On the symmetry peculiarities of Bi2WO6single crystals

Bi2WO6single crystals (a=5.452(1), b=5.433(1), c=16.435(1) Å) were studied by X-ray diffraction (MoKα radiation, diffractometer Xcalibur S, CCD-detector) and electron diffraction techniques. Bi2WO6is an archetypal x=1 member of the Aurivillius family of layered perovskites of general formula Bi2O2Ax+1BxO3x-1. Its high piezoelectric performance and nonlinear optical properties have attracted considerable attention. In addition, these crystals offer high ionic conductivity due to the fast oxygen ion transport. In recent years, this compound has been the subject of intense research in the context of catalytic applications. In this work, the Bi2WO6single crystals were grown from solution in melt of Na2WO4–NaF. There were reflections with indexes 0kl, k=2n+1, in the diffraction pattern, contradicting the sp.gr. P21ab. The structure was solved by direct methods and refined in the sp.gr. R1 (R=3.60%, Rw=3.52%). The group P21ab was found to describe the arrangement of heavy atoms Bi and W only (R=17.5%, Rw=18.68%). The structure can be described by three local groups of symmetry – each atomic layer has inherent symmetry: W atoms and O atoms in equatorial vertices of WO6-octahedra have R11b sp.gr., Bi atoms – Bm11, the rest of O atoms – B11b. The oxygen atoms between two Bi sheets can be also described by B11m sp.gr. Preliminary electron diffraction investigation of the Bi2WO6crystals indicated a presence of small amount of a minority phase B1a1 together with the main P21ab phase. The presence of B1a1 phase can be probably explained by Na content in the crystal originating from the flux. Bi2WO6single crystals were studied earlier [1]. TEM showed coherent intergrowths of two distinct modulated variants having different symmetry. This result couldn't be explained by impurity presence because of investigation of pure crystals grown from melt. The work was done with the partial support of the grant for Leading Scientific Schools NSh-1130.2014.5 and RFBR (proj.14-02-00531a).

Intra-layer ordering and inter-layer disordering of the Li2MnO3phase in Li1.07Mn1.93O4–δcathode materials: electron diffraction investigation andDIFFaXsimulation of X-ray diffraction patterns

A previous transmission electron microscopy (TEM) analysis revealed the existence of monoclinic Li2MnO3in the lithium-rich and oxygen-deficient Li1.07Mn1.93O4−δpowder. Interestingly, the monoclinic phase exhibits different nanoscale lamellar variants involving a rotation of the stacking direction by 120 or 240° along the pseudo-threefold axis,i.e.the [103]M//[111]C(M and C denote the monoclinic and cubic phases, respectively) zone axis. Here, a theoretical X-ray diffraction (XRD) study of Li2MnO3employing theDIFFaXprogram is presented. It is found that, with the occurrence of different stacking configurations, the characteristic superstructure reflections with 2θ between 20 and 35° (Cu Kα) in the XRD pattern become more and more broadened with the increasing degree of stacking disorder, indicating that XRD may fall short in detecting the presence of the monoclinic Li2MnO3phase. Moreover, selective peak asymmetry appears when the stacking sequence becomes extremely disordered. Further selected-area electron diffraction and theoretical neutron diffraction investigation may clarify the similar ambiguity concerning the crystal phases of other structurally related compound cathode materials for lithium-ion batteries (e.g.LiNi1/2Mn1/2O2, LiNi1/3Co1/3Mn1/3O2).

Explanation of the stacking disorder in the β-phase of Pigment Red 170

The β-phase of Pigment Red 170, C26H22N4O4, which is used industrially for the colouration of plastics, crystallizes in a layer structure with stacking disorder. The disorder is characterized by a lateral translational shift between the layers with a componenttyof either +0.421 or −0.421. Order–disorder (OD) theory is used to derive the possible stacking sequences. Extensive lattice-energy minimizations were carried out on a large set of structural models with different stacking sequences, containing up to 2688 atoms. These calculations were used to determine the actual local structures and to derive the stacking probabilities. It is shown that local structures and energies depend not only on the arrangement of neighbouring layers, but also next-neighbouring layers. Large models with 100 layers were constructed according to the derived stacking probabilities. The diffraction patterns simulated from those models are in good agreement with the experimental single-crystal and powder diffraction patterns. Electron diffraction investigation on a nanocrystalline industrial sample revealed the same disorder. Hence the lattice-energy minimizations are able to explain the disorder and the diffuse scattering.