Quasicrystals, crystals and multiple twins in rapidly solidified Al-Cr-Si, Al-Mn-Si and Al-Mn-Cr-Si alloys

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al80Cr14Si6, Al80Mn14Si6 and Al75Mn10Cr5Si10 alloys. The presence of a b.c.c. alpha-AlMnSi phase in Al-Mn-Si and alpha-AlMnSi(Cr) phase in Al-Mn-Cr-Si has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in Al-Mn-Si and Al-Mn-Cr-Si alloys transforms to alpha-AlMnSi at temperatures of 690 and 670 K, respectively. In Al-Cr-Si alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to alpha-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun Al-Cr-Si alloy is presumably due to this metastable phase. Copyright (C) 1996 Acta Metallurgica Inc.

Deffect Structure of LAMNO3

LaMnO3 compositions possessing orthorhombic, rhombohedral and cubic structures and containing 12, 24 and 33% Mn4+ respectively have been investigated by X-ray diffraction, electron diffraction and high resolution electron microscopy. The study shows that the defect structure of LaMnO3 is best described in terms of the random distribution of La and Mn vacancies, rather than by anion excess. Density measurements confirm the cation vacancy model involving an equal number of La and Mn vacancies.

Hydrothermal synthesis, characterization and Raman studies of Eu3+ activated Gd2O3 nanorods

Eu3+ (8 mol%) activated gadolinium oxide nanorods have been prepared by hydrothermal method without and with surfactant, cityl trimethyl ammonium bromide (CTAB). Powder X-ray diffraction (PXRD) studies reveal that the as-formed product is in hexagonal Gd(OH)(3):Eu phase and subsequent heat treatment at 350 and 600 degrees C transforms the sample to monoclinic GdOOH:Eu and cubic Gd2O3:Eu phases, respectively. The structural data and refinement parameters for cubic Gd2O3:Eu nanorods were calculated by the Rietveld refinement. SEM and TEM micrographs show that as-obtained Gd(OH)(3):Eu consists of uniform nanorods in high yield with uniform diameters of about 15 nm and lengths of about 50-150 nm. The temperature dependent morphological evolution of Gd2O3:Eu without and with CTAB surfactant was studied. FTIR studies reveal that CTAB surfactant plays an important role in converting cubic Gd2O3:Eu to hexagonal Gd(OH)(3):Eu. The strong and intense Raman peak at 489 cm(-1) has been assigned to A(g) mode, which is attributed to the hexagonal phase of Gd2O3. The peak at similar to 360 cm(-1) has been assigned to the combination of F-g and E-g modes, which is mainly attributed to the cubic Gd2O3 phase. The shift in frequency and broadening of the Raman modes have been attributed to the decrease in crystallite dimension to the nanometer scale as a result of phonon confinement. (C) 2010 Elsevier B.V. All rights reserved.

Phonon interference in BaTiO3: High-pressure Raman study

Raman experiments have been carried out on single crystals of BaTiO3 as a function of pressure up to 3.5 GPa across the ferroelectric (tetragonal) to paraelectric (cubic) phase transition. The unusual features in the Raman spectra associated with the interference effects due to coupling of the three A1(TO) phonons are studied quantitatively to obtain the pressure dependence of the line shape parameters and the coupling constants. The frequencies of the middle and highest-frequency modes as well as the linewidth of the middle mode show interesting pressure dependence.

Giant magnetoresistance in bulk samples of LaMnO3 with varying Mn4+ content

Magnetoresistance (MR) in bulk samples of LaMnO3 has been investigated by varying the Mn4+ content from 10 to 33 per cent by chemical means, without aliovalent doping. With the increase in Mn4+ content, the structure of LaMnO3 changes first from orthorhombic to rhombohedral and then to cubic and the material becomes increasingly ferromagnetic, exhibiting a resistivity maximum akin to an insulator-metal transition at T-Peak, just below the ferromagnetic T-c. The magnitude of MR is highest in the cubic sample (with 33% Mn4+) around the T-Peak, and negligible in the non-magnetic orthorhombic sample (12% Mn4+).

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from the C-V characteristics. Layers grown from antimony rich melts always exhibit p-type conductivity. In contrast, a type conversion from p- to n- was observed in layers grown from gallium rich melts below 400 degrees C. The electron mobility of undoped n-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated.

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce1 − xZrxO2, X = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m2/g. Calcination of Ce1 −xZrxO2 at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x Image .8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce1 − xZrxO2 has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce0.5Zr0.5O2 was carried out with H2 and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.

Novel Metastable Structures of WO3, MoO3 and W1-xMoxO3 Obtained by the Deintercalation of Layered Amine Adducts

Deintercalation of amines from the layered amine adducts of WO3, MoO3 and W1-xMoxO3 has been employed as a soft chemical route to produce unusual metastable structures of the oxides. After the adducts of WO3, MoO3 and W1-xMoxO3 (x = 0.25, 0.5, 0.75) with amines such as triethylamine (TEA), pyridine, n-butylamine and n-octylamine had been characterized, deintercalation was carried out thermally as well as by acid leaching. Thermal deintercalation yielded novel metastable structures of WO3 and MoO3 that were significantly different from the stable forms, which contain distorted metal-oxygen octahedra. Thus, ReO3-type cubic WO3 was obtained by the thermal deintercalation of WO3 . 0.5 TEA. Acid leaching of the amines gave metastable phases of WO3, MoO3 and W1-xMoxO3, which were different from those obtained thermally. All the metastable phases transformed to the corresponding stable forms at higher temperatures.

