Crystal structure of the uranyl arsenate mineral hügelite, Pb2(UO2)3O2(AsO4)2(H2O)5, revisited: a correct unit cell, twinning and hydrogen bonding

Revisiting the structure of uranyl arsenate mineral hügelite provided some corrections to the available structural data. The previous twinning model (by reticular merohedry) in hügelite has been corrected. Twinning of the monoclinic unit cell [a = 7.0189 (7) Å, b = 17.1374 (10) Å, c = 8.1310 (10) Å and β = 108.904 (10)°], which can be expressed as a mirror in [100], leads to a pseudo-orthorhombic unit cell (a = 7.019 Å, b = 17.137 Å, c = 61.539 Å and β = 90.02°), which is eight times larger, with respect to the unit-cell volume, than a real cell. Moreover, the unit cell of chosen here and the unit cell given by the previous structure description both lead to the same supercell. A new structure refinement undertaken on an untwinned crystal of hügelite resulted in R = 4.82% for 12 864 reflections with I obs > 3σ(I) and GOF = 1.12. The hydrogen-bonding scheme has been proposed for hügelite for the first time.

Redetermination of K2Mg3(OH)2(SO4)3(H2O)2 from single-crystal X-ray data revealing the correct hydrogen-atom positions

In comparison with the previous structure determination of K2Mg3(OH)2(SO4)3(H2O)2, dipotassium trimagnesium dihydroxide tris(sulfate) dihydrate, from laboratory powder X-ray diffraction data [Kubel & Cabaret-Lampin (2013). Z. Anorg. Allg. Chem. 639, 1782–1786], the present redetermination against CCD single-crystal data has allowed for the modelling of all non-H atoms with anisotropic displacement parameters. As well as higher accuracy and precision in terms of bond lengths and angles, the clear localization of the H-atom positions leads also to a reasonable hydrogen-bonding scheme for this hydroxy hydrate. The structure consists of (100) sheets composed of corner- and edge-sharing [MgO6] octahedra and sulfate tetrahedra. Adjacent sheets are linked by the potassium cations and a hydrogen bond of medium strength involving the water molecule. The title compound is isotypic with its CoII and MnII analogues: the three K2 M 3(OH)2(SO4)3(H2O)2 (M = Mg, Co, Mn) structures are quantitatively compared.

Planar Tetracoordinate Fluorine Atoms

Unlike other atoms, planar tetracoordinate fluorines are elusive. So far, there are no theoretical or experimental reports suggesting their existence. Herein, we introduce the first six combinations, whose global minima contain a planar tetracoordinate fluorine. All of them are surrounded exclusively by atoms of group 13. The bonding scheme shown by these species is entirely different from analogous systems with carbon, nitrogen, or oxygen atoms. The magnetic response characterizes these systems mostly σ-aromatic. The planar form is somewhat stabilized by subtle ionic interactions of the fluorine with the peripheral atoms, forming an adequately sized cavity. <br>

Planar Tetracoordinate Fluorine Atoms

Unlike other atoms, planar tetracoordinate fluorines are elusive. So far, there are no theoretical or experimental reports suggesting their existence. Herein, we introduce the first six combinations, whose global minima contain a planar tetracoordinate fluorine. All of them are surrounded exclusively by atoms of group 13. The bonding scheme shown by these species is entirely different from analogous systems with carbon, nitrogen, or oxygen atoms. The magnetic response characterizes these systems mostly σ-aromatic. The planar form is somewhat stabilized by subtle ionic interactions of the fluorine with the peripheral atoms, forming an adequately sized cavity. <br>

Hydrogen bonding in the crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O: single-crystal X-ray study and TORQUE calculations

The crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å3, space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca2+ cations and H2O groups. In contrast to previous studies the approach here reveals five transformer H2O groups (compared to three expected by a previous study) and two non-transformer H2O groups. One of the transformer H2O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H2O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca2(H2 [3]O)5(H2 [4]O)2}[(UO2)3O2(PO4)2], Z = 8.

Graphene-Quantum-Dot-Mediated Semiconductor Bonding: A Route to Optoelectronic Double Heterostructures and Wavelength-Converting Interfaces

A semiconductor bonding technique that is mediated by graphene quantum dots is proposed and demonstrated. The mechanical stability, electrical conductivity, and optical activity in the bonded interfaces are experimentally verified. First, the bonding scheme can be used for the formation of double heterostructures with a core material of graphene quantum dots. The Si/graphene quantum dots/Si double heterostructures fabricated in this study can constitute a new basis for next-generation nanophotonic devices with high photon and carrier confinements, earth abundance, environmental friendliness, and excellent optical and electrical controllability via silicon clads. Second, the bonding mediated by the graphene quantum dots can be used as an optical-wavelength-converting semiconductor interface, as experimentally demonstrated in this study. The proposed fabrication method simultaneously realizes bond formation and interfacial function generation and, thereby, can lead to efficient device production. Our bonding scheme might improve the performance of optoelectronic devices, for example, by allowing spectral light incidence suitable for each photovoltaic material in multijunction solar cells and by delivering preferred frequencies to the optical transceiver components in photonic integrated circuits.