Giant magnetoresistance in lanthanum strontium yttrium manganese oxides (La(1-x)Sr(x-z)YuMnO(3))

Electron transport and magnetic properties of several compositions of the La1-xSx-zYzMnO3 system have been investigated in order to explore the effect of yttrium substitution on the magnetoresistance and related properties of these manganates. Yttrium substitution lowers the T-c and the insulator-metal transition temperature, while increasing the peak resistivity. A comparison of the properties of La1-xSrx-zYzMnO3 with the corresponding La1-xCax-zYzMnO3 compositions shows that the observed properties can be related to the average size of the A-site cations.

Giant magnetoresistance in transition metal oxides

Some materials exhibit large changes in electrical resistance in the presence of a magnetic field, and this change can be used in applications from sensor technology to magnetic data storage. In their Perspective, Rao and Cheetham discuss magnetoresistance in perovskite manganates, where the effect is unusually strong. Much has been learned about these materials, and this understanding is driving the search for new materials with even more impressive properties.

Normal state tunneling conductance of perovskite oxides: Implications for high-Tc superconductors

We have investigated tunneling conductances in disordered, normally conducting perovskite oxides close to the metal�insulator transition. We show that the normal state tunneling conductance of perovskite oxides can be cast in a general form G(V) = G0[1 + curly logical orV/V*curly logical orn] with 1?n?0.5 and where V* is an intrinsic energy scale. The exponent n graduall y increases from 0.5 to 1 as the metal-insulator (M-I) transition is approached. In the high-Tc Bi(2212) cuprates, the normally observed, linear G(V)(n=1) can be made sub-linear (n<1) by substitution of Ca with Y. From the similarity of the linear conductances, we suggest proximity to the M-I transition as a likely cause for this G(V)logical or, bar below V dependence. In systems showing linear conductances (nreverse similar, equals1), we find that ?G/?Vreverse similar, equalsG?0 with ?reverse similar, equals 1 and the intrinsic energy scale V*reverse similar, equals25�75 meV in the different oxides investigated.

Photocatalysis on Fine Powders of Perovskite Oxides

Fine powders of semiconductor oxides have been widely used as photocatalysts for many reactions. Among the various photocatalytic reactions, water splitting has been given much importance, since it is a promising chemical route for solar energy conversion. Perovskite oxides, in particular SrTiO, have been commonly used as photocatalysts because some of them can decompose H,O into H, and 0, without an external bias potential (1). In turn, this is because the conduction band (CB) edges of some of the perovskite oxides are more negative than the H+/H, energy level. Since the catalytic activity is related to the surface properties of the solids, fine powders rather than single crystals are used. Photocatalysis on fine powers can be conveniently discussed in three parts, viz. preparation, characterization and their catalytic activity. Presently, photo-decomposition of water using SrTiO, fine powders is discussed in greater detail, although other photocatalytic reactions on various perovskite oxides are also briefly dealt with.

Insertion of nickel nanoparticles in the interlayers of alpha-zirconium phosphate by the decomposition of the intercalated nickel acetate

By employing EXAFS and magnetic measurements, it is shown that nanoparticles of nickel along with those of NiO are incorporated between the layers of a-zirconium phosphate (ZrP) by the thermal decomposition of nickel acetate intercalated in ZrP. The nickel nanoparticles are superparamagnetic. Hydrogen reduction produces small ferromagnetic nickel particles, most of which appear to be outside the interlayer space of ZrP.

Microwave-assisted selective deoxygenation of layer- and chain-containing oxides

Very rapid (within 5 min), selective, single-step deoxygenation of layer- and chain-containing oxides, MoO3, CrO3, V2O5, alpha-VOPO4 . 2H(2)O and Ag6Mo10O33 has been accomplished using graphitic carbon in a microwave-assisted reaction. The products were found to be MoO2, Cr2O3, VO2, VPO4 and a mixture of (Ag + MoO2), respectively. Products were characterised by X-ray diffraction (XRD), differential scanning calorimetry (DSC), IR and electron paramagnetic resonance (EPR) spectroscopies. Although conventional methods of preparing these materials are tedious, the present method is simple, fast and yields very homogeneous products of good crystallinity. Our results reveal that while layer- and chain-containing oxides undergo rapid microwave-assisted carbothermal reduction, the non-layered materials do not. The high structural selectivity of these reactions is suggestive of the topochemical nature of the fast reduction process.

