ABSTRACT
BaFeCl0.13(2)O2.48(2) has been synthesized and studied. A proper tuning of the synthetic route has been designed to stabilize this compound as a single phase. The thermal stability and evolution, along with the magnetic and structural properties are reported here. The crystal structure has been refined from neutron powder diffraction data, and it is of the type (hhchc)2-10H. It is stable up to a temperature of 900 °C, where the composition reads BaFeCl0.13(2)O2.34(2). The study by electron microscopy shows that the crystal structure suffers no changes in the whole BaFeCl0.13(1)O3-y (2.34 ≤ 3 - y ≤ 2.48) compositional range. Refinement of the magnetic structure shows that the Fe is antiferromagneticaly ordered, with the magnetic moment parallel to the ab plane of the hexagonal structure. At higher temperature, a nonreversible phase transition into a (hchc)-4H structure type takes place with overall composition BaFeCl0.13(1)O2.26(1). Microstructural characterization shows that, in some crystals, this phase intergrows with a seemingly cubic related phase. Differences between these two crystalline phases reside in the chlorine content, which keeps constant through the phase transition for the former and disappears for the latter.
ABSTRACT
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 â¼125 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).
ABSTRACT
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.
Subject(s)
Lead/chemistry , Sulfides/chemistry , Microscopy, Electron, Scanning , Microscopy, Electron, Transmission , Molecular ConformationABSTRACT
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.
ABSTRACT
The synthesis and microstructural characterization, by means of selected-area electron diffraction and high-resolution electron microscopy, of Ba7Rh6O18 is reported. This material, isostructural to Ba7Co6O18, is formed by rows of one trigonal prism and five face-sharing octahedra running parallel to the c-axis of a trigonal unit cell with parameters a = 1.004(5) and c = 3.165(7) nm. The substitution of Sr by Ba is accommodated by means of twin formation due to a rearrangement of the rows of polyhedra.
ABSTRACT
The (alpha =3, beta =2) member of the (A3ABO6) (A3B3O9) 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.
