Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties

MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to tercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which depend on the crystal structure and the method of preparation. The specific capacitance (SC) measured for MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: α(m) > α δ > γ > λ > β. A SC value of 297 F g-1 is obtained for α-MnO2(m), whereas it is 9 F g-1 for β-MnO2. A wide (4.6 Å) tunnel size and large surface area of α-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (7 Å) also facilitates insertion of cations in δ-MnO2 resulting in a SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides a very small SC.

Synthesis, structure and magnetic behavior of a new three-dimensional Manganese phosphite-oxalate: [C2N2H10][Mn-2(II)(OH2)(2)(HPO3)(2)(C2O4)]

A novel manganese phosphite-oxalate, [C2N2H10][Mn-2(II)(OH2)(2)(HPO3)(2)(C2O4)] has been hydothermally synthesized and its structure determined by single-crystal X-ray diffraction. The structure consists of neutral manganese phosphite layers, [Mn(HPO3)](infinity), formed by MnO6 octahedra and HPO3 units, cross-linked by the oxalate moieties. The organic cations occupy the middle of the 8-membered one dimensional channels. Magnetic studies indicate weak antiferromagnetic interactions between the Mn2+ ions. (C) 2009 Elsevier Inc. All rights reserved.

Energy harvesting using ionic electro-active polymer thin films with Ag-based electrodes

In this paper we employ the phenomenon of bending deformation induced transport of cations via the polymer chains in the thickness direction of an electro-active polymer (EAP)-metal composite thin film for mechanical energy harvesting. While EAPs have been applied in the past in actuators and artificial muscles, promising applications of such materials in hydrodynamic and vibratory energy harvesting are reported in this paper. For this, functionalization of EAPs with metal electrodes is the key factor in improving the energy harvesting efficiency. Unlike Pt-based electrodes, Ag-based electrodes have been deposited on an EAP membrane made of Nafion. The developed ionic metal polymer composite (IPMC) membrane is subjected to a dynamic bending load, hydrodynamically, and evaluated for the voltage generated against an external electrical load. An increase of a few orders of magnitude has been observed in the harvested energy density and power density in air, deionized water and in electrolyte solutions with varying concentrations of sodium chloride (NaCl) as compared to Pt-based IPMC performances reported in the published literature. This will have potential applications in hydrodynamic and residual environmental energy harvesting to power sensors and actuators based on micro-andn nano-electro-mechanical systems (MEMS and NEMS) for biomedical,maerospace and oceanic applications.

Metal-spinel-corundum three phase equilibria in the system Ni-Cr-Al-O at 1373 K

The tie-lines representing the inter-crystalline ion exchange equilibria between the NiCr2O4-NiAl2O4 spinet solid solution and Cr2O3-Al2O3 corundum solid solution are determined by electron microprobe andEDAX pointcountanalysis of the oxide phases equilibrated with metallic Ni at 1373 K. The component activities in the spinet solid solution are derived from the tie-lines and thermodynamic data for Cr2O3-Al2O3 solid solution available in the literature. The Gibbs energy of mixing of the spinet solid solution calculated from the experimental data is discussed in relation to the values derived from the cation distribution models which assume random mixing of cations on both tetrahedral and octahedral sites. Positive deviation from the models is observed indicating significant positive enthalpy contribution arising form the size mismatch between Al+3 and Ni+2 ions on the tetrahedral site and Al+3, Ni+2 and Cr+3 on the octahedral site. Variation of the oxygen potential for threephase equilibrium involving metallic nickel, spinet solid solution and corundum solid solution is computed as a function of composition of the solid solutions at 1373 K. The oxygen potential exhibits a minimum at aluminum cationic fraction eta(Al)/(eta(Al) + eta(Cr)) = 0.524 in the oxide solid solutions.

Chain structures in alkali metal borophosphates: synthesis and characterization of K3[BP3O9(OH)3] and Rb3[B2P3O11(OH)2]

Two new alkali metal borophosphates, K-3[BP(3)o(9)(OH)(3)] and Rb-3[B2P3O11(OH)(2)], were synthesized by applying solvothermal techniques using ethanol as solvent. The crystal structures were solved by means of single-crystal X-ray diffraction (K-3[BP3O9(OH)(3)], monoclinic, C2/c (No. 15), a = 2454.6(8) pm, b = 736.3(2) pm, c = 1406.2(4) pm, beta = 118.35(2)degrees, Z = 8; Rb-3[B2P3O11(OH)(2)], monoclinic, P2(1)/c (No. 14), a = 781.6(2) pm, b:= 667.3(2) pm, c = 2424.8(5) pm, beta = 92.88(1)degrees, Z = 4). Both crystal structures comprise borophosphate chain anions. While for the rubidium compound a loop-branched chain motif is found as common for most of the chain anions in alkali metal borophosphates, the crystal structure of the potassium phase comprises the first open-branched chain with the highest phosphate content found so far in this group of compounds. Both chain anions are Closely related to known anhydrous or hydrated phases, and the structural relations are discussed in terms of how the presence of OH groups and hydrogen bonds as well as number, charge, and size of charge balancing cations influence the 3D structural arrangement. The anionic entities are classified in terms of general principles of structural systematics for borophosphates.

