Preparation and Magnetic Properties of?(La1?xSr1+x)(Mn0.5Co0.5)O4

A series of single phase (La_1−x Sr_1+x ) (Mn_0.5Co_0.5)O₄ (0≤x≤0.40) materials with a tetragonal K₂NiF₄ structure was prepared by a solid state reaction method at 1400 or 1450 °C with a short period of heating time. Powder X-ray diffraction (XRD) and the Rietveld refinement method were employed for the structural analysis. Unit cell a- and c-axes decrease with increasing amount of Sr substitution. A discrepancy between the zero-field-cooled and the field-cooled magnetization is observed in all samples investigated below a characteristic temperature, T ^*, which likely arises from the canted nature of spins or the random freezing of spins. It appears that T ^* decreases with increasing amount of Sr substitution, which is possibly due to the enhancement of chemical pressure induced by Sr and a corresponding increases of the valence of Mn and/or Co.     

Magnetic and transport properties of Mn3+xGa1?xN compounds

Mn_3+x Ga_1−x N compounds with x = 0.0 and 0.1 were prepared by re-sintering Mn₂N_0.86, Ga bulk and Mn powders. These compounds are deduced to be the N-deficiency ones. In Mn₃GaN, a step-like magnetic transition, from frustrated antiferromagnetism to paramagnetism with increasing temperature, occurs at 370 K, while the same magnetic transition of Mn_3.1Ga_0.9N is far above 380 K. The enhanced magnetization of Mn₃GaN at low temperatures is ascribed to the fast lowering of antiferromagnetism. The electrical resistivity of Mn₃GaN exhibits a typically metallic conducting behavior with a positive magnetoresistance of 4–7%.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba1 ? x(Zn1/2W1/2)O3 ? x and Ba(Zn1/2 ? yW1/2)O3 ? y/2

We have synthesized Ba_{1 − x} (Zn_1/2W_1/2)O_{3 − x} and Ba(Zn_{1/2 − y} W_1/2)O_{3 − y/2} barium tungstates with different deviations from cation stoichiometry (x = 0.01–0.05, y = 0.01–0.05), determined the phase composition of ceramics fabricated from the tungstates, and investigated their electrical properties. Even slight deviations from cation stoichiometry in Ba(Zn_1/2W_1/2)O₃ lead to the formation of the scheelite phase BaWO₄, and its content increases with heat-treatment temperature. Barium or zinc deficiency in the systems studied improves the sintering behavior of Ba(Zn_1/2W_1/2)O₃ and increases the degree of 1: 1 B-site cation ordering, which in turn ensures an increase in microwave quality factor, Q.     

Synthesis and properties of Ca1 ? xTiSi1 ? xFe2xO5 solid solutions

We have studied the CaTiSiO₅-Fe₂TiO₅ system. Ca_{1 − x} TiSi_{1 − x} Fe_2x O₅ solid solutions were prepared by low-temperature plasma synthesis in a hydrogen-oxygen flame. The CaTiSiO₅-Fe₂TiO₅ system contains two phases of variable composition: α (monoclinic structure) and β (orthorhombic structure). We have determined the homogeneity ranges, unit-cell parameters, and electrical parameters of the synthesized solid solutions. The Ca and Si in CaTiSiO₅ can be replaced by Fe³⁺. Fe³⁺ substitution increases the conductivity of the material by up to five orders of magnitude.     

Lattice parameters and dielectric properties of (1 ? x)Bi(Mg1/2Ti1/2)O3 · xBiCoO3 perovskite solid solutions

(1 − x)Bi(Mg_1/2Ti_1/2)O₃ · xBiCoO ceramics have been prepared at high pressures (6 GPa) and temperatures (1370–1570 K). Perovskite solid solutions have been obtained in the composition ranges 0 < x < 0.6 and 0.8 < x < 1. The Bi(Mg_1/2Ti_1/2)O₃-based perovskite phase (x < 0.6) has an orthorhombic structure (sp. gr. Pnnm), which persists up to the decomposition temperature of the material. The BiCoO₃-based phase (x > 0.8) has a tetragonal structure (sp. gr. P4mn). In the range 0.6 < x < 0.8, the ceramics consisted of the two perovskite phases. The lattice parameters of the solid solutions with x < 0.6 are linear functions of composition. The dielectric properties of the orthorhombic solid solutions have been studied using impedance spectroscopy. The temperature dependences of dielectric permittivity ɛ′ and dielectric loss tanδ exhibit different behaviors at low and high temperatures. With increasing x, the boundary between these regions shifts from about 450 to 300 K. At high temperatures, both ɛ′ and tanδ rise steeply. The dc conductivity of the solid solutions with x < 0.6 exhibits Arrhenius behavior. The activation energies for charge transport in the ceramics studies are presented.     

