Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Structure and Reactivity of Perovskite-Type Oxides

Low crystalline iron hydroxides such as goethite (alpha-FeOOH) and akaganeite (beta-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients (K(d)) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl-, NO3-, SO4(2-) << CO3(2-), HPO4(2-) for goethite, and Cl-, CO3(2-) < NO3- < SO4(2) << HPO4(2-) for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.

Phosphate adsorption on synthetic goethite and akaganeite.

Future improvements in the development of highly volume-efficient multilayer ceramic capacitors (MLCC) require manufacturing processes that allow for a reduced dielectric thickness well below 1 μm. Obviously, such thin dielectric layers can only be produced if nanosized dielectric powders are applied and deposited by techniques more advanced than tape casting and screen printing. These processes require high-purity, homogeneous, weakly agglomerated ultrafine powders. Over the past few years, significant progress has been made in the synthesis of nanocrystalline BaTiO3-based powders for use in MLCC. The present review article aims to summarize the present state of the art with respect to some of the technically and industrially most relevant preparation routes, including methods based on solid-state reactions and, more importantly, solution-based approaches.

Progress in the Synthesis of Nanocrystalline BaTiO3 Powders for MLCC

Synthesis of Fe3O4 nanoparticles at 100 °C and its magnetic characterization

3.35Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry

Thermal decomposition of titanyl oxalates—III

A wet chemical route is developed for the preparation of Sr2CeO4 denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO2.xH(2)O, 40<x<75) by the addition of ammonium carbonate. During calcination, CeO2.xH(2)O dehydroxylation is followed by the reaction with SrCO3 to form Sr2CeO4 with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr2CeO4 originates from the Ce4+-O2- charge-transfer (CT) transition resulting from the interaction of Ce4+ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce4+-O2- CT emission is studied by doping with Eu3+, Sm3+ or Yb3+ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce4+-O2- CT state to trivalent lanthanide ions (Ln(3+)) if the latter has CT excited states, leading to sensitizer-activator relation, through non-resonance process involving exchange interaction. Yb3+-substituted Sr2CeO4 does not show any line emission because the energy of Yb3+-O2- CT level is higher than that of the Ce4+-O2- CT level. Sr2-xEuxCeO4+x/2 shows white emission at xless than or equal to0.02 because of the dominant intensities of D-5(2)-F-7(0-3) transitions in blue-green region whereas the intensities of D-5(0)-F-7(0-3) transitions in orange-red regions dominate at concentrations xgreater than or equal to0.03 and give red emission. The appearance of all the emissions from D-5(2), D-5(1) and D-5(0) excited states to the F-7(0-3) ground multiplets of Eu3+ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu3+. White emission of Sr2-x SmxCeO4+x/2 at xless than or equal to0.02 is due the co-existence of Ce4+-O2- CT emission and (4)G(4)(5/2)-H-6(J) Sm3+ transitions whereas only the Sm3+ red emission prevails for xgreater than or equal to0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce4+. Quenching of Ce4+-O2- CT emission by other Ln(3+) is due to the ground state crossover arising out of the NNN interactions.

https://pubs.rsc.org/en/content/articlepdf/2003/jm/b207756f

Photoluminescence of Sr2 − xLnxCeO4 + x/2(Ln = Eu, Sm or Yb) prepared by a wet chemical method

A method for the preparation of acicular hydrogoethite (α-FeOOH·xH2O, 0.1 < x < 0.22) particles of 0.3–1 μm length has been optimized by air oxidation of Fe(II) hydroxide gel precipitated from aqueous (NH4)2Fe(SO4)2 solutions containing 0.005–0.02 atom% of cationic Pt, Pd or Rh additives as morphology controlling agents. Hydrogoethite particles are evolved from the amorphous ferrous hydroxide gel by heterogeneous nucleation and growth. Preferential adsorption of additives on certain crystallographic planes thereby retarding the growth in the perpendicular direction, allows the particles to acquire acicular shapes with high aspect ratios of 8–15. Synthetic hydrogoethite showed a mass loss of about 14% at ∼280 °C, revealing the presence of strongly coordinated
water of hydration in the interior of the goethite crystallites. As evident from IR spectra, excess H2O molecules (0.1–0.22 per formula unit) are located in the strands of channels formed in between the double ribbons of FeO6 octahedra running parallel to the c-axis. Hydrogoethite particles constituted of multicrystallites are formed with Pt as additive, whereas single crystallite particles are obtained with Pd (or Rh). For both dehydroxylation as well as H2 reduction, a lower reaction temperature (∼220 °C) was observed for the former (Pt treated) compared to the latter (Pd or Rh) (∼260 °C). Acicular magnetite (Fe3O4) was prepared either by reducing hydrogoethite (magnetite route) or dehydroxylating hydrogoethite to hematite and then reducing it to magnetite (hematite–magnetite route). According to TEM studies, preferential
dehydroxylation of hydrogoethite along leads to microporous hematite. Maghemite (γ-Fe2O3 − δ, 0 < δ < 0.25) was obtained by reoxidation of magnetite. The micropores are retained during the topotactic transformation to magnetite and finally to maghemite, whereas cylindrical mesopores are formed due to rearrangement of the oxygen sublattice from hexagonal to cubic close packing during the conversion of hydrogoethite to magnetite and then to maghemite. Accordingly, three different types of maghemite particles are realized: strongly oriented multicrystalline particles, single crystalline acicular particles with micropores or crystallites having mesopores. Higher values of saturation magnetization (σs = 74 emu g−1) and coercivity (Hc = 320 Oe)
are obtained for single crystalline mesoporous particles. In the other cases, the smaller size of particles and larger distribution of micropores decreases σs considerably (<60 emu g−1) due to relaxation effects of spins on the surface atoms as revealed by Mossbauer spectroscopy.

Synthesis of acicular hydrogoethite (α-FeOOH·xH2O; 0.1 < x < 0.22) particles using morphology controlling cationic additives and magnetic properties of maghemite derived from hydrogoethite

Thermal decomposition of titanyl oxalates—I: Barium titanyl oxalate

Coarse Bn+On/2·xH2O (10 < x < 120) gels, free of anionic contaminants react with A(OH)2 solutions under refluxing conditions at 70–100 °C giving rise to nanoparticles of multicomponent oxides (A = Ba, Sr, Ca, Mg or Pb; B = Zr, Ti, Sn, Fe, Al or Cr). These include ABO3 perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel–crystallite (G–C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A2+ ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B—(OH)—B or B—O—B, leading to the breakdown of the gel into crystallites. G–C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity.

T. R. Narayanan Kutty

Papers

Thermal decomposition of titanyl oxalates—III

Photoluminescence of Sr2 − xLnxCeO4 + x/2(Ln = Eu, Sm or Yb) prepared by a wet chemical method

Synthesis of acicular hydrogoethite (α-FeOOH·xH2O; 0.1 < x < 0.22) particles using morphology controlling cationic additives and magnetic properties of maghemite derived from hydrogoethite

Thermal decomposition of titanyl oxalates—I: Barium titanyl oxalate

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion