Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Synthesis,Structure, and Properties of the High‐Pressure Modification of CePdSn – a 5 K Antiferromagnet

The high‐pressure (HP) modification of CePdSn was synthesized under multianvil high‐pressure (10.5 GPa) high‐temperature (1100 °C) conditions from the normal‐pressure (NP) modification. The structures of both modifications were studied by X‐ray powder and single crystal diffraction: TiNiSi type, Pnma, a = 754.1(2), b = 470.6(1), c = 798.4(3) pm, wR2 = 0.0333, 945 F² values, 20 variables for NP‐CePdSn and ZrNiAl type, , a = 760.03(5), c = 416.06(3) pm, wR2 = 0.0443, 248 F² values, 13 variables for HP‐CePdSn. The structural chemistry of both modifications is goverened by platinum centered trigonal prisms. The platinum and tin atoms in NP‐CePdSn and HP‐CePdSn build up a three‐dimensional [PdSn] network in which the cerium atoms fill channels. Susceptibility measurements on HP‐CePdSn reveal an experimental magnetic moment of 2.55(1) μ_B/Ce atom in the paramagnetic region. At 5 K a paramagnetic‐to‐antiferromagnetic transition is evident from magnetization and specific heat measurements.     

Origin of charge density wave formation in insulators from a high resolution photoemission study of BaIrO3

We investigate the origin of charge density wave (CDW) formation in insulators by studying BaIrO3 using high-resolution (1.4 meV) photoemission spectroscopy. The spectra reveal the existence of localized density of states at the Fermi level, E(F), in the vicinity of room temperature. These localized states are found to vanish as the temperature is lowered, thereby, opening a soft gap at E(F), as a consequence of CDW transition. In addition, the energy dependence of the spectral density of states reveals the importance of magnetic interactions, rather than well-known Coulomb repulsion effect, in determining the electronic structure thereby implying a close relationship between ferromagnetism and CDW observed in this compound. Also, Ba core level spectra surprisingly exhibit an unusual behavior prior to CDW transition.     

Neodymium(III)-substituted bismuth titanate thin film generation using metal alkoxo, acyloxo, and beta-diketonato precursors employing a sol-gel route and using 4f-4f transition spectra as probes to explore kinetic performance

Heterotrimetallic lanthanide-substituted bismuth titanate (BLT, where lanthanide is neodymium) with stoichiometry Nd(0.75)Bi(3.25)Ti(3)O(12) has been obtained in both highly homogenized crystalline and amorphous thin film forms using three different multicomponent precursors, A (formed from Nd(OC(3)H(7)(i))(3), Bi(OOC.CH(3))(3), and Ti(OC(3)H(7)(i))(4) taken in the desired stoichiometry), B (formed from Nd(OC(3)H(7)(i))(2)(acac), Bi(OOC.CH(3))(3), Ti(OC(3)H(7)(i))(3)(acac)), and C (formed from Nd(OC(3)H(7))(acac)(2), Bi(OOC.CH(3))(3), Ti(OC(3)H(7))(3)(acac), in the desired stoichiometry), and employing controlled acidic hydrolysis during the sol-gel method. Paramagnetic Nd(III), an f(3) metal ion, gives characteristic 4f-4f transition spectra in the visible and near infrared region. The sensitivity of 4f-4f transitions to minor coordination changes around paramagnetic Nd(3+) has been used to monitor hydrolysis during the progress of the sol-gel process of the multicomponent BLT precursors. The variation of intensities (oscillator strengths) of 4f-4f bands during hydrolysis, as well as the variation of Judd-Ofelt intensity parameters, has helped in following the preliminary kinetics of hydrolysis. Highly complex polycondensation reactions occurred during sol-gel hydrolysis of three BLT precursors. Rates of hydrolysis with respect to five 4f-4f transitions of Nd(III) were determined. The different types of multicomponent BLT precursors have shown different rates of hydrolysis, following the reactivity trend A > B > C.     

Praseodymium(III)-substituted bismuth titanate thin-film generation using metallo-organic precursors with an M-O-C skeleton and sol-gel technique and employing 4f-4f transition spectra as a probe to explore kinetic performance

