Aurea Varela

Unknown aspects of self-assembly of PbS microscale superstructures.

A lot of interesting and sophisticated examples of nanoparticle (NP) self-assembly (SA) are known. From both fundamental and technological standpoints, this field requires advancements in three principle directions: (a) understanding the mechanism and driving forces of three-dimensional (3D) SA with both nano- and microlevels of organization; (b) understanding disassembly/deconstruction processes; and (c) finding synthetic methods of assembly into continuous superstructures without insulating barriers. From this perspective, we investigated the formation of well-known star-like PbS superstructures and found a number of previously unknown or overlooked aspects that can advance the knowledge of NP self-assembly in these three directions. The primary one is that the formation of large seemingly monocrystalline PbS superstructures with multiple levels of octahedral symmetry can be explained only by SA of small octahedral NPs. We found five distinct periods in the formation PbS hyperbranched stars: (1) nucleation of early PbS NPs with an average diameter of 31 nm; (2) assembly into 100-500 nm octahedral mesocrystals; (3) assembly into 1000-2500 nm hyperbranched stars; (4) assembly and ionic recrystallization into six-arm rods accompanied by disappearance of fine nanoscale structure; (5) deconstruction into rods and cuboctahedral NPs. The switches in assembly patterns between the periods occur due to variable dominance of pattern-determining forces that include van der Waals and electrostatic (charge-charge, dipole-dipole, and polarization) interactions. The superstructure deconstruction is triggered by chemical changes in the deep eutectic solvent (DES) used as the media. PbS superstructures can be excellent models for fundamental studies of nanoscale organization and SA manufacturing of (opto)electronics and energy-harvesting devices which require organization of PbS components at multiple scales.

Synthesis, structure and gas sensitivity properties of pure and doped SnO2

Polycrystalline SnO2 showing small particle size has been synthesized by means of pyrolysis in a tubular furnace of an aerosol produced by ultrahigh-frequency spraying of a solution. Pd nanoparticles have also been dispersed on the surface of cassiterite grains. In both cases, three solution precursors have been used: chloride, oxalate and sulfate. An SEM study indicates that two kinds of textures are obtained depending on the precursor solution: hollow spherical particles or quasi-spherical grain agglomerates. Cold pressing and annealing modify such morphologies. Conductance G = f(T) curves were studied from 50 to 500 °C under pure air (G = G0), 80 ppm ethanol or 300 ppm CO. G = f(T) curves under polluted air show different behaviors. Most of them show a pronounced maximum and corresponding sensitivity S = (G - G0)/G0 varies between 10 and 390.

Ordered rock-salt related nanoclusters in CaMnO2.

Oxygen engineering techniques performed under adequate controlled atmosphere show that the CaMnO(3)-CaMnO(2) topotactic reduction-oxidation process proceeds via oxygen diffusion while the cationic sublattice remains almost unaltered. Extra superlattice reflections in selected area electron diffraction patterns indicate doubling of the CaMnO(2) rock-salt cell along the cubic directions of a distorted rhombohedral cell originated by ordering of Ca(2+) and Mn(2+) ions distributed in nanoclusters into a NaCl-type matrix, as evidenced by dark field electron microscope images. The local nature of the information provided by the transmission electron microscopy techniques used to characterize the rock-salt type Ca(1-x)Mn(x)O(2) solid solution clearly hints at the existence of subtle extra ordering in other upper oxides of the Ca-Mn-O system. The combination of local characterization techniques like electron microscopy with more average ones like powder X-ray and neutron diffraction allows a very complete characterization of the system.

Structure Determination of the α = 3, β = 6 Term of the (A3B2O6)α(A3B3O9)β Homologous Series (A = Ba, B = Rh) by a Combination of Powder X-ray Diffraction and Electron Microscopy

The α = 3, β = 6 term of the series (A3B2O6)α (A3B3O9)β has been synthesized in polycrystalline form. The structural characterization by powder X-ray diffraction, electron diffraction and high-resolution electron microscopy indicates that this material is isostructural with Ba9Co8O24. In the structure, seven face-sharing octahedra alternate with a trigonal prism defining chains running parallel to the c axis of a rhombohedral unit cell (Rc), with parameters a = 1.00766(4) nm and c = 4.1571(2) nm. The structural refinement shows the presence of a small percentage of Rh vacancies leading to the composition Ba9Rh7.92O24. Pauli paramagnetic behaviour is observed. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

SrMnO3 Thermochromic Behavior Governed by Size-Dependent Structural Distortions

The influence of particle size in both the structure and thermochromic behavior of 4H-SrMnO3 related perovskite is described. Microsized SrMnO3 suffers a structural transition from hexagonal (P63/mmc) to orthorhombic (C2221) symmetry at temperature close to 340 K. The orthorhombic distortion is due to the tilting of the corner-sharing Mn2O9 units building the 4H structural type. When temperature decreases, the distortion becomes sharper reaching its maximal degree at ∼125 K. These structural changes promote the modification of the electronic structure of orthorhombic SrMnO3 phase originating the observed color change. nano-SrMnO3 adopts the ideal 4H hexagonal structure at room temperature, the orthorhombic distortion being only detected at temperature below 170 K. A decrease in the orthorhombic distortion degree, compared to that observed in the microsample, may be the reason why a color change is not observed at low temperature (77 K).