Potentiometric determination of the gibbs energies of formation of SrZrO3 and BaZrO3

The Gibbs free energies of formation of strontium and barium zirconates have been determined in the temperature range 960 to 1210 K using electrochemical cells incorporating the respective alkaline-earth fluoride single crystals as solid electrolytes. Pure strontium and barium monoxides were used in the reference electrodes. During measurements on barium zirconate, the oxygen partial pressure in the gas phase over the electrodes was maintained at a low value of 18.7 Pa to minimize the solubility of barium peroxide in the monoxide phase. Strontium zirconate was found to undergo a phase transition from orthorhombic perovskite to) with space group Cmcm; D-2h(17) to tetragonal perovskite (t) having the space group 14/mcm; D-4h(18) at 1123 (+/- 10) K. Barium zirconate does not appear to undergo a phase transition in the temperature range of measurement. It has the cubic perovskite (c) structure. The standard free energies of formation of the zirconates from their component binary oxides AO (A = Sr, Ba) with rock salt (rs) and ZrO2 with monoclinic (m) structures can be expressed by the following relations:SrO (rs) + ZrO2 (m) --> SrZrO3 (o) Delta G degrees = -74,880 - 14.2T (+/-200) J mol(-1) SrO (rs) + ZrO2 (m) --> SrZrO3 (t) Delta G degrees = -73,645 - 15.3T (+/-200) J mol(-1) BaO (rs) + ZrO2 (m) --> BaZrO4 (c) Delta G degrees = -127,760 - 1.79T (+/-250) J mol(-1) The results of this study are in reasonable agreement with calorimetric measurements reported in the literature. Systematic trends in the stability of alkaline-earth zirconates having the stoichiometry AZrO(3) are discussed.

Preparation and dielectric properties of zinc and cadmium doped barium tantalates, Ba3Zn1?xCdxTa2O9

Members of the Ba2Zn1-xCdxTa2O9 (0 less than or equal to x less than or equal to 1) series have been synthesized by solid state reactions at 1473K. Powder x-ray diffraction studies show a cubic perovskite cell with a similar to 4.1 Angstrom which increases with increase in x. Electron diffraction studies show the presence of hexagonal ordered perovskite structure in addition to the cubic structure seen by x-rays, the x = 0.5 composition showing more ordered crystallites. These samples show high dielectric constants with a maximum (epsilon(r) = 30 at 1 kHz) for the x = 0.5 member. The dielectric loss increases with increase in x at all the frequencies under study.

Mesoporous aluminoborates

Hexagonal, cubic and lamellar aluminoborate mesophases containing octahedral aluminium and tetrahedral boson are prepared and characterized for the first time.

Enhancement of uniaxial magnetic anisotropy in Fe thin films grown on GaAs(001) with an MgO underlayer

The understanding and control of anisotropy in Fe films grown on cubic systems such as GaAs and MgO has been of interest from the point of view of applications in devices. We report magnetic anisotropy studies on Fe/GaAs(001) and Fe/MgO/GaAs(001) prepared by pulsed laser deposition. In Fe/GaAs(001), magneto optical Kerr effect (MOKE) measurements revealed a dominant uniaxial anisotropy for Fe thickness less than 20 monolayers (ML) and this was confirmed by ferromagnetic resonance (FMR) studies. Multiple steps in the hysteresis loops were observed for Fe films of thickness 20 and 25 ML. Whereas, in Fe/MgO/GaAs(001), even at 25 ML of Fe, the uniaxial anisotropy remained dominant. The anisotropy constants obtained from FMR spectra have shown that the relative strength of uniaxial anisotropy is higher as compared to the cubic anisotropy constant in the case of Fe/MgO/GaAs(001). (C) 2011 American Institute of Physics. doi:10.1063/1.3556941]

Sensors based on the dissipation characteristics of n-BaTiO3 ceramics and its solid solutions

The humidity, heat flux and mass flow sensing capability of n-BaTiO3 and its solid solutions were evaluated based on their dissipation characteristics. The cubic/tetragonal phase content of the ceramics seem to play an important role in their sensitivity towards the measurand. The humidity-sensitive characteristics of these perovskites were studied with respect to different moisture sensitive coating materials. The sensor was also used to determine the heat of hydration during the curing process of cements and the mass flow rate of the gases. For all these applications, suitable operating points have been fixed from the highly non-linear I-V characteristics with the retention of good stability and high sensitivity. (C) 1997 Elsevier Science S.A.

Synthesis of Ti, Ga, and V nitrides: Microwave-assisted carbothermal reduction and nitridation

A simple, novel, and fast method of preparation of metal nitride powders (GaN, TiN, and VN) using microwave-assisted carbothermal reduction and nitridation has been demonstrated. The procedure uses the respective oxides and amorphous carbon powder as the starting materials. Ammonia gas is found to be more effective in nitridation than high-purity N-2 gas. Complete nitridation is achieved by the use of a slight excess of amorphous carbon. Metals themselves are not found to be effectively nitrided. The products were characterized using XRD, TEM, and SAED and found to possess good crystallinity and phase purity. The method can be of general applicability for the preparation of metal nitrides.