetic moments at C positions are disordered. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic analysis and technology of preparation of high-Tc superconducting system YBa2Cu3O7−x

By modeling the equilibrium state of the phase formation reaction of High-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) and using the results of P-T−x correlation investigation of some authors, the variation of Gibbs free energy (ΔGm) versus temperature has been calculated. On the basis of the variation of ΔGm, the technology procedure of preparation of phase 1:2:3 with desired x has been established to meet requirement of basic investigation and application of High-T_c superconductors YBa₂Cu₃O_7−x.     

Optical absorption spectra of epitaxial layer - gradient layer - substrate systems

Theoretical relations are derived for the photon energy dependence of the absorption coefficient of epitaxial layer — gradient layer — substrate systems assuming a spatially linear variation of the gap energy in the gradient layer. It is shown that optical absorption measurements on such systems can be used to determine the gap energy of the epitaxial layer. Numerical calculations for the system GaAs_0.3P_0.7—GaAs_1—xP_x—GaP are presented.     

Growth of bulk zinc chalcogenide single crystals from the vapour phase and their use for laser screens of projection colour television

Results on ZnSe, ZnSe_xS_1−x and ZnS crystal growing from the vapour phase up to 7.5 cm³ in volume are described. Crystals were grown on sapphire, ZnS, ZnSe_xS_1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10³ cm⁻² were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m⁻² TV image of 1.5 × 2 m² in area.     

Transport Coefficients of (Bi1-xSbx)2Se3 Single Crystals

(Bi_1-xSb_x)₂Se₃(x = 0.0 to 0.2) single crystals were prepared using a modified Bridgman method. Measurements of the Hall coefficient R_H, electrical conductivity σ and Seebeck coefficient α showed that incorporation of the antimony atoms into the Bi₂Se₃ crystal lattice results in an increase in the free electron concentration for a low antimony content, whereas the free electron concentration is suppressed in the range of a high antimony content. This effect is explained qualitatively by changes in the concentrations of point defects in the (Bi_1-xSb_x)₂Se₃ crystals. We asume that the substitution of Bi atoms by Sb atoms results in a decrease in the concentration of Se vacancies V and antisite defects Bí_Se. The course of the dependences of In (R_Hσ) vs In T manifests that in the temperature region of 100–400 K transport properties of the studied crystals are characterized by a mixed transport mechanism of free carriers, mainly on acoustical phonons and ionized impurities.     

Dislocation density in InxGa1–xAs films grown on GaAs substrates with intermediate buffer layer InGaAsP of variable composition

In_xGa_1–xAs films with x = 0.03 and 0.05 were grown from an In Ga As P liquid phase. Because of high value of distribution coefficient of P we have heterojunction GaAs In_yGa_1–yP_zAs_1–x–In_xGa_1–xAs. The influence of Phosphorus atom fraction (X_p) in liquid phase on dislocation density in the top In_xGa_1–xAs layer was studied. It was found that dislocation density (N_d) as a function of X_p is a curve with some minima. The minima of N_d for substrates of (111) A and (111) B orientations are observed in the different intervals of X_p axis. — The width of N_d minimum is decreased if the substrate is misoriented from the (111) plane. — It was supposed that the clusters exist in the liquid phase. On the basis of this assumption one can explain the influence of substrate position over or under the melt on the film perfection. The diameter of these clusters is estimated to be about 500 Å.     

Investigation of InAs—InSb solid solution heteroepitaxial layers and structures

The results of investigations concerning the peculiarities of obtaining InAs—InAs_1−xSb_x heterostructure and of its properties revealed that the quality of epitaxial layers as well as photoelectric and electroluminescent device structure characteristics are to a great extent determined by a disagreement magnitude of heterostructure materials. — To decrease the magnitude of disagreement at the layer-substrate boundary and to achieve the composition with x > 0.1 at some distance the method of layer-by-layer growth (ANDREWS et al.) and the obtaining a graded heterojunction with optimization of component distribution along epilayer thickness seems to be promising.     

Optical Reflection Spectra of Silicon Clathrate Compounds Ba8AgxSi46−x

Abstract

Optical reflection spectra are measured for Ba₈Ag_xSi_46?x (0 ≤ x ≤ 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature T_c decreases. An origin of the decrease in T_c is assigned to the decrease in the density of states at the Fermi level due to the decrease in the carrier concentration.     

Te-rich Ge–Te–Se glass for the CO2 infrared detection at 15 μm

Te_80−xGe₂₀Se_x glasses have been prepared along the GeSe₄–GeTe₄ axis using the classical method in silica tube under vacuum. A phase separation domain appears for composition around Te₄₀Ge₂₀Se₄₀. Our attention was turned toward the Te-rich compositions corresponding to 1 < x < 5 at.%. These glasses are transparent from 4 to about 20 μm without any purification of the starting elements. Furthermore the difference ΔT between the crystallization temperature T_x and the vitreous transition temperature T_g lies at about 110 °C that is to say 30 °C higher than for the GeTe₄ reference glass. Finally the introduction of a few percentages of Se makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers for example. Taking into account their transparency window, encompassing the CO₂ absorption band around 15 μm, the Te_80−xGe₂₀Se_x with 1 < x < 5 at.% could become matchless composition for the CO₂ infrared detection as planed by the Darwin mission of the European Space Agency.     

Effect of Lithium Doping in the Bi-2212 Superconducting System

Samples with the nominal composition Bi₂Sr₂Ca_1–xLi_xCu₂O_y (x = 0.2,0.4, and 0.6) and Bi₂Sr₂CaCu_2−x L1_xO_z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of L1 at both Ca and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the L1-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T_c and the L1-doped at the Cu site gives T_c(0) above the liquid nitrogen temperature. It was observed that the L1 doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method.     

Chemical bonding and optical properties of germanium–carbon alloy films prepared by magnetron co-sputtering as a function of substrate temperature

Amorphous non-hydrogenated germanium carbide (a-Ge_{1 − x}C_x) films have been prepared by magnetron co-sputtering system designed by ourselves. The chemical bonding and microstructure have been analyzed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The optical properties of the films have been investigated by means of spectroscopic ellipsometry. The relationship between the chemical bonding and the optical properties has been explored. It has been found that all films with the constant carbon content are amorphous. The sp² CC and sp³ GeC bonds increase with T_s, and some sp² CC bonds gain infrared activity. The fraction of sp³ GeC bonds rises with T_s, but the fraction of sp³ GeGe bonds gradually drops down. In addition, the refractive index and extinction coefficient increase with T_s. The film optical gap is seen to reach 1.15 eV when T_s is 200 °C. However, the optical properties of a-Ge_{1 − x}C_x films almost remain stable with the substrate temperature.     

Homogenitätsuntersuchungen und Bestimmung der molaren Konzentration x an CdxHg1–xTe mit Hilfe der Elektronenstrahlmikrosonde

The semiconductor Cd_xHg_1–xTe serves as a starting material for the production of infrared detectors with a high detectivity and a small time constant. Cd_xHg_1–xTe single crystals are necessary to manufacture detectors efficiently. The single crystals should not contain segregations and have to include a homogeneous molar concentration x. An electron microprobe X-ray analyzer meets all requirements expected from the measuring method for checking the homogeneity of Cd_xHg_1–xTe crystals. — Results of qualitative and quantitative studies in the microscopical and macroscopical range are presented. Typical defects of the crystals were detected by using the qualitative electron microprobe X-ray analysis. The quantitative analysis was carried out with standard samples. The method for determination of the molar concentration x of the standard samples is described.