Non-linear optical response of rutile-related oxides, LiM(V)M(VI)O(6), and their derivatives obtained by ion-exchange and intercalation

Polycrystalline samples of oxides of the general formula LiM(V)M(VI)O(6) (M(V) = Nb, Ta; M(VI) = Mo, W), crystallizing in a non-centrosymmetric (space group P (4) over bar 2(1)m) trirutile structure, exhibit second harmonic generation (SHG) of 1064 nm radiation with efficiencies 15-45 times that of alpha-quartz; interestingly, the SHG response is retained by the protonated derivatives HM(V)M(VI)O(6) . xH(2)O, and their n-alkylamine intercalates as well.

Phase equilibria in the system CaO-CoO-SiO2 and Gibbs energies of formation of the quaternary oxides CaCoSi2O6, Ca2CoSi2O7, and CaCoSiO4

Phase relations in the pseudoternary system CaO-CoO-SiO2 have been established at 1323 K. Three quaternary oxides were found to be stable: CaCoSi2O6 with clinopyroxene (Cpx), Ca2CoSi2O7 with melilite (Mel), and CaCoSiO4 with olivine (Ol) structures. The Gibbs energies of formation of the quaternary oxides from their component binary oxides were measured using solid-state galvanic cells incorporating yttria-stabilized zirconia as the solid electrolyte in the temperature range of 1000-1324 K. The results can be summarized as follows: CoO (rs) + CaO (rs) + 2SiO(2) (Qtz) --> CaCoSi2O6 (Cpx), Delta G(f)(0) = -117920 + 11.26T (+/-150) J/mol CoO (rs) + 2CaO (rs) + 2SiO(2) (Qtz) --> Ca2CoSi2O7 (Mel), Delta G(f)(0) = -192690 + 2.38T (+/-130) J/mol CoO (rs) + CaO (rs) + SiO2 (Qtz) --> CaCoSiO2 (Ol), Delta G(f)(0) = -100325 + 2.55T (+/-100) J/mol where rs = rock salt (NaCl) structure and Qtz = quartz. The uncertainty limits correspond to twice the standard error estimate. The experimentally observed miscibility gaps along the joins CaO-CoO and CaCoSiO4-Co2SiO4 were used to calculate the excess free energies of mixing for the solid solutions CaxCo1-xO and (CayCo1-y)CoSiO4:Delta G(E) = X(1 - X)[31975X + 26736 (1 - X)] J/mol and Delta G(E) = 23100 (+/-250) Y(1 - Y) J/mol. A T-X phase diagram for the binary CaO-CoO was computed from the thermodynamic information; the diagram agrees with information available in the literature. The computed miscibility gap along the CaCoSiO4-Co2SiO4 join is associated with a critical temperature of 1389 (+/-15) K. Stability fields for the various solid solutions and the quaternary compounds are depicted on chemical-potential diagrams for SiO2, CaO, and CoO at 1323 K.

Role of zirconium addition on stability of UAl3 phase in Al-U system

The microstructural changes of Al-22 wt%U and Al-46 wt%U alloys containing 3 wt% Zr were investigated after heat treatment at 620 degrees C for 1 to 45 days, Though it is reported that addition of similar to 3 wt% Zr stabilizes the (U,Zr)Al-3 phase at room temperature, the present investigation shows that the (U,Zr)Al-3 phase is not stable but slowly transforms to the U0.9Al4 phase, The high temperature creep curves generated for these ternary alloys showed a wavy pattern which also suggests that the (U,Zr)Al-3 phase is not stable.

Synthesis, structure and IR absorption studies of LnBaCuCoO(5) (Ln equal earth) oxides

A series of oxides LnBaCuCoO(5) (Ln = Pr, Nd, Sm, Dy, Gd, Ho and Er) have been synthesized by ceramic method. The oxides crystallize in a tetragonal structure, isostructural to YBaCuCoO5. All the oxides in the series are semiconducting. IR spectra of these oxides show distinct absorption bands at 630 cm(-1), 550 cm(-1) and 330 cm(-1) which are assigned to E, A(2) and A(1) modes respectively. Doping of holes in these oxides, by calcium substitution in Er1-xCaxBaCuCoO5-x (up to x similar or equal to 0.3) was done but, these oxides did not show metallic behaviour.