Suppression of spinel formation to induce reversible thermal behavior in the layered double hydroxides (LDHs) of Co with Al, Fe, Ga, and In

The layered double hydroxides (LDHs) of Co with trivalent cations decompose irreversibly to yield oxides with the spinel structure. Spinel formation is aided by the oxidation of Co(II) to Co(III) in the ambient atmosphere. When the decomposition is carried out under N-2, the oxidation of Co(II) is suppressed, and the resulting oxide has the rock salt structure. Thus, the Co-Al-CO32-/Cl- LDHs yield oxides of the type Co1- Al-x(2x/3)rectangle O-x/3, which are highly metastable, given the large defect concentration. This defect oxide rapidly reverts back to the original hydroxide on soaking in a Na2CO3 solution. Interlayer NO3- anions, on the other hand, decompose generating a highly oxidizing atmosphere, whereby the Co-Al-NO3- LDH decomposes to form the spinel phase even in a N-2 atmosphere. The oxide with the defect rock salt structure formed by the thermal decomposition of the Co-Fe-CO32- LDH under N2, on soaking in a Na2CO3 solution, follows a different kinetic pathway and undergoes a solution transformation into the inverse spinel Co(Co, Fe)(2)O-4. Fe3+ has a low octahedral crystal field stabilization energy and therefore prefers the tetrahedral coordination offered by the structure of the inverse spinel rather than the octahedral coordination of the parent LDH. Similar considerations do not hold in the case of Ga- and In-containing LDHs, given the considerable barriers to the diffusion of M3+ (M=Ga, In) from octahedral to tetrahedral sites owing to their large size. Consequently, the In-containing oxide residue reverts back to the parent hydroxide, whereas this reconstruction is partial in the case of the Ga-containing oxide. These studies show that the reversible thermal behavior offers a competing kinetic pathway to spinel formation. Suppression of the latter induces the reversible behavior in an LDH that otherwise decomposes irreversibly to the spinel.

Structure of Ce1-xSnxO2 and its relation to oxygen storage property from first-principles analysis

CeO2-SnO2 solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce1-xSnxO2 structure (x=0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO2, and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO2, SnO2, and Ce1-xSnxO2 indicates that fluorite structure is the most stable for Ce1-xSnxO2 solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO2 in fluorite structure is highly unstable while CeO2 in rutile structure is only weakly unstable. Thus, Sn in Ce1-xSnxO2-fluorite structure is associated with high local structural distortion whereas Ce in Ce1-xSnxO2-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M=Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce1-xSnxO2 show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence similar to 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce1-xSnxO2 solid solution. (C) 2010 American Institute of Physics.

Potassium phosphate as a reagent for the gravimetric estimation of lithium

Previous attempts for the quantitative estimation of lithium as orthophosphate, employing an alkali metal phosphate, have not been successful. A method, is described for the estimation of lithium as trilithium phosphate from 60% ethyl alcohol solution at 65° to 70° C., employing potassium phosphate reagent, at pH 9.5. The method is applicable in the presence of varying amounts of sodium and/or potassium cations and chloride, sulfate, nitrate, and phosphate anions.

Role of entropy in the stability of cobalt titanates

The standard molar Gibbs free energy of formation of Co2TiO4, CoTiO3,and CoTi2O5 as a function of temperature over an extended range (900 to 1675) K was measured using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte, with CoO as reference electrode and appropriate working electrodes. For the formation of the three compounds from their component oxides CoO with rock-salt and TiO2 with rutile structure, the Gibbs free energy changes are given by:Delta(f)G degrees((ox))(Co2TiO4) +/- 104/(J . mol(-1)) = -18865 - 4.108 (T/K)Delta(f)G degrees((ox))(CoTiO3) +/- 56/(J . mol(-1)) = -19627 + 2.542 (T/K) Delta(f)G degrees((ox))(CoTi2O5) +/- 52/(J . mol(-1)) = -6223 - 6.933 (T/K) Accurate values for enthalpy and entropy of formation were derived. The compounds Co2TiO4 with spinel structure and CoTi2O5 with pseudo-brookite structure were found to be entropy stabilized. The relatively high entropy of these compounds arises from the mixing of cations on specific crystallographic sites. The stoichiometry of CoTiO3 was confirmed by inert gas fusion analysis for oxygen. Because of partial oxidation of cobalt in air, the composition corresponding to the compound Co2TiO4 falls inside a two-phase field containing the spinet solid solution Co2TiO4-Co3O4 and CoTiO3. The spinel solid solution becomes progressively enriched in Co3O4 with decreasing temperature. (c) 2010 Elsevier Ltd. All rights reserved.

