Neutron diffraction study of (NH4I)0.73(KI)0.27

Powder and single crystal neutron diffraction experiments were performed on (NH₄I)_0.73(KI)_0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH₄ sites of the four NH₄ molecules per unit cell. The NH₄ group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH₄ molecules are 0.18 Å off center and exhibit C_3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.     

Crystal structure of the quaternary compound CuTa2InTe4 from X-ray powder diffraction

The crystal structure of the new quaternary compound CuTa₂InTe₄ was studied using X-ray powder diffraction data. The powder pattern refined by the Rietveld method indicates that this material crystallizes in the tetragonal system with space group I-4¯2m (No. 121), Z=2, and unit cell parameters a=6.1963(2) Å, c=12.4164(4) Å, c/a=2.00 and V=476.72(3) Å³. The structural and instrumental refinement of 28 parameters led to R_p=10.4%, R_wp=11.1%, R_exp=6.8% and χ²=2.7 for 96 independent reflections.     

Synthesis and magnetic properties of nickel and cobalt nanoparticles obtained in DMF solution

A simple solution-phase approach has been developed to fabricate nickel (fcc) and cobalt (hcp) nanoparticles with narrow size distribution at 160 ^°C, in which dimethylformamide (DMF) acts as not only as solvent but also as reductant. The resultant samples are characterized by XRD and TEM to depict the phase and morphology. The hysteresis loops of the obtained samples reveal the ferromagnetic behaviors and the enhanced coercivity (Hc) and decreased saturation magnetization (Ms) in contrast to their respective bulk materials.     

Demineralisation of a semianthracite char with molten salts/HCl: A study by X-ray diffraction

The effects of chemical heat treatments of a semianthracite char (AC) on the composition of the mineral fraction of the material are investigated. The starting char was first treated with a mixture of LiCl/KCl or LiCl/KCl/CaO at 743, 873 or 1173 K and the products obtained were then washed thoroughly with distilled water. A small fraction of these samples were treated with 10⁻³ M HCl solution. The composition changes were studied by X-ray diffraction. The predominant mineral components initially present in the starting char are quartz, mullite, muscovite and/or kaolinite and oldhamite. The treatments of AC resulted in significant changes in the mineral fraction of the material, in particular when LiCl/KCl/CaO was used. In this case, spurrite, γ-calcium orthosilicate and gehlenite were formed, which were eliminated by treatment with 10⁻³ M HCl solution.     

Synthesis of crystalline SrMoO4 nanowires from polyoxometalates

Crystalline SrMoO₄ nanowires were synthesized via a facile hydrothermal process at 180 °C for 10 h. α-(NH₄)₆-P₂Mo₁₈O₆₂·nH₂O, one of polyoxometalates with Dawson structure, was employed as the source of molybdates. The diameter and length of the obtained SrMoO₄ nanowires are about 20 nm and 5-10 μm, respectively. HRTEM results show that the SrMoO₄ nanowires are of high crystallinity with rough surface. However, when Na₂MoO₄·2H₂O was used, there are only SrMoO₄ nanorods with smaller aspect ratio (200/70 nm) in the similar hydrothermal process. The probable growth mechanism was discussed.     

Synthesis and characterization of nanocrystalline TaN

Tantalum nitride (TaN) nanocrystals have been successfully synthesized at 650 °C through a solid-state reaction in an autoclave. The X-ray powder diffraction pattern indicates that the product is a mixture of hexagonal and metastable cubic TaN. Transmission electron microscopy images and selected area electron diffraction patterns show that the hexagonal TaN crystallites consist of nanorod with a typical size of about 50×1000 nm and the cubic TaN crystallites are composed of uniform particles with an average size of about 30 nm.     