Superconducting Properties of Carbon and Zn Double-doped Mg1?xZnx(B1?xCx)2

A series of polycrystalline samples of Mg_1−x Zn _x (B_1−x C _x )₂ (x=0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) were synthesized by a conventional solid-state reaction method under a background pressure about 10⁻³ Pa. Phase identification, crystal structure and superconducting transition temperature (T _c) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicated that the lattice parameters a and c show no clear trend in their changes with increasing doping level, and it turned out that the dopants had a marked effect on the crystal-lattice parameters and changed the crystal structure of the samples. The T _c for Mg_1−x Zn _x (B_1−x C _x )₂ decreased with C and Zn doping, but the rate of decrease was slower than single C-doped. We propose that the suppression of T _c by doping originates largely from the structural change.     

Structural and dielectric properties of Na1?xBaxNb1?x(Sn0.5Ti0.5)xO3 ceramics

Lead-free (1 − x)NaNbO₃/xBa(Ti_0.5Sn_0.5)O₃ (x = 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3) ceramics were elaborated by the conventional ceramic technique. Sintering has been made at 1523 K for 2 h. The crystal structure was investigated by X-ray diffraction with CuKα radiation at room temperature. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. Dielectric measurements show that the materials have a classical ferroelectric behavior for compositions in the range 0.10 ≤ x ≤ 0.15 and relaxor one for compositions in the range 0.15 < x ≤ 0.30. Temperatures T _C or T _m decrease as x content increases. The ferroelectric behavior has been confirmed by hysteresis characterization. For x = 0.1, a piezoelectric coefficient d ₃₁ of 42.146 pC N⁻¹ was obtained at room temperature. The evolution of the Raman spectra was studied as a function of temperature for x = 0.1.     

Effect of Zn2TiO4 and ZnB2O4 additions on the microstructure and dielectric properties of AgNb1 ? xTaxO3 solid solutions

We report the preparation and characterization of AgNb_0.6Ta_0.4O₃ (ANT) based materials. The addition of Zn₂TiO₄ and ZnB₂O₄ influences the sintering temperature, phase composition, and microstructure of ANT ceramics. ANT doped with 1 wt % Zn₂TiO₄ or ZnB₂O₄ has high dielectric permittivity (400–470), low dielectric losses (tanδ ∼ 10⁻³), and a nonlinearity coefficient n _R ≅ 3–9% (at a field strength E = 3 × 10⁶ V/m).     

Magnetic and dielectric properties of Co–Zn ferrite

Nanocrystalline powders of Co substituted Zn ferrite with the chemical formula Co_xZn_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) were synthesized by sol–gel autocombustion method using tartaric acid as fuel agent. The samples were sintered in static air atmosphere for 7 h at 773 K, 7 h at 973 K and 10 h at 1173 K. The organic phase extinction and the spinel phase formation were monitored by means of Fourier transform infrared spectroscopy. The X-ray diffraction patterns analysis confirmed the spinel single phase accomplishment. Crystallite size, average grains size, lattice parameter and cation distribution were estimated. Magnetic behavior of the as-obtained samples by means of M-H hysteresis measurements was studied at room temperature. Permeability and dielectric permittivity at room temperature versus frequency was the subject of a comparative study for the Co_xZn_1−xFe₂O₄ series. In agreement with the proposed cation distribution the sample with Co_0.8Zn_0.2Fe₂O₄ formula exhibits the optimal magnetic and dielectric properties.     

Synthesis, and study of magnetic properties, of Bi1?xCdxFeO3

Ceramic Bi_1−x Cd _x FeO₃ (x = 0.0, 0.05, and 0.1) samples were prepared by a citrate-gel method. The as-prepared compounds were calcined at 600 °C for 3 h to obtain nearly single-phase materials. The crystal structure, examined by X-ray powder diffraction (XRD) and Rietveld analysis, confirmed that the samples crystallize in a rhombohedral (space group, R-3c no. 161) structure. Magnetic measurements were carried out on the resultant powders from 300 to ~2.5 K. Magnetic hysteresis loops showed a significant increase in magnetization as a result of Cd doping in BiFeO₃.     

Magnetic order and evolution of the electronic state aroundx=0.12 in La2?xBaxCuO4 and La2?xSrxCuO4

Muon spin rotation ( ⁺SR) measurement provides clear evidence of the antiferromagnetic order of Cu moments below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the change of the electronic state coupled with the lattice instability is relevant to the local suppression of SC and freezing of spin fluctuations of the Cu moment.     

X-ray diffraction study of multicomponent oxides in the Zn2 ? x(TiaZrb)1 ? xFe2xO4 system

The multicomponent refractory oxide system Zn_{2 − x} (Ti _a Zr _b )_{1 − x} Fe_2x O₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 2: 1, 3: 1, 4: 1; x = 0–1.0; Δx = 0.05) has been studied by X-ray diffraction, using samples prepared by melting appropriate metal oxide mixtures in a low-temperature hydrogen-oxygen plasma. Two phases, both with wide homogeneity ranges, have been identified: α-phase, with a cubic inverse spinel structure, and β-phase, with a tetragonal inverse spinel structure. The phase boundaries in the system have been determined. Structural data are presented for about 100 solid solutions.     

