Magnetic and structural transition of Ni50+xMn25−x/2Ga25−x/2 (x=2–5) alloys

Magnetic and structural transitions of non-stoichiometric Ni_50+xMn_25−x/2Ga_25−x/2 (x=2–5) alloys are systematically investigated. Differential scanning calorimetry and modified thermogravimetry (TG) are used to measure magnetic and structural transitions simultaneously. The structural transition temperatures increase monotonically with increasing Ni substitution for Mn and Ga. Different magnetic transition sequences on heating were observed from ferromagnetic martensite to ferromagnetic autensite, then to paramagnetic autensite, from ferromagnetic martensite to paramagnetic austensite or from ferromagnetic martensite to paramagnetic martensite, respectively. Three kinds of NiMnGa alloys were obtained according to the sequence of the structural and magnetic transition, whose structural transition temperatures are lower, equal to or higher than the magnetic transition temperatures.     

Preparation and characterization of Al and La co-doped (Ce1−x−y Al x LayO2-(x+y)/2) ceria

Effect of co-doping of lanthanum and aluminum on ionic conductivity of CeO₂ for its use as a solid electrolyte in intermediate temperature solid oxide fuel cells has been studied. A few compositions in the system Ce_1???x???y Al _x La_yO_{2???(x?+?y)/2} (CAL) with x?=?0.05, 0.025, and 0.00 and y?=?0.00, 0.025, and 0.05 such that x?+?y?=?0.05 are prepared by auto-combustion method. X-ray diffraction patterns show that all the synthesized samples are ceria-based solid solutions. Microstructures of thermally etched samples have been studied by scanning electron microscope. To determine the contribution of grains, σ _g and grain boundaries, σ _gb to the total conductivity, σ _t impedance is measured in the frequency range 1–1 MHz at different steady temperatures in the range 250–500 °C. It has been found that conductivity of the co-doped composition is more than singly doped with Al but less than singly doped lanthanum composition.     

Oxidation of Si(100)2x1 and Gex Si1−x (100)2x1

High resolution electron energy loss Spectroscopy (HREELS), X-ray photoelectron Spectroscopy (XPS) and low energy electron diffraction (LEED) have been used to characterize initial stages of oxidation of Si(100)2x1 and Ge_x Si_1−x (100)2x1 surfaces. Different oxidation stages of Si were identified by measuring Si 2p core level shifts and characteristic vibrations of Si-O-Si complexes. The latter may be described in the framework of continuous random network models.Then,from the experimentally determined frequencies, force constants,average bond angles and bond lengths are determined. Complementary,marker experiments with atomic hydrogen as a probe in conjunction with HREELS were used to determine the local, atomic structure for different stages of oxidation from chemical shifts of Si-H and Ge-H stretching mode intensities, respectively.     

Structural,morphologic and optical characterization of In(2−x)Al x S3

Aluminum-doped indium sulfide thin films are deposited on glass by spray pyrolysis technique. The structure and the surface morphology of these films were characterized by X-ray diffraction and atomic force microscopy. The effects of aluminum ratio z and substrate temperature T _s, on the film structure and grain size are discussed. The influence of aluminum ratio on surface morphology is revealed by scanning electron microscope. Besides, energy dispersive spectrometry technique is used to compare atomic aluminum concentration in the film with aluminum ratio z in spray solution. Optical properties are studied by a spectrophotometer in the wavelength range 350–850 nm, at room temperature. Optical transmission and grain size are found to be maximal for z = 1.8 %. Moreover, band-gap energy is found to increase with aluminum ratio.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

Studies on the synthesis and characterization of Zn1− x Cd x S and Zn1− x Cd x S:Mn2+ semiconductor quantum dots

Quantum dots (3–4?nm) of Zn_{1? x} Cd _x S (both free of Mn²⁺ and with Mn²⁺ incorporated) were synthesized through a novel solvothermal-microwave irradiation technique. Detailed structural analysis of the Zn_{1? x} Cd _x S and Zn_{1? x} Cd _x S:Mn²⁺ (x?=?0, 0.25, 0.5, 0.75 and 1) materials was carried out using powder X-ray diffraction technique. For all the compositions, the crystallite size was controlled to less than 1.5?nm. The optical energy gap for Zn_{1? x} Cd _x S was found to vary from 3.878 to 2.519?eV and for Zn_1?x Cd _x S:Mn²⁺ it varies from 3.830 to 2.442?eV when x is increased from 0 to 1. Overall, the optical energy gap could be tuned from a minimum of 2.442?eV to a maximum of 3.878?eV. DC conductivity analysis (from 40°C to 150°C) and electrical energy gap analysis for all the compositions were also performed. The dc conductivity for Zn_{1? x} Cd _x S solid solutions varies from 0.3840?×?10^?10 to 8.7782?×?10^?10?mho/m at 150°C and for Zn_{1? x} Cd _x S:Mn²⁺ it varies from 0.5751?×?10^?10 to 9.8078?×?10^?10 mho /m at 150°C (for x?=?0 to x?=?1). The method of synthesis and the results observed in this investigation may assist in the fabrication of optical devices when the required operational performance falls under the range observed in the study.     