Optical properties of Eu2+-ions immersed as substitutional impurities in a novel spatially-coherent composite host obtained from a melt of KCl,KBr and KI

The optical properties of doping Eu²⁺-ions in a novel composite host, consisting of a spatially-coherent aggregate of crystallites of KBr(0.097):KI(0.903) and KBr(0.459):KCl(0.511):KI(0.030), are investigated. The absorption spectrum consists of two broad absorption bands peaking at 353 and 279 nm while the fluorescence spectrum has a single emission band peaking at 422 nm. These spectra, formed by the spectral contributions from the phases in the composite, are similar in overall shape to the spectra of the Eu²⁺-doped alkali halides used as mother salts, indicating that they are similar in electronic origin. However, in relation to these alkali halides, the phases in the Eu²⁺-doped composite have low 10Dq-splittings (5684 and 8034 cm^?1), low 5d-level barycentre shifts (corresponding to decrements of about ?3351, ?2839, and ?1823 cm^?1, respectively, for one of the phases in the composite, and ?2411, ?1899 and ?916 cm^?1, respectively, for the other) and low Stokes shifts (4632 and 5496 cm^?1). Such low values are discussed to be due to the effect of the mixed ionic character of the impurity environment on the local crystal field as well as to an impurity preference for host cation lattice sites where an iodide ion is nearby to lie at.     

Microstructure and properties of manganese dioxide films prepared by electrodeposition

Nanostructured manganese dioxide films were obtained by galvanostatic, pulse and reverse pulse electrodeposition from 0.01 to 0.1 M KMnO₄ solutions. The deposition yield was investigated by in situ monitoring the deposit mass using a quartz crystal microbalance (QCM). Obtained films were studied by electron microscopy, X-ray diffraction analysis, energy dispersive spectroscopy, thermogravimetric and differential thermal analysis. The QCM and electron microscopy data were utilized for the investigation of deposition kinetics and film formation mechanism. It was shown that the deposition rate and film microstructure could be changed by variation of deposition conditions. The method allowed the fabrication of dense or porous films. The thickness of dense films was limited to ∼0.1 μm due to the insulating properties of manganese dioxide and film cracking, attributed to drying shrinkage. Porous and crack-free 1-2 μm films were obtained using galvanostatic or reverse pulse deposition from 0.02 M KMnO₄ solutions. It was shown that film porosity is beneficial for the charge transfer during deposition and crack prevention in thick films. Moreover, porous nanostructured films showed good capacitive behavior for applications in electrochemical supercapacitors. The porous nanostructured films prepared in the reverse pulse regime showed higher specific capacitance (SC) compared to the SC of the galvanostatic films. The highest SC of 279 F/g in a voltage window of 1 V was obtained in 0.1 M Na₂SO₄ solutions at a scan rate of 2 mV/s.     

Structure and thermoelectric properties of nanocomposite bismuth telluride prepared by melt spinning or by partially alloying with IV–VI compounds

Bismuth telluride samples are compared with respect to the evolution of their thermoelectric material parameters like thermal and electrical conductivity. The Seebeck coefficient is discussed in dependence on the melt spinning fabrication technique. The melt spinner used is only able to produce small thin ribbon shaped specimens, some as thin as 10 μm. This limits melt spinning to mainly production of research specimens for alloys with high critical cooling rate, which are difficult to fabricate with other techniques. Additional parameters are alloying or doping of the base material by comparing the properties as prepared to different annealing conditions. The intrinsic p‐ and n‐doped material was alloyed with up to 0.5% lead telluride by rapidly cooling the bulk material to improve the thermoelectric properties analysed from RT up to about 600 K. A Seebeck coefficient of well above 200 µV/K could be obtained for p‐ and n‐type materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rational synthesis of α-MnO2 and γ-Mn2O3 nanowires with the electrochemical characterization of α-MnO2 nanowires for supercapacitor

An acidification-hydrothermal method was developed to synthesize α-MnO₂ nanowires, which was subsequently treated with ethanol, resulting in γ-Mn₂O₃ nanowire bundles on a large scale. The electrochemical characterization was carried out by cyclic voltammetry, which indicated that the α-MnO₂ nanowires in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte was of an excellent electrode material for supercapacitor at the scan rate of 10 mV S⁻¹ in the range of 0.0-0.85 V.     

