M. Parras

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.

Nonstoichiometry in BaFeO3−y (0.35 < y < 0.50)

BaFeO3−y compositions (0.35 < y < 0.50) annealed in the temperature range 980 to 1050°C have been investigated by means of electron diffraction and microscopy to resolve contradictory results from previous X-ray diffraction data. For 0.46 < y < 0.50, the nonstoichiometry is accommodated through the formation of microdomains. Each of these domains shows a superstructure of the cubic perovskite structure with either a monoclinic symmetry (as observed in Ba2Fe2O5) or an orthorhombic symmetry which is typical of a phase of composition 0.44 < y ≤ 0.46. With increase in the amount of Fe4+ (0.35 < y ≤ 0.44), BaFeO3−y consists of a two-phase mixture of a 6H hexagonal phase and of the above orthorhombic phase. The limit of pure cubic stacking seems to be y ⋍ 0.46.

Sur le système BaFeO3−y (0 < y ≤ 0.50)

Abstract Because of previous contradictory results, the BaFeO3−y system has been reinvestigated. Phases corresponding to 0.07 ≤ y ≤ 0.50 have been prepared and characterized. For y ≤ 0.35, the lattice packing has mainly hexagonal symmetry leading to 12H and 6H nonstoichiometric perovskite-related structures, while for higher y values it becomes cubic, this transition being related to a change of the Goldschmidt factor. In the hexagonal-type structures, the oxygen vacancies seem to be disordered. In the cubic domain (y > 0.35), vacancy ordering occurs leading, for y = 0.50, to a new monoclinic phase Ba2Fe2O5, the structure of which seems to be different from the well-known brownmillerite structure.

A reassessment of Ba2Fe2O5

Abstract Electron diffraction on samples of pure Ba 2 Fe 2 O 5 shows this phase to be monoclinic with cell parameters multiple of a perovskite basic cell.

Oxygen vacancy distribution in 6HBaFeO3−y (0.20 ≤ y ≤ 0.35)

Abstract An electron diffraction and microscopy study in the BaFeO 3− y (0.20 ≤ y ≤ 0.35) system has been performed. For 0.25 ≤ y ≤ y 0.35 values, a phase of average composition BaFeO 2.75 intergrows, in a disordered way, with a cubic phase of composition close to BaFeO 2.50 . For 0.20 ≤ y ≤ 0.25 only a single 6H-type structure is observed. On the basis of these results and of a previous Mossbauer resonance study, several models of vacancy ordering are discussed.

Structural aspects and Mössbauer resonance investigation of Ba2Fe2O5

Abstract Barium ferrite Ba 2 Fe 2 O 5 has been studied using various techniques. X-ray diffraction and TEM have shown the phase to crystallize with a monoclinic symmetry, the unit cell being a complex supercell of a cubic perovskite cell. The Mossbauer resonance spectrum exhibits five sextuplets which have been assigned to Fe 3+ in O h , T d , and fivefold coordinated sites. Obviously Ba 2 Fe 2 O 5 does not adopt the brownmillerite structure of homologous calcium (or strontium) ferrite and the oxygen vacancy ordering is more complex.

Anionic vacancy distribution in reduced barium-lanthanum ferrites: BaxLa1−xFeO3−x2 (12 ≤ x ≤ 23)

Abstract An electron diffraction and microscopy study of the Ba x La 1−x FeO 3− x 2 ( 1 2 ≤ x ≤ 2 3 ) samples to be formed by three-dimensional microdomains. The unit cell corresponding to each domain is related to the cubic perovskite structure by the expression a c × a c × 2 a c , a c being the cubic perovskite unit cell parameter. A structural model in which octahedral and square pyramidal layers intergrow in an ordered way following the double axis is proposed.

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.

Compositional variations and structural disorder in the BaMnO3−y system

Abstract An electron diffraction and high resolution electron microscopy study of the BaMnO 3− y system shows that in the reduction process of BaMnO 3 , the oxygen loss is accompanied with structural changes related to the introduction of cubic layers in the starting hexagonal close packing. Several ordered hexagonal types (2H, 15H, 8H, 6H, 10H and 4H) can be obtained, for a given oxygen content, as a function of the synthetic route. The synthesis and structural characterization of the 6H-type is described.

A high temperature study of the BaFeO3-y system

Abstract A study by means of powder X-ray diffraction at high temperature of Ba2Fe2O5 shows the existence of a monoclinic-cubic transition as a consequence of the disappearance of the ordering of anionic vacancies in the starting monoclinic material. A similar transition has been encountered during transmission electron microscopy observations.