A study of the microstructure of La1 − x Ca x MnO3 (x = 0.5, 0.8) solid solutions

HREM and XPD methods are used to study the microstructure of La_{1 − x} Ca _x MnO₃ perovskite-like oxides synthesized by pyrolysis of polymer-salt compositions. The XPD data show that the samples studied are single-phase solid solutions. Morphological transitions are observed in series of samples depending on the substitution parameter x. At 0 ≤ x ≤ 0.4 the samples can be characterized by rhombic symmetry (space group Pnmb); at 0.5 ≤ x ≤ 0.8 the symmetry increases to the tetragonal space group I4/mmm; at 0.9 ≤ x ≤ 1 the symmetry lowers to the monoclinic space group P1121. Heating of samples with x = 0.5 and 0.8 up to 1200°C in air does not lead to noticeable changes in the sample structure, which indicates their high thermal stability. When the La_0.2Ca_0.8MnO₃ sample is heated to 1100°C in vacuum nanocrystalline states with particle disintegration into microphases of different structures form because of a partial decomposition of the solid solution. As a result, a system of Mn₃O₄ nanoparticles appears, which is coherently bound with the perovskite phase of a defective structure.     

Neutron diffraction study of the oxide conducting δ1-phase of (Bi2O3)1−x(Y2O3)x (x=0.25)

The atomic parameters of the average cell of δ¹-Bi(Y)O_1.5 were determined by powder neutron diffraction. The evidence indicates that the disorder in the structure has many features in common with the structure of the oxygen deficient zirconia. Most oxygens (78%) are in sites displaced 0.335 Å along 〈100〉 directions from the normal fluorite positions, while a smaller proportion (22%) are displaced 0.80 Å along 〈111〉 directions and no oxygen remains in normal positions. In addition to the anion displacements a smaller displacement (0.25 Å) along the 〈111〉 direction was found for the cations. Comparison with the structure of β-Bi₂O₃ suggests that the displacements may be precursors to a δ→β phase transition.     

High-Moment-Low-Moment Description of Magnetovolume Effects in Y(Mn x Al 1− x ) 2 and Y x Sc 1− x Mn 2

Based on the assumption of a high-moment-low-moment instability of the Mn atom, we construct a simple spin model with coupled magnetic and spatial degrees of freedom to describe the Laves phase systems Y(Mn x Al 1 m x ) 2 and Y x Sc 1 m x Mn 2 . Monte Carlo simulations of this model qualitatively reproduce anomalies observed in these materials like a discontinuous giant volume change and anomalous thermal expansion behavior.     

Heat capacity and dielectric properties of multiferroics Bi1 − x Gd x FeO3 (x = 0–0.20)

The heat capacity and the permittivity of multiferroics Bi_{1 ? x} Gd _x FeO₃ (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.     

Mössbauer characterization of Nd0.7Sr0.3(Mn1−x57Fex)O3 (x=0.02–0.15)

The magnetic structure of colossal magneto-resistive Nd_0.7Sr_0.3(Mn_1−x⁵⁷Fe_x)O₃ (x=0.02, 0.05, 0.10, and 0.15) has been studied by Mössbauer spectroscopy in magnetic fields up to 60 kG. ⁵⁷Fe ions appear to spread over two different magnetic phases. One of them remains ferromagnetic for all values of x, but the other one changes from canted to anti-ferromagnetic with increasing x. For x=0.02 and 0.05, the latter magnetic phase is comprised of small clusters, which are very sensitive temperature and strongly influenced by the applied fields.     

