Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Antiferromagnetism and the Effect of Exchange Bias in LaCr1−x Fe x O3 (x=0.40 to 0.60)

LaCr_1?x Fe _x O₃ (x=0, 0.4, 0.45, 0.6 and 1.0) have been prepared in single phase form with Pbnm space group. The x=0 sample exhibits antiferromagnetic (AFM) transition at Néel temperature T _N =290 K and it is found to reduce with increase in doping concentration up to x=0.45. Magnetization reversal below a compensation temperature of 100 K was observed in the x=0.45 sample and it is explained in terms of competition between two AFM interactions and associated magnetic anisotropy. Exchange bias was observed in the x=0.45 sample and the temperature dependence of exchange bias field and vertical magnetization shift were analyzed.     

Electrical properties of (Ni1−x )Li x )O (x=0.1 and 0.2) under various relative humidities

Rocksalt-type (Ni_1–x Li _x )O (x=0.1 and 0.2) was synthesized at 1350° C in air and its electrical resistivity (R) was measured under various relative humidities (H). R increases with increasing H in the range 0H79%, reaches a maximum value, then decreases in the range 79%<H100%. The increase in R is explained by an electron boundary layer model. On the other hand, the decrease in R is explained by ionic conductivity.     

Synthesis and properties of LiZr2(AsO4)3 and LiZr2(AsO4) x (PO4)3 − x

The LiZr₂(AsO₄)₃ arsenate and LiZr₂(AsO₄) _x (PO₄)_{3 ? x} solid solutions have been prepared through precipitation followed by heat treatment, and characterized by X-ray diffraction, X-ray structure analysis, IR spectroscopy, and impedance spectroscopy. We have established conditions for the crystallization of the arsenate and a continuous series of arsenate phosphate solid solutions (0 ≤ x ≤ 3), which have been obtained as two polymorphs: monoclinic and hexagonal. Using the Rietveld method, we have refined the crystal structures of the polymorphs of LiZr₂(AsO₄)₃ (sp. gr. P2₁/n, a = 9.1064(2), b = 9.1906(2), c = 12.7269(3) Å, β = 90.844(2)°, V =1065.03(5) ų, Z = 4; sp. gr. R $\bar 3$ c, a = 9.1600(4), c = 22.9059(13) Å, V = 1664.44(14) Å, Z = 6) and LiZr₂(AsO₄)_1.5(PO₄)_1.5. Their structural frameworks are built up of AsO₄ tetrahedra—or (As,P)O₄ tetrahedra occupied by arsenic and phosphorus atoms at random—and ZrO₆ octahedra, with the lithium atoms in between. The ionic conductivity of the materials has been measured. The cation conductivity of monoclinic LiZr₂(AsO₄) _x (PO₄)_{3 ? x} with 0 ≤ x ≤ 1 has been shown to exceed the conductivity of lithium zirconium phosphate.     

Grain orientation and anisotropy in the physical properties of SrBi2(Nb1 − x V x )2O9 (0 ≤ x ≤ 0.3) ceramics

SrBi₂(Nb_{1 – x} V _x )₂O₉ (0 x 0.3 in molar ratio) ceramics have been fabricated via conventional sintering at elevated temperatures. Interestingly sintering the pellets in the 1320–1470 K temperature range yielded partially grain oriented ceramics. The orientation factor (f) monitored via X-ray powder diffraction studies was found to increase with increasing V₂O₅ content and reached 83% for x = 0.3. The increment in (f) was not that significant with increase in sintering temperature and its duration. The microstructural studies suggest that V₂O₅ has a truncating effect on the abnormal platy growth of SBN grains. The dielectric constant (_r) and loss (D) measurements as functions of both temperature and V₂O₅ content have been carried out along the directions parallel (_rp) and perpendicular (_rn) to the cold pressing axis of the pellet. The anisotropy (_rn/_rp) associated with _r was found to be 1.11 at 300 K and 2.1 at the Curie temperature, (T _c) respectively. Different dielectric mixture formulae that were employed to analyze the effective dielectric constants of these samples with varying porosity confirmed that the experimental value of _r was comparable with that obtained using Wiener's formula. Impedance spectroscopy was employed to rationalize the electrical behavior of these ceramics. The pyroelectric coefficients along the directions parallel and perpendicular to the pressing axis of the grain oriented (83%) SBN ceramic at 300 K were 0.13145 mC/m²K and 0.26291 mC/m²K respectively. The ferroelectric properties of these grain-oriented ceramics were better in the direction perpendicular to the pressing axis than those in the parallel direction.     

