Correlation of the Madelung constant and I—M—I bonding angle with cohesive energy contributions in layered metal diiodides (MI2) with CdI2 (2H polytype) structure

This study uses theoretically methods to investigate, for metal diiodides MI2 (M = Mg, Ca, Mn, Fe, Cd, Pb) with CdI2 (2H polytype) structure, the mutual correlation between the structure-characterizing parameters (the flatness parameter of monolayers f, the Madelung constant A, and bonding angle I—M—I) and correlation of these parameters with contributions of the Coulomb and covalent energies to cohesive energy. The energy contributions to cohesive energy are determined with the use of empirical atomic potentials. It is demonstrated that the parameters f and A, and the bonding angle I—M—I are strictly correlated and increase in the same order: FeI2 < PbI2 < MnI2 < CdI2 < MgI2 < CaI2. It is found that with an increase of parameter A and bonding angle I—M—I the relative contribution of the Coulomb energy to cohesive energy increases, whereas the relative contribution of the covalent energy decreases. For a hypothetical MX 2 layered compound with the CdI2 (2H polytype) structure, composed of regular MX 6 octahedra (angle X—M—X = 90°), the flatness parameter and the Madelung constant are found to be f reg = 2.449 and A reg = 2.183, respectively. Correlation of the covalent energy with the type of distortion of MI6 octahedra (elongation or compression) with respect to regular configuration (angle I—M—I = 90°) is also analyzed.

Order-disorder Twinning of Malachite

Malachite has a layered structure composed of triangular CO3 groups and CuO2(OH)2 coordination squares of two types. In 2D projection, each layer can be decomposed into two OD strips: the A strip, containing CO3 groups and square co-ordinations of Cu2, with idealized symmetry pgm2, and the B strip, without CO3 groups, with symmetry p2 (the b axis of malachite is the infinite strip direction). In the 3D structure, these strips form the ‘A' OD-layers with symmetry reduced by coordination requirements to approximately p2212, which alternate with the ‘B' OD-layers with layer symmetry . The B-layer can assume two positions, which are related by a 180° rotation axis running through the C1 and O1 atoms of the CO3 group in the A-layer. This axis is oriented close to [100]*. The fit of the A-layer and of the coplanar rotated B-layer is hindered by the presence of important partial gaps and overlaps at the boundary. Inclining these two structure portions towards one another, together with small positional adjustments, appears to alleviate the misfit problems; the two layer portions so related form an angle of about 124°, i.e., they form a twin relationship. This desymmetrization of the malachite structure away from the ideal model results in occasional twinning instead of the fully developed monoclinic polytype structure which is derived in this paper. In the structure of the Cu-Zn analogue, rosasite, the above-described match problem is solved in a different way, by shifting a Zn coordination octahedron (which replaces Cu2 in malachite) into the interlayer space. Malachite polytypes 1 and 2 and rosasite illustrate three different orientations of P21/a symmetry elements with respect to the structural layers, resulting in three different structure configurations.

Draft Genome Sequence of Broad-Spectrum Antibiotic-Producing Marine Verrucosispora sp. Strain FIM060022

Verrucosispora sp. strain FIM060022 shows multiple biological activities and produces polytype structure compounds, including abyssomicins, proximicin A, lumichrome, denosine, and desferrioxamine-like compounds.