Structural phase transition of ScSb and YSb with a NaCl-type structure at high pressures

By use of synchrotron radiation, powder X-ray diffraction of ScSb and YSb with a NaCl-type structure has been studied up to 45 GPa at room temperature. A first-order phase transition from the NaCl-type (B1) to a CsCl-type structure (B2) began to occur at around 28 GPa for ScSb and at around 26 GPa for YSb. Crystal data of the high-pressure phase of both antimonides are obtained. The high-pressure structural behavior of ScSb and YSb is similar to that of heavier LnSb (Ln=Dy-Lu). The B1-B2 transition for ScSb and YSb can be understood according to the rigid sphere model. The bulk moduli of ScSb and YSb are about 58 GPa at ambient pressure.     

Pressure-induced phase transitions of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure

By use of synchrotron radiation the powder X-ray diffraction of lanthanide monoarsenides LaAs and LuAs with a NaCl-type structure has been studied up to 60 GPa at room temperature. First-order phase transitions with the crystallographic change were found at around 20 GPa for LaAs, and 57 GPa for LuAs. The high-pressure form of LaAs is a tetragonal structure and can be viewed as a distorted CsCl-type structure. The atoms in the tetragonal structure are located at La: 0, 0, 0; As: 1/2, 1/2, 1/2. The space group is P4/mmm. The structural change to the tetragonal structure occurs with the volume collapse of about 10%. The structure of these high-pressure phases of LuAs is unknown. The volume vs. pressure curves for LaAs and LuAs are fitted by a Birch equation of state. The bulk moduli of both arsenides are 92±6 GPa for LaAs and 85±3 GPa for LuAs. The high-pressure structural behavior of LaX (X=P, As and Sb) and LnAs (Ln=lanthanide) with the NaCl-type structure is discussed.     

Synthesis, crystal structure and thermoelectric properties of IrSn1.5Te1.5 -based skutterudites

The mixed anion skutterudite IrSn_1.5Te_1.5 has been synthesized and characterized by X-ray powder diffraction, thermopower and electrical resistivity measurements. Structural analysis reveals that Sn and Te order in layers perpendicular to the [111] direction of the skutterudite unit cell, and a distortion of the anion sublattice is evident. The thermopower (S) is 160 μV/K at room temperature, while the electrical resistivity (ρ) is . The effects of chemical substitutions on the Ir site (Ru, Pd) and Sn site (In, Sb) have been investigated. The power factor (S²/ρ) was found to improve with In substitution but, at 0.9 μW/K² cm, is too small for this material to be useful for thermoelectric applications.     

Structural, electrical and magnetic characterization of the double perovskites Sr2CrMO6 (M=Mo, W): B′ 4d-5d system

We report on the preparation and characterization of the variation of B′-site transition metal in Sr₂CrMO₆ (M=Mo, W) with double perovskites structure. The magnetic susceptibility shows that Sr₂CrMoO₆ and Sr₂CrWO₆ are antiferromagnets with T_N=40 and 30 K at H=1 T, respectively. In addition, a large magnetoresistance ratio (MR) of ∼38% (H=3 T) at 5 K was observed in the Sr₂CrWO₆ compound. However, the Sr₂CrMoO₆ compound does not show any significant MR even at high fields (MR∼4%; H=3 T and 5 K). The measured O K-edge X-ray absorption is in agreement with the calculated O p-density of states for both compounds.     

Calculated elastic properties of M2AlC (M=Ti, V, Cr, Nb and Ta)

M₂AlC phases, where M is a transition metal, are layered ternary compounds that possess unusual properties. In this paper, we have calculated the elastic properties of M₂AlC, with M=Ti, V, Cr, Nb and Ta, by means of ab initio total energy calculations using the projector augmented-wave method. We have derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂AlC aggregates. We have estimated the elastic modulus of Cr₂AlC with 357.7 GPa while the values of all other phases are in the range 309±10 GPa. We suggest that this can be understood based on the calculated bond energies for the M-C bonds. Furthermore, our results indicate a profound elastic anisotropy of M₂AlC even compared to materials with a well-established anisotropic character such as α-alumina. Finally, we have estimated the Debye temperatures of M₂AlC from the average sound velocity.     