Strategies to stabilize new members of the (A3A'BO6)α(A3B3O9)β homologous series in the Sr-Rh-O system: Structure of the one-dimensional (α=3,β=2) [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh]O24 oxide

The (α=3, β=2) member of the (A3A′BO6)α (A3B3O9)β homologous series has been stabilised in the Sr-Rh-O system for a [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh]O24 composition. The structural characterisation has been performed by powder X-ray and electron diffraction measurements and high-resolution electron microscopy. In this structure, three face-sharing [RhO6] octahedra linked by one [Rh/SrO6] trigonal prism comprise the infinite one-dimensional chain that runs parallel to the c axis of a trigonal unit cell (Pc1), with parameters a=9.6411(1) and c=21.2440(4) A.

Structure–property relations in anion deficient 5H- and 3C-polytype Ba(Ti,Co)O3−δ perovskites

The polytypism and physical properties of BaTi1−yCoyO3−δ for y = 0.8 and 0.9 prepared with different δ values are reported. Samples quenched from 1293 K are strongly reduced, 3C-type perovskites (space group Pmm) with δ ∼ 0.4 and an average oxidation state of ∼Co3+ whereas slow cooled (4 °C min−1) samples are 5H-type perovskites (space group Pm1 and stacking sequence (ccchh)) with δ = 0.2 and an average oxidation state of ∼Co3.5+. An unusual feature of the 5H polymorph is the presence of ∼15 to 20% Ti(IV) on the tetrahedral (M3) sites. The 3C-polytypes are antiferromagnetic semiconductors with Neel temperatures 285 K. The reasons for the changes in polymorphism and physical properties with cation and anion content are rationalised on the basis of the different chemical nature and concentration of the Co(III), Co(IV) and Ti(IV) ions on the B-sites of these perovskites. The results are discussed in context to behaviour observed in related mixed-valent B-site perovskites to obtain a better understanding of the structure–composition–property relationships in these materials.

Transition from the Layered Sr2RhO4 to the Monodimensional Sr4RhO6 Phase

Study of the structural changes occurring during the reduction process of the Sr2RhO4+delta, (214), n=1 term of the Ruddlesden and Popper series, shows that for delta <0.02 values, this material dissociates into the Sr4RhO6 (416) monodimensional phase, alpha = infinity, beta = 0 compound of the (A3B2O6)alpha-(A3B3O9)beta family, and Rh metal. During the first stage, this process occurs by the formation of an intergrowth between the (214) and (416) materials which can be only detected by high resolution electron microscopy and is easily interpreted on the basis of the structural relationship established between them. Further reduction allows the segregation of both phases as separated entities, which coexist with Rh metal. The dissociation process is reversible and, under oxidizing conditions, a layered material with anionic composition delta =0.06 is always obtained. This behaviour seems to be a general way of accommodating the compositional changes in layered A2BO4 phases where the B cation is always in a octahedral environment. The structural mechanism of this transformation is proposed, and the structural relationship between these two low-dimensional oxides is established.

Atomically Resolved Short-Range Order at the Nanoscale in the Ca–Mn–O System

The elucidation of the reaction mechanisms involving redox processes in functional transition-metal oxides, which usually start in areas of very few nanometers in size, is yet a challenge to be satisfactorily achieved. Atomically resolved HAADF and EELS have provided both chemical and structural information at the nanoscale, which reveal the preservation of short-range cationic order in areas of 2-3 nm length as the driving force behind the reversibility of the Ca2Mn3O8-Ca2Mn3O5 redox process. Oxygen evolution is accommodated by cationic diffusion along the Ca and Mn layers of the cation-deficient Ca2Mn3O8 delafossite related structure, whereas Mn remains octahedrally coordinated.

Phase identification and superconducting transitions in Sr-doped Pr{sub 1.85}Ce{sub 0.15}CuO{sub 4+{delta}}

Sr-doped Pr{sub 1.85}Ce{sub 0.15}CuO{sub 4+{delta}} samples have been prepared with accurate control of the oxygen content. The stability of both T{sup {prime}} and T{sup {asterisk}} phases is strongly dependent on Sr and oxygen content. An electron diffraction study indicates that, in some cases, anionic vacancies are ordered leading to a pseudo-tetragonal superlattice with unit cell parameters 2{radical}2{bold a}{sub t}{times}{bold c}{sub t}. Structural transitions and superconducting phases created by hole doping in such a system are also reported. {copyright} {ital 1997 Materials Research Society.}