Three samples of bluish chromium-bearing dickite and chromium-bearing kaolinite were examined by X-ray powder diffraction, chemical analysis, electron microprobe, optical, and infrared techniques to determine whether chromium is part of the mineral structure or present in an impurity phase. Two of the samples studied contain a single dominant chromium-bearing phase (either dickite or kaolinite); the third contains equal proportions of both minerals. The optical absorption and infrared spectra are consistent with the presence of octahedrally coordinated chromium. The range of Cr3+-Al3+ substitution is rather limited: up to 0.06 atoms per unit cell. The electron microprobe study revealed the presence of very rare, minute grains of chromite, as well as a uniform distribution of chromium in dickite and kaolinite, indicating that chromium occupies octahedral sites in the structure of these minerals.

The crystal chemistry of allactite from Långban, Värmland (Sweden) was investigated by single-crystal X-ray and neutron diffraction, optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and electron microprobe analysis by wavelength-dispersive spectroscopy (EPMA-WDS). The optical spectra indicate the presence of Mn in valence state 2+ only. Assuming 16 O atoms per formula unit, arsenic as As5+ and the (OH) content calculated by charge balance, the resulting formula based on the EPMA-WDS data is (Mn2+ 6.73Ca0.13Mg0.12Zn0.02)∑7.00(As5+)2.00O16H8, very close to the ideal composition Mn7(AsO4)2(OH)8. In the unpolarized FTIR spectrum of allactite, fundamental (OH)-stretching bands are observed at 3236, 3288, 3387, 3446, 3484, 3562 and 3570 cm–1, suggesting that a number of OH environments, with different hydrogen bond strengths, occur in the structure. The neutron structure refinement shows that four independent H sites occur in allactite with full site occupancy, all as members of hydroxyl groups. The complex hydrogen-bonding scheme in the allactite structure is now well defined, with at least nine hydrogen bonds energetically favourable with mono-, bi- and trifurcated configurations.

Many-body perturbation theory is applied to compute the quasiparticle electronic structures and the optical absorption spectra (including excitonic effects) for several transparent conducting oxides (TCOs). We discuss HSE+G0W0 results (based on the hybrid exchange-correlation functional by Heyd, Scuseria, and Ernzerhof, and quasiparticle corrections from approximating the electronic self energy as the product of the Green’s function and the screened Coulomb interaction) for band structures, fundamental band gaps, and effective electron masses of magnesium oxide, zinc oxide, cadmium oxide, tin dioxide, tin oxide, indium (III) oxide and silicon dioxide. The Bethe–Salpeter equation (BSE) is solved to account for excitonic effects in the calculation of the frequency-dependent absorption coefficients. We show that the HSE+G0W0 approach and the solution of the BSE are very well suited to describe the electronic structure and the optical properties of various TCOs in good agreement with experiment.

In this work we present results of X-ray diffraction using powder method, on natural alexandrite samples from Minas Gerais State (Brazil), as a function of a sequence of annealing. From these measurements we determine lattice parameters before (a=9.405 Å, b=5.471 Å, c=4.409 Å) and after annealing, and its structure is confirmed as orthorhombic. Measurements done after an annealing of 15 minutes at 700 °C and for 5 hours at 1000 °C indicate the migration of atoms present in the sample through different phases, which were also identified by Microprobe Analysis (WDS). However we have verified that such migration does not modify the structure. X-ray diffraction measurements have been carried out in conjunction with optical absorption in the UV–Vis as a function of annealing.

The formation pathway to double-walled carbon nanotubes (DWNTs) from C60 encased within single-walled carbon nanotubes (peapods) is introduced in this article. Onedimensionally arranged C60 molecules coalesce gradually within the nanotube and change the structure to C60 dimers, trimers, tetramers, and so on as intermediates. In addition to these interesting structural transformations visualized in the nanotube space, the nanotube itself is very stable, and this structural stability is very important when using the interior of the nanotube as the reaction field or the space for molecular storage. In terms of optical absorption, the lowest energy absorption band for DWNTs, ∼0.65 eV, shows broadened and downshifted features as compared with that of SWNTs.We expect that this opticalabsorption feature will lead to the use of DWNTs in absorbing devices for optical-fiber communications. The Raman experiments give new information about the frequency of the C-C stretching-mode vibration for nanotubes with diameters of less than ∼1 nm, which shows a decrease in vibration frequency with decreasing tube diameter. This diameter dependence can be explained by an admixture of sp3 character in the C-C interaction. Therefore, the electronic and mechanical properties of nanotubes with diameters of <1 nm are expected to be different from nanotubes of the ∼1-nm-diameter class, and we anticipate that new phenomena will occur in small-diameter tubes.