Calculation of the electronic structure and hyperfine fields for Fe1 − x Co x B and (Fe1 − x Co x )2B compounds by the Korringa-Kohn-Rostoker method

The magnetic properties of Fe_{1 ? x} Co _x B and (Fe_{1 ? x} Co _x )₂B disordered compounds were investigated using first-principles calculations of the electronic structure in the framework of the density functional theory with the Korringa-Kohn-Rostoker method. The concentration dependences of the magnetic moments and the electron density were calculated for the Fe_{1 ? x} Co _x B solid solutions. The results obtained were used to analyze in detail and to interpret the transition from a magnetic phase to a nonmagnetic phase, which was previously revealed from the experiments in the compounds under investigation. The performed analysis of the calculated hyperfine fields induced by the electronic shells at the iron and cobalt atoms in the (Fe_{1 ? x} Co _x )₂B borides made it possible to explain the experimentally observed magnetic anisotropy.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

High-pressure synthesis of superconducting Nb1−xB2 (x=0–0.48) with the maximum Tc=9.2 K

Superconductivity with T_c above 9 K was found in metal-deficient NbB₂ prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB₂. The superconductivity was observed above x=0.04 in Nb_1−xB₂, and the lattice parameters also changed abruptly at x=0.04. As x increased, the transition temperature T_c slightly rose and fell with the maximum value of 9.2 K at x=0.24 for the samples sintered at 5 GPa and 1200 °C. The T_c-value changed in the range from 7 to 9 K, depending on the sintering pressure. A series of Ta_1−xB₂ (0⩽x⩽0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.     

Optical spectroscopy and electronic structure of compounds HoNi5 − x Al x (x = 0, 1, 2)

The optical properties of the compounds HoNi_{5 ? x} Al _x (x = 0, 1, 2) have been investigated using the ellipsometric method in the wavelength range from 0.22 to 16 μm. The electronic structure of these intermetallic compounds has been calculated in the local electron-spin density approximation with the correction for strong electronic interactions in the 4f shell of the holmium ions. The experimental dispersion dependences of optical conductivity in the region of interband light absorption have been interpreted based on the results of the calculation of the electron density of states. The plasma and relaxation frequencies of electrons have been determined.     

Thermoelectric power of Ba(Fe1−x Co x )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rh x )2As2 (0 ≤ x ≤ 0.171)

Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.05) and Ba(Fe_1?x Rh _x )₂As₂ (0?≤?x?≤?0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe_1?x Co _x )₂As₂ delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0?≤?x?≤?0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0?≤?x?≤?0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe_1?x Rh _x )₂As₂ show very similar behavior to that of Co substituted BaFe₂As₂. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.     

A computational study of the electron detachment energies of Al2As2 − and Al3As3 −

Ab initio and density functional theory calculations are reported for the low-lying electronic states of Al₂As₂ ^?, Al₂As₂, Al₃As₃ ^?, and Al₃As₃. The ²B_2g ground electronic state of Al₂As₂ ^? has a rhombic structure with the Al atoms occupying the shorter diagonal. In contrast, the As atoms occupy the shorter diagonal of the ground state rhombic structure of Al₂As₂. Electron detachment energies computed for Al₂As₂ ^? are presented and discussed. The adiabatic electron affinity of Al₂As₂ ^? is calculated to be 2.1 eV at the CCSD(T) level, using B3LYP and MP2 optimized geometries. The ground states of both Al₃As₃ ^? (²A₁′) and Al₃As₃ (¹A₁′) have planar hexagonal D_3h geometry. Electron detachment energies computed for the anion are reported. At the CCSD(T)//B3LYP level, the electron affinity of Al₃As₃ is calculated to be 2.47 eV.     

Electron-microscopy and electron-diffraction study of order-disorder phenomena in the intercalate C60(I2)2−x

Single crystals of iodine-intercalated C₆₀ have been examined by transmission electron microscopy. Tilting series of Electron Diffraction Patterns (EDP) confirm that it has a simple hexagonal structure with c/a1 in accordance with the results of X-ray diffraction, which allows higher precision c/a=1.0024. Cooling and heating experiments were performed in the vacuum of the microscope to investigate the order — disorder phenomena. High-Resolution Electron Microscopy (HREM) images and electron diffraction patterns reveal that the c/2-displacement along the c-direction plays an important role in the transition from the simple hexagonal structure into the fcc structure when iodine is lost from the specimen or vice versa. The orientation relationship between the intercalate and the C₆₀ structure is [01 10]_h[1 10]_fcc and (001)_h(110)_fcc. A model is proposed to interpret the features observed in EDP and HREM consistently.On leave from Department of Materials Science and Engineering, Zhejiang University, People's Republic of ChinaOn leave from Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, People's Republic of China