Electrical and magnetic properties of the diluted magnetic semiconductors Cd1 ? xMnxGeP2 and Cd1 ? xMnxGeAs2 at high pressures

The structural and magnetic phase transitions and magnetoresistance of the diluted magnetic semiconductors Cd_{1 − x} Mn _x GeAs₂ and Cd_{1 − x} Mn _x GeP₂ have been studied at high hydrostatic pressures, up to 7 GPa. The normal and anomalous Hall coefficients of the samples have been determined graphically from experimental data.     

Electrical properties of p-Zn1 ? xCdxGeAs2?Mn?

The electrical resistivity and Hall coefficient of the high-temperature ferromagnetic semiconductor p-Zn_{1 − x} Cd _x GeAs₂〈Mn〉 have been measured as a function of pressure near room temperature. The features in the curves obtained attest to a structural phase transition at particular pressures. With increasing manganese content, the phase transition shifts to lower pressure. The features observed in the pressure-dependent Hall coefficient of the crystals containing more than 0.06 wt % manganese, in which ferromagnetic behavior is more pronounced, are probably due to the anomalous component of the Hall effect and its modulation in the presence of narrow impurity bands.     

Electrical, thermal, and dielectric properties of PbZr1?xSnxO3 (0?≤?x?≤?0.3) single crystals

PbZrO₃ and mixed PbZr_1−x Sn _x O₃ single crystals were grown by means of high temperature solution growth technique. A spontaneous crystallization was carried out in a Pt crucible, with PbO as a solvent. Electrical as well as thermal and dielectric properties were investigated in terms of Sn concentration. The measurements have been made in the large temperature range between 120 K and 800 K. The temperature dependences of c _p for investigated crystals can be approximated by the Einstein function and at high temperatures (cubic phase) c _p reaches the classical Dulong–Petit limit value ~125 J/mol K. From electrical measurements the ac and dc conductivity and the activation energies have been calculated and attributed to different types of electrical conductivity.     

Preparation, characterization and conductivity studies of Li3?2xAl2?xSbx(PO4)3

New NASICON type materials of composition, Li_3−2x Al_2−x Sb _x (PO₄)₃ (x = 0·6 to 1·4), have been prepared and characterized by powder XRD and IR. D.C. conductivities were measured in the temperature range 300–573 K by a two-probe method. Impedance studies were carried out in the frequency region 10²−10⁶ Hz as a function of temperature (300–573 K). An Arrhenius behaviour is observed for all compositions by d.c. conductivity and the Cole-Cole plots obtained from impedance data do not show any spikes on the lower frequency side indicating negligible electrode effects. A maximum conductivity of 4·5 × 10⁻⁶ S cm⁻¹ at 573 K was obtained for x = 0·8 of the Li_3−2x Al_2−x Sb _x (PO₄)₃ system.     

Frequency effect on the electrical and dielectric properties of (TlGaS2) 1 ? x(TlInSe2)x (x = 0.005, 0.02) single crystals

The electrical properties (loss tangent (tanδ), real (ɛ) and imaginary (ɛ″) parts of complex dielectric permittivity, and ac conductivity across the layers (σ_ac)) of (TlGaS₂)_{1 − x} (TlInSe₂) _x (x = 0.005, 0.02) layered single crystals have been studied in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the crystals has a relaxation nature. Almost throughout the frequency range studied, their ac conductivity follows the relation σ_ac ∼ f ^0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (N _F ), the spread of their energies, the mean hop time τ and distance R, and the concentration of deep traps determining the ac conductivity of the crystals (N _t ) have been estimated. With increasing x in (TlGaS₂)_{1 − x} (TlInSe₂) _x , N _F and N _t increase, while τ and R decrease.     

Synthesis and abrasive properties of nanoparticulate MoO2-modified Al2 ? xFexO3 and Fe2 ? yAlyO3 solid solutions

X-ray diffraction, IR spectroscopy, particle-size analysis, and chemical analysis are used to elucidate the general mechanisms of the formation of nanoparticulate molybdenum-dioxide-modified Al_{2 − x} Fe _x O₃ and Fe_{2 − y} Al _y O₃ solid solutions prepared via heat treatment of ammonium hydroxycarbonate complexes, (NH₄)₂Al₂Fe(OH)₅(CO₃) · nH₂O. The addition of molybdenum dioxide (within 0.005 mol %) is shown to enhance the polishing performance of the oxides for final polishing of nonferrous metals and alloys (copper and brass) by a factor of 6–7 relative to unmodified aluminum iron oxides, which is attributable to the increased chemical activity of the abrasive material. The surface roughness value R _a achieved is below 0.005 μm.     

M?ssbauer Spectroscopy Determination of Iron Foreign Phases in?the Superconducting Systems; RAsFeO1?x, RAsFeO1?xFx, and?Sr1?xKxFe2As2

The recently discovered superconducting—spin density wave materials, containing Fe and As, have raised huge interest. However, most materials prepared to date, suffer from a varying degree of content of foreign Fe–As phases, Fe₂As, FeAs₂, and FeAs, which can lead to wrong conclusions concerning the properties of these materials. We show here that M?ssbauer spectroscopy is able to determine quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of seven samples of superconducting or spin density wave materials, prepared in three different laboratories.