Different stacking sequences of graphene are investigated a using combination of experimental and theoretical methods. The high-resolution transmission electron microscopy (HRTEM) of the stacking sequence of several layers of graphene, formed on the C-terminated 4H-SiC(0001) surface, was used to determine the stacking sequence and the interlayer distances. These data prove that the three metastable configurations exist: ABAB, AAAA, ABCA. In accordance to these findings, those three cases were considered theoretically, using Density Functional Theory calculations comparing graphene sheets, freestanding and positioned on the SiC(0001) substrate. The total energies were calculated, the most stable structure was identified and the electronic band structure was obtained. The four graphene layer electron band structure depends crucially on the stacking: for the ABAB and ABCA stacking, the bands, close to the K point, are characterized by the hyperbolic dispersion relation while the AA stacking the dispersion in this region is linear, similar to that of a single graphene layer. It was also shown that the linear dispersion relation is preserved in the presence of the SiC substrate, and also for different distances between adjacent carbon layers.
Crystal structure of arsenolite, the cubic polymorph of arsenic(III) oxide, has been determined by single crystal X-ray diffraction up to 30 GPa. The bulk of the crystal is monotonically compressed with no detectable anomalies, to 60% of the initial volume at 30 GPa. In the structure the most compressed are As•••As contacts which contrasts with increased intramolecular As•••As distance in the deformed molecule. The ratio between As•••As inter-and intramolecular distances decreases from 1.47 at 0.1 MPa to 1.03 at 30 GPa. The As4O6 molecules are deformed to become more tetrahedron-like. Pressure above 3 GPa favours the formation of As4O6•2He inclusion compound in the surface layer increasingly deeper with pressure. The experimental As4O6 crystal compression has been compared with various theoretical models within the DFT framework. According to band-structure calculations the arsenolite band gap falls from 4.2 eV at ambient pressure to 2.7 eV at 27.8 GPa.
An original room-temperature and ambient-pressure method
of claudetite
II crystallization consisting in ammonium arsenate(III) decomposition
is presented. Claudetite II is characterized by single crystal and
powder X-ray diffraction, Raman spectroscopy and differential scanning
calorimetry. Claudetite I and II equations of state were obtained
by DFT calculations of their energies incorporating dispersive contribution
accounted for by the Grimme method. It has been shown that claudetite
II is metastable in the investigated pressure range and a new high-pressure
As2O3 polymorph has been predicted. Presented
computational results indicate that the contribution of van der Waals
interactions to the systems’ energy cannot be neglected.
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