Synthesis, structure and magnetic properties of Sr2Fe1−xGaxMoO6 (0 ≤ x ≤ 0.6) double perovskites

Polycrystalline Sr₂Fe_1−xGa_xMoO₆ (0 ≤ x ≤ 0.6) materials have been synthesized by solid state reaction method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in the tetragonal symmetry in the space group I4/m. The anti-site (AS) defects concentration increases with Ga doping, giving rise to highly B-site disordered materials. Ga doping at the Fe-site decreases the cell volume. The evolution of bond lengths and the cation oxidation states was determined from the Rietveld refinement data. The saturation magnetization and Curie temperature decreased with the increasing Ga content in the samples. Low temperature neutron diffraction data analysis and magnetization measurements confirm the magnetic interaction as ferrimagnetic in the sample.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.     

Sintering behavior, microstructure and microwave dielectric properties of Li2+xTiO3 (0 ≤ x ≤ 0.2)

Sintering behavior, microstructure and microwave dielectric properties of Li_2+xTiO₃ (0 ≤ x ≤ 0.2) ceramics have been studied by X-ray diffraction (XRD), scan electron microscopy (SEM), Raman spectra, dilatometery and microwave resonant measurement in this research. Homogeneous non-stoichiometric composition with rock salt structure existed for Li_2+xTiO₃ (0 ≤ x ≤ 0.2) ceramics. The sintering temperature was successfully reduced and highly densified sample could be obtained with appropriate excessive amount of lithium (x = 0.08). A transient reactive liquid phase sintering mechanism was proposed. The preferred orientation of grain growth and micro-cracks existed in the Li₂TiO₃ (x = 0) sample disappeared in the lithium excessive samples with x ≥ 0.08. The microwave dielectric properties varied significantly with the excessive amount of lithium. Optimized microwave dielectric properties were obtained for the x = 0.08 composition: ?_r = 24.6, Q × f = 66,000 GHz, and τ_f = 22.1 ppm/°C.     

Crystal structure and electrical properties of new tungsten bronzes: BxWO3 (0.01 ≤ x ≤ 0.08)

Boron tungsten bronzes B_xWO₃ (0.01 ≤ x ≤ 0.08) were synthesized by hybrid microwave method from mixtures of WO₃ and amorphous boron powder. With the increase of boron content, the crystal structure of B_xWO₃ transforms from orthorhombic (x = 0.01) to tetragonal α (x = 0.048) and then to tetragonal β (0.07 ≤ x ≤ 0.08). The average size of crystallites is in the range of 1-10 μm. All samples show semiconducting behaviour in their temperature dependence of resistivity. The conduction behaviour above 80 K for samples with x = 0.01 and 0.08 can be explained using the variable-range hopping and thermally activated mechanism, respectively. Comparative experiments showed that boron bronze phases cannot be obtained by the microwave heating of pure WO₃ powder or a mixture of B₂O₃ and WO₃ under the same conditions.     

Rietveld refinements of the solid solution Li(1−2x)NixTiO(PO4) (0 ≤ x ≤ 0.50)

Li_(1−2x)Ni_xTiO(PO₄) oxyphosphates with 0 ≤ x ≤ 0.10 crystallize in the orthorhombic system with the space group Pnma, those with 0.10 < x ≤ 0.25 crystallize in the monoclinic system with the space group P2₁/c and compositions with 0.25 < x < 0.50 present a mixture of the limit of the solid solution Li_0.50Ni_0.25TiO(PO₄) and Ni_0.50TiO(PO₄). The structure of the compositions 0 ≤ x ≤ 0.25 is based on a three-dimensional anionic framework constructed of chains of alternating TiO₆ octahedra and PO₄ tetrahedra, with the lithium and nickel atoms in the cavities in the framework. The dominant structural units in the compositions are chains of tilted corner-sharing TiO₆ octahedra running parallel to one of the axis. The oxygen atoms of the shared corners, not implied in (PO₄) tetrahedra, justify the oxyphosphate designation. Titanium atoms are displaced from the geometrical center of the octahedra resulting in alternating long (≈2.25 Å) and short (≈1.71 Å) TiO(1) bonds. The four remaining TiO bond distances have intermediate values ranging from 1.91 to 2.06 Å.     