Positional and orientational ordering in La(1−x) Ga2(1+x) and formation of LaGa4

Positional and orientational ordering of Ga pairs were observed in the La-Ga system, which leads to the formation of a new compound, LaGa₄, having an orthorombic cell, space group Pmmm, and lattice parameters of a₀ = 0.434nm and c₀ = 0.441nm. A structural relation exists between LaGa₂, La_(1−x) Ga_2(1+x), LaGa₄, LaGa₆ and Ga and the structure obtained depends on the degree of substitution of the La atoms by Ga-like molecules.     

Growth of perfect-crystal Si-Si1−x Gex-(Ge2)1−x (InP)x structures from the liquid phase

Structures comprising Si-Si_1−x Ge_x-(Ge₂)_1−x (InP)_x with an intermediate Si_1−x Ge_x buffer layer were grown on silicon substrates. Morphological examinations, scanning patterns and diffraction spectra, and also the electrophysical and luminescence properties of the heterostructures were used to show that the crystal perfection of these structures depends on the choice of liquid-phase epitaxy conditions. Pis’ma Zh. Tekh. Fiz. 25, 37–40 (December 26, 1999)     

Crystal Structures and Magnetic Properties of the Co2Mn1−x V x Sb (0 ≤ x ≤ 1) Heusler Compounds

Crystal structure and magnetic properties of the Co₂Mn_1?x V _x Sb (0 ≤ x ≤ 1) Heusler compounds have been studied by X-ray powder diffraction (XRD), magnetometric measurements, and full-potential linearized augmented plane wave (FP–LAPW) method. All compounds crystallize in a cubic Cu₂MnAl-type crystal structure with the space group Fm–3m. The samples for x<0.8 have the Curie temperatures above room temperature, while the Curie temperature is observed at 68 K for the sample with x = 0.8. The saturation magnetization at 5 K decreases linearly with increasing vanadium concentration x. The values of the saturation magnetization obtained by FP–LAPW–local density approximation (LDA) calculations are in better agreement with the experimental results compared with the results obtained by FP–LAPW–generalized gradient approximation (GGA) calculations.     

Effect of CuO on the sintering and dielectric characteristics of (Ba1−x Sr x ) (Ti0.9Zr0.1)O3 ceramics

In order to lower the temperature required to density (Ba_1–x Sr _x ) (Ti_0.9Zr_0.1) O₃ (BSTZ-series) and to avoid the formation of a low dielectric constant phase, copper oxide is added as liquidphase promotor after BSTZ-series are calcined. Thea-axis lattice constant at room temperature elongates with both increasing amount of CuO added and higher sintering temperature, while thec-axis lattice constant elongates only slightly under the same condition. The dielectric constant increases with sintering temperature. However, for different amounts of CuO added, the dielectric constant increases with increasing amount of CuO at lower sintering temperatures (below 1100°C). When a higher sintering temperature is used (above 1200°C), the dielectric constant reaches a maximum at 1 wt % CuO added, and decreases slightly on further addition of CuO.     

Laser deposition and study of CuInS2x Se2(1−x) crystals and films

Single-phase films of CuInS_2x Se_2(1−x) solid solutions with a chalcopyrite structure were deposited by laser evaporation of CuInS_2x Se_2(1−x) crystal targets (0≤x≤1). The structural and optical characteristics of the films depend on the chalcogen concentrations.     

Influence of the cation sublattice defectness on the electronic thermoelectric power of Li x Cu(2−x)−δ (x ≤ 0.25)

This paper presents an experimental study of the electronic thermoelectric power as a function of copper content for Li _x Cu_(2?x)?δS (x ≤ 0.25) electronic-ionic mixed conductors. Using a vacancy model for lattice defects, we calculate their electronic thermoelectric power as a function of x. By adjusting the hole effective mass, we reach satisfactory agreement with experimental data.     

The thermal conductivity and Lorenz function of the tin-lead alloy system Sn x Pb1 − x (x = 0.85, 0.7 0.5 0.3)

The thermal conductivity and electrical resistivity of the tin-lead alloy system Sn _x Pb_{1 – x} (x = 0.85, 0.7 0.5 0.3) were measured in the temperature range 7–300 K. The thermal conductivity was analyzed at temperature T 20K, assuming 1//T+Tⁿ, where , , and n are constants. The analysis shows that n < 2, and not n 2, which would be expected for a normal pure metal. The electrical resistivities could be represented by a T ⁵ relation for temperatures up to about 60 K. The characteristic temperature _R appears to decrease with increasing weight percent of lead. The total Lorenz functions were high, indicating the presence of phonon conductivity. The phonon conductivity _g appears to vary with T, and can be represented by _g=a/Tⁿ (a > 0) and n2.Work supported by the Universiti Sains Malaysia.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

Mixtures of (N2)1−x : (O2) x at High Pressures and Low Temperatures

We performed Raman measurements at 18 K and pressures up to 25 GPa in order to construct a tentative phase diagram of the (N ₂)_1–x :(O ₂)_x –system at low temperatures. We varied the composition of the mixed system over the whole concentration range. Here we focused on the systems with high nitrogen concentration and pressures above 2 GPa. It is known that at room temperature oxygen is highly solvable in the –phase of N ₂. The experimental results show that oxygen suppresses the disorder–order transition –N ₂.     