A Simple and General Strategy for the Design of Fluorescent Cation Sensor Beads

Chenodeoxycholic acid based PET sensors for alkali metal ions have been immobilized on Merrifield resin and on Tentagel. The fluorescence of the sensor beads is enhanced upon binding the cations. The modular nature of the sensor allows designing different sensors based on this concept.

Reversible cation/anion extraction from K2La2Ti3O10: Formation of new layered titanates, KLa2Ti3O9.5 and La2Ti3O9

A new soft-chemical transformation of layered perovskite oxides is described wherein K2O is sequentially extracted from the Ruddlesden-Popper (R-P) phase, K2La2Ti3O10 (I), yielding novel anion-deficient KLa2Ti3O9.5 (II) and La2Ti3O9 (III). The transformation occurs in topochemical reactions of the R-P phase I with PPh4Br and PBu4Br (Ph = phenyl; Bu = n-butyl). The mechanism involves the elimination of KBr accompanied by decomposition of PR4+ (R = phenyl or n-butyl) that extracts oxygen from the titanate. Analysis of the organic products of decomposition reveals formation of Ph3PO, Ph3P, and Ph-Ph for R = phenyl, and Bu3PO, Bu3P along with butane, butene, and octane for R = butyl. The inorganic oxides II and III crystallize in tetragonal structures (II: P4/mmm, a = 3.8335(1) angstrom, c = 14.334(1) angstrom; III: /4/ mmm, a = 3.8565(2) angstrom, c = 24.645(2) angstrom) that are related to the parent R-P phase. II is isotypic with the Dion-Jacobson phase, RbSr2Nb3O10, while III is a unique layered oxide consisting of charge-neutral La2Ti3O9 anion-deficient perovskite sheets stacked one over the other without interlayer cations. Interestingly, both II and III convert back to the parent R-P phase in a reaction with KNO3. While transformations of the R-P phases to other related layered/three-dimensional perovskite oxides in ion-exchange/metathesis/dehydration/reduction reactions are known, the simultaneous and reversible extraction of both cations and anions in the conversions K2La2Ti3O10 reversible arrow KLa2Ti3O9.5 reversible arrow La2Ti3O9 is reported here for the first time.

Effect of coordinated ions on structure and flexibility of parallel G-quadruplexes: A molecular dynamics study

ingle tract guanine residues can associate to form stable parallel quadruplex structures in the presence of certain cations. Nanosecond scale molecular dynamics simulations have been performed on fully solvated fibre model of parallel d(G(7)) quadruplex structures with Na+ or K+ ions coordinated in the cavity formed by the O6 atoms of the guanine bases. The AMBER 4.1 force field and Particle Mesh Ewald technique for electrostatic interactions have been used in all simulations. There quadruplex structures are stable during the simulation, with the middle four base tetrads showing root mean square deviation values between 0.5 to 0.8 Angstrom from the initial structure as well the high resolution crystal structure. Even in the absence of any coordinated ion in the initial structure, the G-quadruplex structure remains intact throughout the simulation. During the 1.1 ns MD simulation, one Nai counter ion from the solvent as well as several water molecules enter the central cavity to occupy the empty coordination sites within the parallel quadruplex and help stabilize the structure. Hydrogen bonding pattern depends on the nature of the coordinated ion, with the G-tetrad undergoing local structural variation to accommodate cations of different sizes. in the absence of any coordinated ion. due to strong mutual repulsion, O6 atoms within G-tetrad are forced farther apart from each other, which leads to a considerably different hydrogen bonding scheme within the G-tetrads and very favourable interaction energy between the guanine bases constituting a G-tetrad. However, a coordinated ion between G-tetrads provides extra stacking energy for the G-tetrads and makes the quadruplex structure more rigid. Na+ ions, within the quadruplex cavity, are more mobile than coordinated K+ ions. A number of hydrogen bonded water molecules are observed within the grooves of all quadruplex structures.

Role of the environment in the interaction of nonintercalators with Z-DNA

Interaction of the DNA binding nonintercalators Netropsin, Distamycin and the mPD derivative with Z-DNA has been studied. It has been found that environmental factors like the solvent and added cations significantly modulate the interaction of these ligands with Z-DNA. However no definite Z to B transition in presence of these ligands was found in any case, in contrast to previously reported results (Ch. Zimmer, C. Marck and W. Guschlbauer, FEBS Lett. 154, 156-160 (1983)).

Electronegativities of constituent atoms and Tc of superconductors

In the superconducting state of high Tc oxides, it is possible to conceive that the mobility of the charge carrier pairs is a consequence of the absence of a net chemical force on them. On this assumption, we have examined a heuristic relation between Tc and a simple function of electronegativities of constituent atoms. We find that Tc varies approximately linearly with the fractional electronegativity of all cations considered together.

Corrected spacegroup for M0.5Ti2P3O12 compounds

The space group of the low thermal expansion phosphates, belonging to NASICON structural family, having divalent cations has been reassigned as RImage based on powder X-ray diffraction studies in the system M0.5Ti2P3O12. This implies further ordered distribution of M2+ cations and vacancies along the hexagonal ‘c’ direction of NASICON structure.