Influence of SnO2 addition on the thermoelectric properties of Zn1 − xSnxO (0.01 ≤ x ≤ 0.05)

The grain size and the density of the Zn_1 − xSn_xO (0 ≤ x ≤ 0.05) samples decreased with increasing SnO₂ content. The addition of a small amount of SnO₂ (x ≤ 0.01) to ZnO led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in a significant increase in the power factor. The thermoelectric power factor was maximized to a value of 1.25 × 10⁻³ Wm⁻¹ K⁻² at 1073 K for the Zn_0.99Sn_0.01O sample.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

Electron paramagnetic resonance (EPR) of antiferromagnetic nanoparticles of La1−xSrxCrO3 (0.000 ≤ x ≤ 0.020) synthesized by combustion reaction

Nanocrystalline particles of La_1−xSr_xCrO₃ (0.000 ≤ x ≤ 0.020) compounds were synthesized in order to investigate the antiferromagnetic (AFM) to paramagnetic (PM) phase transition temperature, g-factor, line width and intensity by electron paramagnetic resonance (EPR). All samples were synthesized by combustion reaction method using strontium nitrate, lanthanum nitrate, chromium nitrate and urea as fuel without subsequent heat treatment. X-ray diffraction patterns of all systems showed broad peaks consistent with orthorhombic structure of LaCrO₃. The absence of extra reflections in the diffraction patterns of as-prepared materials ensures the phase purity. The average crystallite sizes determined from the prominent (1 1 2) peak of the diffraction using Scherrer's equation was independent of the addition of Sr²⁺ ions; being ca. 31–29 nm for x = 0.000 and 0.020, respectively. The EPR line width and intensity were found to be dependent on Sr²⁺ addition and temperature. However, the AFM–PM transition temperature was found to be independent of strontium concentration, being ca. 296 K. In the PM phase, g-factor was nearly temperature independent with increasing of x. The EPR results indicated that the addition of Sr²⁺ ions may induce creation of Cr³⁺–Cr⁴⁺ clusters.     

Synthesis and characterization of Ba1−xNdxF2+x (0.00 ≤ x ≤ 1.00)

A series of mixed fluorides with general composition Ba_1−xNd_xF_2+x (0.00≤x≤1.00) was prepared by vacuum heat treatment of the mixture of starting fluorides, and analyzed by powder XRD. From the XRD analysis, the low temperature phase equilibria in BaF₂-NdF₃ system is elucidated. The initial compositions in this series, that is, up to the nominal composition Ba_0.65Nd_0.35F_2.35 (x≤0.35) exist as cubic fluorite-type solid solution. Beyond the solid solution limit, that is, x>0.35, a rhombohedral fluorite related ordered phase is observed. Further, NdF₃-rich compositions (x≥0.50) exist as a mixture of rhombohedral ordered phase and NdF₃ (tysonite)-type phase. About 10 mol% of BaF₂ could be retained in the NdF₃ lattice, forming a tysonite-type solid solution, under the short annealed and slow cooled conditions.     

Structural and optical properties of polycrystalline CdSexTe1−x (0 ≤ x ≤ 0.4) thin films

CdSe_xTe_1−x (0 ≤ x ≤ 0.4) ternary thin films have been deposited on quartz substrates at room temperature by a single source thermal evaporation. X-ray diffraction patterns and transmission electron microscope micrographs of these films showed that the films were of polycrystalline texture over the whole range studied and exhibit predominant cubic (zinc blende) structure with strong preferential orientation of the crystallites along (1 1 1) direction. Linear variation of the lattice constant with mole fraction x is observed obeying Vegard's law. The dependence of the optical constants, the refractive index n and extinction coefficient k, of the films on the mole fraction x was studied in the spectral range of 400-2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple-DiDomenco single-oscillator model. CdSe_xTe_1−x thin films of different composition have two direct and indirect transitions corresponding to energy gaps and . The variation in either or with x indicates that this system belongs to the amalgamation type. The variation follows a subquadratic dependence and the bowing parameters were found to be 0.36 and 0.48 eV for the direct, and indirect energy gaps, respectively. Direct linear variation of the ratio N/m^* with x is observed.     

Study on the optical properties of Zn1−xMgxS (0 ≤ x ≤ 0.55) quantum dots prepared by precipitation method

Zn_1−xMg_xS (0 ≤ x ≤ 0.55) quantum dots (QDs) were successfully synthesized by precipitation method. The crystal structures, microstructures, and optical properties of the Zn_1−xMg_xS QDs were investigated using X-ray diffraction, scanning electron microscopy, and ultraviolet-visible and photoluminescence (PL) spectroscopy. The Zn_1−xMg_xS QDs were found to have a cubic crystal structure and an average crystallite size of 6.40-7.96 nm. It has been shown that an increase in doping Mg²⁺ concentration in Zn_1−xMg_xS QDs led to a gradual widening of the band gap and a weakening in the PL intensity of the Zn_1−xMg_xS QDs.     

Synthesis of spherical nanostructured LiMxMn2−xO4 (M = Ni, Co, and Ti; 0 ≤ x ≤ 0.2) via a single-step ultrasonic spray pyrolysis method and their high rate charge-discharge performances

LiM_xMn_2−xO₄ (M = Ni²⁺, Co³⁺, and Ti⁴⁺; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn₂O₄ could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn₂O₄ in terms of capacity, cycleability and rate capability.     