ChemInform Abstract: Slicing the Perovskite Structure into Layers: Synthesis of Novel Three- Dimensional and Layered Perovskite Oxides, ALaSrNb2M(II)O9 (A: Na, Cs)

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Study of the reactions of pyridinium poly(hydrogen fluoride) with some ternary oxides

The products of the reaction of pyridinium poly(hydrogen fluoride), PPHF, with KIO3, Na2SnO3, NaBiO3, K2CrO4, Na2MoO4 and Na2WO4 were KIO2F2; Na2SnF6; NaHF2, BiF3; K3CrF6, KHF2, (PyH)(3)CrF6; NaHF2, (PyH)(2)MoO2F4.2NaHF(2); and (PyH)(2)WO2F4.2NaHF(2), respectively, while KClO3, KBrO3 and KlO(4) react with complete decomposition to form KHF2 as the fluorinated product. This differential reactivity and mode of reaction has been discussed in terms of the oxidation state of the central atom, the nature and strength of the bonds and the complex behaviour of the formed intermediate or fluorinated products that undergo complexation or solvation with pyridine and/or hydrogen fluoride.

Metal-insulator transition in perovskite oxides: Tunneling experiments

In this paper we have investigated the composition-driven metal-insulator (MI) transitions in two ABO3 classes of perovskite oxides (LaNixCo1-xO3 and NaxTayW1-yO3) in the composition range close to the critical region by using the tunneling technique. Two types of junctions (point-contact and planar) have been used for the investigation covering the temperature range 0.4 K

Plate-shaped transformation products in zirconium-base alloys

Plate-shaped products resulting from martensitic, diffusional, and mixed mode transformations in zirconium-base alloys are compared. in the present study. These alloys are particularly suitable for the comparison in view of the fact that the lattice correspondence between the parent beta (bcc) and the product alpha (hcp) or gamma-hydride (fct) phases are remarkably similar for different types of transformations. Crystallographic features such as orientation relations, habit planes, and interface structures associated with these transformations have been compared:, with a view toward examining whether the transformation mechanisms have characteristic imprints on these experimental observables. Martensites exhibiting dislocated lath, internally twinned plate, and self-accommodating three-plate cluster morphologies have been encountered in Zr-2.5Nb alloy. Habit planes corresponding to all these morphologies have been found to be consistent with the predictions based on the invariant plane strain (IFS) criterion. Different morphologies have been found to reflect the manner in which the neighboring martensite variants are assembled. Lattice-invariant shears (LISs) for all these cases have been identified to be either {10 (1) over bar 1}(alpha) ((1) over bar 123)(alpha) slip or twinning on (10 (1) over bar 1)(alpha) planes. Widmanstatten alpha precipitates, forming in a step-quenching treatment, have been shown to have a lath morphology, the alpha/beta interface being decorated with a periodic array of (c + a) dislocations at a spacing of 8 to 10 nm. The line vectors of these dislocations are nearly parallel to the invariant lines. The alpha precipitates, forming in the retained beta phase on aging, exhibit an internally twinned structure with a zigzag habit plane. Average habit planes for the morphologies have been found to lie near the {103}(beta) - {113}(beta) poles, which are close to the specific variant of the {112}(beta) plane, which transforms into a prismatic plane of the type {1 (1) over bar 00}(alpha). The crystallography of the formation of the gamma-hydride phase (fct) from both the alpha and beta phases is seen to match the IFS predictions. While the beta-gamma transformation can be treated approximately as a simple shear on the basal plane involving a change in the stacking sequence, the alpha-gamma transformation call be conceptually broken into a alpha --> beta transformation following the Burgers correspondence and the simple beta-gamma shear process. The active eutectoid decomposition in the Zr-Cu system, beta --> alpha + beta', has been described in terms of cooperative growth of the alpha phase from the beta phase through the Burgers correspondence and of the partially ordered beta' (structurally similar to the equilibrium Zr2Cu phase) through an ordering process. Similarities and differences in crystallographic features of these transformations have been discussed. and the importance of the invariant line vector in deciding the geometry of the corresponding habit planes has been pointed out.