A hetero-trimetallic lanthanide-substituted bismuth titanate (BLT, where lanthanide is praseodymium) with stoichiometry Pr(0.75)Bi(3.25) Ti(3)O(12) has been obtained as both highly homogenized crystalline and amorphous thin films using three different BLT precursors: (i). precursor A-(Pr(OC(3)H(7)(i))(3),Bi(OOCCH(3))(3),Ti(OC(3)H(7)(i))(4)); (ii). precursor B-(Pr(OC(3)H(7)(i))(2)(acac),Bi(OOCCH(3))(3),Ti(OC(3)H(7)(i))(3)(acac)); and (iii). precursor C-(Pr(OC(3)H(7)(i))(2)(acac),Bi(OOCCH(3))(3),Ti(OC(3)H(7)(i))(2)(acac))(2). These three BLT precursors (A, B, C) are prepared by reacting constituent monometallic precursors in the desired stoichiometry and by employing controlled acidic hydrolysis via the sol-gel method. Paramagnetic Pr(III), being a f(2) ion, gives characteristic 4f-4f transition bands (3H(4)-->3P(2), 3H(4)-->3P(1), 3H(4)-->3P(0), and 3H(4)-->1D(2)) in the visible region, the intensities of which have been found to be highly sensitive to even minor changes in the immediate coordination around Pr(III), occurring as a result of the progress of polycondensation reactions of three multicomponent BLT sols. We have used the changes with time in the intensities (represented by oscillator strengths of these four 4f-4f bands) and the magnitude and variation of the spectral parameters evaluated from the observed spectra with a view toward monitoring the sol-gel reactions of BLT precursors A, B, and C. 4f-4f transition spectra of the aliquots, withdrawn from the hydrolyzing A, B, and C sols at different time intervals, represent the changes occurring in the Pr(III) environment with the progress of sol-gel hydrolysis of BLT, and are used to investigate the kinetic performance in hydrolysis of the three precursors. Kinetics of hydrolysis of precursors A, B, and C indicate that all four f-f transition bands of Pr are almost equally sensitive to precursor hydrolysis in the order A>B>C.     

Structural, magnetic, and spectroscopic studies of YAgSn, TmAgSn, and LuAgSn

The rare earth-silver-stannides YAgSn, TmAgSn, and LuAgSn were synthesized from the elements by arc-melting and subsequent annealing. The three stannides were investigated by X-ray powder and single-crystal diffraction: NdPtSb type, P6₃mc, Z=2, a=468.3(1), pm, wR₂=0.0343, 353 F² values, 12 variables for YAgSn, and ZrNiAl type, P6¯2 m, a=726.4(2), , wR₂=0.0399, 659 F² values, 15 variables for TmAgSn, and a=723.8(2), , wR₂=0.0674, 364 F² values, 15 variables for LuAgSn. Besides conventional laboratory X-ray data with monochromatized Mo radiation, the structures were also refined on the basis of synchrotron data with , in order to clarify the silver-tin ordering more precisely. YAgSn has puckered, two-dimensional [AgSn] networks with Ag-Sn distances of 278 pm, while the [AgSn] networks of TmAgSn and LuAgSn are three-dimensional with Ag-Sn distances of 279 and 284 pm for LuAgSn. Susceptibility measurements indicate Pauli paramagnetism for YAgSn and LuAgSn. TmAgSn is a Curie-Weiss paramagnet with an experimental magnetic moment of 7.2 μ_B/Tm. No magnetic ordering is evident down to 2 K. The local environments of the tin sites in these compounds were characterized by ¹¹⁹Sn Mössbauer spectroscopy and solid-state NMR (in YAgSn and LuAgSn), confirming the tin site multiplicities proposed from the structure solutions and the absence of Sn/Ag site disordering. Mössbauer quadrupolar splittings were found in good agreement with calculated electric field gradients predicted quantum chemically by the WIEN2k code. Furthermore, an excellent correlation was found between experimental ¹¹⁹Sn nuclear magnetic shielding anisotropies (determined via MAS-NMR) and calculated electric field gradients. Electronic structure calculations predict metallic properties with strong Ag-Sn bonds and also significant Ag-Ag bonding in LuAgSn.     

Dehydration of aqueous acetonitrile solution by pervaporation using PVA–iron oxide nanocomposite membrane

Pervaporative performances were investigated for dehydration of water–acetonitrile using nanocomposite metal oxide and Pervap^® 2202 membranes. Poly (vinyl alcohol) based nanocomposite metal oxide membranes were prepared through co-precipitation of different amounts of Fe (II) and Fe (III). The freestanding nanocomposite metal oxide membranes were characterized by Transmission electron microscopy and X-ray diffraction. Sorption studies evaluated the extent of interaction and degree of swelling of the membranes. Fe containing PVA polymer matrix showed improved flux and selectivity. In order to observe simultaneous effect of flux and selectivity, pervaporation separation index showed 10 wt.% iron oxide containing membrane is the most amongst all tested. The diffusion coefficients were calculated using pervaporation results and sorption kinetics data. An attempt was made to predict sorption selectivity thermodynamically. PV separation factor was observed to be governed by sorption and/or diffusion phenomena and sorption selectivity was found to be higher than PV separation factor. Prediction of concentration profile in the membrane was also attempted and the results showed that water concentration in the membrane drops down with increase in membrane thickness.     

Neutron diffraction studies on La2−x Dy x Ca2xBa2Cu4+2x Oz superconductors

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La_2-xDy_xCa_2xBa₂Cu_4+2xO_z stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.