Synthesis, structural and magnetic studies of the CuCr1−xRhxO2 delafossite solid solution with 0 ≤ x ≤ 0.2

The CuCr_1−xRh_xO₂ series is investigated by X-ray diffraction, magnetization measurements and Raman spectroscopy on ceramic samples. It is found that a delafossite solid solution is maintained up to x = 0.2 in CuCr_1−xRh_xO₂. The small observed variation in cell parameters is consistent with the small difference between the ionic radii of Cr³⁺ and Rh³⁺. A significant broadening of X-ray reflections is observed and when analyzed using the Williamson-Hall relationship showed that the strain generated by Rh substitution is strongly anisotropic, affecting mainly (Cr,Rh)-O bonds in the ab plane. Room temperature Raman spectra displayed three main Raman active modes. All modes shift to lower frequency and undergo significant changes in intensity with increasing Rh content, showing the effect of Rh atoms on the M³⁺-O bond strength. The magnetic behavior of CuCr_1−xRh_xO₂ samples was investigated as a function of temperature and applied field. At high temperature paramagnetic behavior, and at low temperature, evidence for weak ferromagnetism, reinforced by a hysteresis loop at 4 K is observed. The magnetic behavior of CuCr_1−xRh_xO₂ is attributed to the disorder of Cr and Rh in octahedral sites resulting in short-range Cr-O-Cr and Cr-O-Rh interactions, which give rise to short-range weak ferromagnetism.     

Double subband occupation of the two-dimensional electron gas in InxAl1 − xN/AlN/GaN/AlN heterostructures with a low indium content (0.064 ≤ x ≤ 0.140) barrier

We present a carrier transport study on low indium content (0.064 ≤ x ≤ 0.140) In_xAl_1 − xN/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33-300 K) and a magnetic field (0-1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger-Poisson equations.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Effect of substitution in the scheelite-like series, AxBa1−xBi2Mo4O16 (A = Ca, Sr, Pb)

A new series of solid solutions of the type, A_xBa_1−xBi₂Mo₄O₁₆ (A = Ca, Sr, Pb) have been isolated. The domain of the solid solutions is very narrow and is in the range (0.01 ≤ x ≤ 0.10). All the phases were synthesized by the solid-state technique. The crystal structure is similar to that of the monoclinic scheelite type BaBi₂Mo₄O₁₆. The structure consists of layers of [Bi₂O₂] units separated by (Ba/AO)₁₀ units and isolated MoO₄ tetrahedra. Their semiconductor band gaps render them as potential materials for photocatalysis.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Dielectric properties of Ba(Ti1 − xZrx)O3 solid solutions

This paper reports the structural and dielectric properties of Ba(Ti_1 − xZr_x)O₃ (x = 0-0.3) ceramics. Single-phase solid solutions of the samples were determined by X-ray diffraction. Microscopic observation by scanning electron microscope revealed dense, single-phase microstructure with large grains (20-60 μm). The evolution of dielectric behavior from a sharp ferroelectric peak (for x ≤ 0.08) to a round dielectric peak (for 0.15 ≤ x ≤ 0.25) with pinched phase transitions and successively to a ferroelectric relaxor (for x = 0.3) was observed with increasing Zr concentration. Compared with pure BaTiO₃, broaden dielectric peaks with high dielectric constant of 25,000-40,000 and reasonably low loss (tanδ: 0.01-0.06) in the Ba(Ti_1 − xZr_x)O₃ ceramics have been observed, indicating great application potential as a dielectric material.     

Phase relation of Na1−xKxMgF3 (0 ≤ x ≤ 1) perovskite-type solid-solutions

Na_1−xK_xMgF₃ (0≤x≤1) solid-solutions were synthesized and the phase diagram of NaMgF₃-KMgF₃ system was determined by high temperature X-ray powder diffraction experiments and differential thermal analysis (DTA). This system is characterized by a complete series of solid solutions, with a minimum in the solidus at 30 mol% KMgF₃ and 1279 K. No immiscibility gap was found. The crystal system changes from orthorhombic to cubic at x=0.22 at room temperature. The volume change of the unit cell as a function of composition shows a large deviation (excess volume) from Vegard’s law for solid solution. The transition temperatures of NaMgF₃ and Na_0.9K_0.1MgF₃ from orthorhombic to cubic are 1043 and 723 K, respectively. The transition temperature decreases rapidly by the effect of replacing Na by K. Axial ratios of b/a and c/√2a in orthorhombic NaMgF₃ and Na_0.9K_0.1MgF₃ decrease linearly with temperature toward the transition and then discontinuously changes to cubic at the transition point.