The design and properties of pseudobinary alloys (PbTe)1−x–(SnTe)x with gradient composition

The pseudobinary thermoelectric alloys (PbTe)_1−x–(SnTe)_x (0≤x≤0.4) doped with 0.02 mol% Ag concentration were prepared by pressureless sintering (PS), and their properties were optimized to fit the preparation of functionally graded materials (FGM) with gradient composition. The calculation shows that the FGM can raise the maximal power output to approximately 200.0 (W m⁻²), at least 21% greater than that of the best monolithic alloys (PbTe)_0.6–(SnTe)_0.4. The measurement shows that the maximum power output of FGM is about 175.0 (W m⁻²), and at least increases by about 16% as compared to those of any monolithic materials (PbTe)_1−x–(SnTe)_x (0≤x≤0.4) without or with different Ag concentration doping.     

Preparation and Structure of Ga2 – x Sc x O3 (0.42 ≤ x ≤ 0.52)

The structure of a scandium gallium oxide single crystal grown by the Czochralski process from a charge of composition Ga_1.5Sc_0.5O₃ is determined by x-ray diffraction. The structure is shown to be a superstructure to -Ga₂O₃. The superstructure parameters are found to vary both along the axis of the crystal and radially, which is due to variations in Ga : Sc ratio. A structural model of the Ga_1.5Sc_0.5O₃ crystal is proposed.     

Preparation of the single phase γ′-(Fe1−x Ni x )4N compounds (0 ≤x ≤ 0.6)

Single phase -Fe₄N type (Fe_1–xNi_x)₄N compounds (0 x 0.6 have been synthesized for the first time by controlled heat treatment of iron-nickel oxalates in a gaseous flow of NH₃ xand H₂. The preparation processes were investigated using differential scanning calorimetry (DSC), X-ray diffraction and Mössbauer spectroscopy. The results confirmed that annealing of oxalates in the NH₃ and H₂ atmosphere included the processes of dehydration, decomposition and reduction, nitrogenation and thermal decomposition of the nitrides. The decomposition and reduction occur simultaneously. The final products depend on the flow rate ratio of NH₃ H₂ and the annealing temperature. The formation conditions for the single phase -Fe₄N type (Fe_1–xNi_x)₄N compounds are related to the nickel concentration, with increasing nickel content, the nitrogenation temperature decreased, in contrast the flow rate ratio of NH₃ H₂ increased.     

Negative Magnetization and Superconductivity in the (LaPt2Asx (x = 1, 2) and BaPt2As2 Compounds

Journal of Superconductivity and Novel Magnetism - We have investigated the electrical and magnetic properties of the compounds LaPt2As, LaPt2As2, and BaPt2As2 prepared by conventional solid-state...     

Metastability of the K2NiF4 type structure of the solid solution LaCa(Cr x Al1−x )O4(0⩽x⩽0.10)

Metastability of the K₂NiF₄ type aluminate LaCaAlO₄ and its chromium diluted solid solution LaCaCr _x Al_1–x O₄ (x0.10) was evidenced at 1400C in air, in terms of demixing into the parent structures of the 11 intergrowth, i.e. the perovskite and rocksalt type LaAlO₃ (LaCr _x Al_1–x O₃) and CaO, respectively. This behaviour is discussed comparatively with YCaAlCO₄ and LaSrAlO₄ whose structures are stable under the same thermodynamic conditions. The results of a structure analysis are used to emphasize the role of the nine-fold co-ordination of the (A ³⁺=Y³⁺, La³⁺; A²⁺=Ca²⁺, Sr²⁺) cations as mixing the twelve-fold co-ordination of A ³⁺ in A ³⁺AlO₃ perovskite and the six-fold one of A ²⁺ in A ²⁺O rocksalt. Calcium-oxygen underbonding in the (La, Ca)-O-Al bridge is assumed to trigger the metastability of the intergrowth structure at high temperature.     

Growth and Properties of CuAlS2x Se2(1 – x) Single Crystals

CuAlS₂, CuAlSe₂, and CuAlS_2x Se_{2(1 – x)} (0 < x < 1) single crystals were grown by chemical vapor transport in a close-spaced geometry and characterized by x-ray diffraction and differential thermal analysis. The results were used to map out the CuAlSe₂–CuAlS₂ phase diagram. The density of the crystals was found to vary linearly with x, while microhardness shows a maximum. The transmission and reflection data obtained near the intrinsic edge were used to determine the band gap E _g of CuAlS₂, CuAlSe₂, and the solid solutions. E _g was found to vary nonlinearly with composition.