Crystallographic and magnetic properties of (Nd,Dy)3Fe27.5(Ti,Mo)1.5 compounds

A systematic study of the formation, structure and magnetic properties of (Nd,Dy)₃Fe_27.5(Ti,Mo)_1.5 compounds has been performed. Rietveld analyses of the X-ray patterns of the samples indicate that the concentrations of Ti and Mo affect the formation and structural properties slightly, whereas different rare-earth (Nd and Dy) contents influence them significantly. It is found that high Dy contents make it difficult to form the 3:29-type structures. The Curie temperatures of Nd_2.1Dy_0.9Fe_27.5Ti_1.5−xMo_x decrease monotonically as more Ti was replaced by Mo but their saturation magnetizations remain almost unchanged; in contrast, for Nd_3−yDy_yFe_27.5TiMo_0.5, their saturation magnetizations decrease monotonically with increasing Dy contents while their Curie temperatures are constant.     

Extended X-ray absorption fine-structure (EXAFS) of a complex oxide structure: a full multiple scattering analysis of the Au L3-edge EXAFS of Au2O3

Crystalline Au₂O₃ was obtained by hydrothermal synthesis at 3000 atm and its extended X-ray absorption fine-structure (EXAFS) at the Au L₃-edge was measured at room temperature. A detailed full multiple scattering (MS) analysis using FEFF8 theory shows that only a small number of scattering paths contribute significantly to the EXAFS of Au₂O₃. Because of the complex unit cell (low local symmetry) of the Au₂O₃ structure, contributions of MS paths are almost negligible. The results indicate that FEFF8 theory provides a good reference for the analysis of Au-O phases.     

Synthesis, structure and luminescence of LaSi3N5:Ce phosphor

In this work, new LaSi₃N₅:Ce³⁺ phosphors have been synthesized by solid-state reaction. Rietveld refinement of the crystal structure of La_1−xCe_xSi₃N₅ reveals that Ce atoms substituted for La atoms occupy 4a crystallographic positions. Broad emission and excitation bands observed were attributed to the transitions between the doublet ground state of the 4f¹ configuration and the crystal field components of the 5d¹ excited state. At 77 K, the centroid and crystal field splitting ε_cfs of the 5d levels of Ce³⁺ in LaSi₃N₅:Ce³⁺ compounds were valuated at 33.4×10³ and 11.3×10³ cm⁻¹, respectively. The zero-phonon line and the Stokes shift were measured to be 26.0×10³ and 5.0×10³ cm⁻¹, respectively.     

Preparation and magnetic properties of La-substituted Zn–Cu–Cr ferrites via a rheological phase reaction method

Nanocrystalline La-substituted Zn–Cu–Cr ferrites Zn_0.6Cu_0.4Cr_0.5La_xFe_1.5−xO₄ (x=0.00, 0.02, 0.04, 0.06) were prepared by a rheological phase reaction method. The obtained powders were characterized by X-ray diffractometer, transmission electron microscopy and vibrating sample magnetometer. Permeability of the samples was investigated using an impedance analyzer. The results indicated that ferrite samples had the single spinel phase at low La content. Lattice parameter increased with increasing La content, while particle size calculated from Scherrer's formula decreased with increasing La content in La-substituted ferrite samples. The magnetic properties of La-substituted ferrites were strongly affected by La content. The saturation magnetization decreased, while coercivity increased with increasing La content. The variation of real permeability with La content was investigated in the frequency range of 1 MHz–1 GHz.     

Effects of B′-site transition metal on the properties of double perovskites Sr2FeMO6 (M=Mo, W): B′ 4d-5d system

The crystal structure, magnetic and magnetotransport properties of the variation of B′-site transition metal in Sr₂FeMO₆ (M=Mo, W) with double perovskites structure have been investigated systematically. Measurements of magnetization vs. temperature at H=5 T show that Sr₂FeMoO₆ is a ferromagnet and Sr₂FeWO₆ is an antiferromagnet with T_N∼35 K. Additionally, the large magnetoresistance ratio (MR) of ∼22% (H=3 T) at room temperature (RT) was observed in the Sr₂FeWO₆ compound. However, the Sr₂FeMoO₆ compound did not show any significant MR even at high fields and RT (MR∼1%; H=3 T and 300 K). The implications of these findings are supported by band structure calculations to explain the interaction between the 4d(Mo) and 5d(W) orbitals of transition metal ions and oxygen ions.