Jinshajiangite: structure, twinning and pseudosymmetry

The crystal structure of jinshajiangite based on a sample from its original discovery location is studied using single-crystal X-ray diffraction and transmission electron microscopy methods. Jinshajiangite is a titanium silicate mineral with an ideal chemical formula of BaNaFe4Ti2(Si2O7)2O2(OH)2F. The structure of jinshajiangite is of P\bar 1 symmetry (triclinic system), with a = 8.7331 (2) Å, b = 8.7366 (2) Å, c = 11.0404 (3) Å, α = 81.477 (1)°, β = 110.184 (1)°, γ = 104.384 (1)° and V = 764.03 (3) Å3, instead of the previously proposed C\bar 1 cell [a = 10.7059 (5) Å, b = 13.7992 (7) Å, c = 20.760 (1) Å, α = 90.008 (1)°, β = 94.972 (1)°, γ = 89.984 (1)°, V = 3055.4 (4) Å3]. The basic topology of the new structure is similar to the previously proposed C\bar 1 structure, except there is only one type of titanium silicate and intermediate cation layer in the structure (instead of two types), which are all related by the translation along the c-axis. Even though there is a significant amount of Mn in the chemical composition, no obvious ordering between Fe and Mn is observed in the structure. All the mineral species of the perraultite-type structure (jinshajiangite, perraultite, surkhobite and bobshannonite) should have the same P\bar 1 structure as jinshajiangite with ∼10 Å d 001 spacing, and all the previously proposed monoclinic space groups were pseudosymmetry generated by nanoscale polysynthetic twinning on the (001) composition plane. The similar phenomenon observed in bafertisite is also discussed in the paper with an alternative polytype structure model proposed.

Defects and Polytype Instabilities

A model based on the generation and recombination of defect was developed to describe the stability of stacking faults and basal plane dislocation loops in crystals with layered polytype structures. The stability of the defects configuration was analysed for stacking faults surrounded by Shockley and Frank partial dislocation as well as Shockley dislocation dipoles with long range elastic fields. This approach allows the qualitative prediction of defect subsystems in polytype structure in external fields.

Micro-FTIR and EPMA Characterisation of Charoite from Murun Massif (Russia)

Combined micro-Fourier transform infrared (micro-FTIR) and electron probe microanalyses (EPMA) were performed on a single crystal of charoite from Murun Massif (Russia) in order to get a deeper insight into the vibrational features of crystals with complex structure and chemistry. The micro-FTIR study of a single crystal of charoite was collected in the 6000–400 cm−1 at room temperature and after heating at 100°C. The structural complexity of this mineral is reflected by its infrared spectrum. The analysis revealed a prominent absorption in the OH stretching region as a consequence of band overlapping due to a combination of H2O and OH stretching vibrations. Several overtones of the O-H and Si-O stretching vibration bands were observed at about 4440 and 4080 cm−1 such as absorption possibly due to the organic matter at about 3000–2800 cm−1. No significant change due to the loss of adsorbed water was observed in the spectrum obtained after heating. The occurrence of well-resolved water bending vibration bands at about 1595 and 1667 cm−1 accounts for more than one structural water molecule as expected by charoite-90 polytype structure model from literature. The chemical composition of the studied crystal is close to the literature one.

Stacking Fault Formation via 2D Nucleation in PVT Grown 4H-SiC

Synchrotron white beam x-ray topography (SWBXT), synchrotron monochromatic beam x-ray topography (SMBXT), and high resolution transmission electron microscopy (HRTEM) studies have been carried out on stacking faults in PVT grown 4H-SiC crystal. Their fault vectors were determined by SWBXT to be 1/3<-1100>, 1/2<0001>, 1/6<-2203>, 1/12<4-403>, 1/12<-4403>. HRTEM studies reveal their similarity in stacking sequences as limited numbers of bilayers of 6H polytype structure. Simulation results of the two partial dislocations associated with the stacking faults in SMBXT images reveal the opposite sign nature of their Burgers vectors. A mechanism for stacking fault formation via 2D nucleation is postulated.

Atomic structure of metal-free and catalyzed Si nanowires

ABSTRACTMetal-free and Au-catalyzed silicon nanowires (Si-NWs) grown at low temperatures have been analyzed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystalline phase studied. All the observed nanowires are crystalline, grow along two different directions, <110> or <112>, and contain high density of planar defects, such as stacking faults (SFs) and twins. The defect size is comparable to the wire diameter for the metal-free process whilst it is much larger than the wire diameter for the Aucatalyzed Si-NWs. In this latter case parallel SFs may re-arrange and transform in a metastable rhombohedral 9R polytype structure whose formation mechanism is discussed.