The electronic and optical properties of sodalite (Na(8)Al(6)Si(6)O(24)Cl(2)) from first principles

This study reports the results of ab initio electronic and optical calculations for pure socialite crystal using the linear augmented plane wave (LAPW) method within density functional theory (DFT). The calculated electronic structure revealed predominantly orbital characters of the valence band and the conduction band, and enabled us to determine the type and the value of the fundamental gap of the compound. The imaginary part of the dielectric tensor, extinction coefficient and refraction index were calculated as functions of the incident radiation wavelength. It is shown that the O 2p states and Na 3s states play the major role in optical transitions as initial and final states, respectively. The absorption spectrum is localized in the ultraviolet range between 40 and 250 nm. Furthermore, we concluded that the material does not absorb radiation in the visible range. (C) 2009 Elsevier Ltd. All rights reserved.

Electronic structure and optical properties of BaMoO(4) powders

Barium molybdate (BaMoO(4)) powders were synthesized by the co-precipitation method and processed in microwave-hydrothermal at 140 degrees C for different times. These powders were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis) absorption spectroscopies and photoluminescence (PL) measurements. XRD patterns and FT-Raman spectra showed that these powders present a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 850.4 cm(-1), which is associated to the Mo-O antisymmetric stretching vibrations into the [MoO(4)] clusters. UV-vis absorption spectra indicated a reduction in the intermediary energy levels within band gap with the processing time evolution. First-principles quantum mechanical calculations based on the density functional theory were employed in order to understand the electronic structure (band structure and density of states) of this material. The powders when excited with different wavelengths (350 nm and 488 nm) presented variations. This phenomenon was explained through a model based in the presence of intermediary energy levels (deep and shallow holes) within the band gap. (C) 2009 Elsevier B.V. All rights reserved.

The low-lying electronic states of BeP: a reliable and accurate quantum mechanical prediction

A very high level of theoretical treatment (complete active space self-consistent field CASSCF/MRCI/aug-cc-pV5Z) was used to characterize the spectroscopic properties of a manifold of quartet and doublet states of the species BeP, as yet experimentally unknown. Potential energy curves for 11 electronic states were obtained, as well as the associated vibrational energy levels, and a whole set of spectroscopic constants. Dipole moment functions and vibrationally averaged dipole moments were also evaluated. Similarities and differences between BeN and BeP were analysed along with the isovalent SiB species. The molecule BeP has a X (4)Sigma(-) ground state, with an equilibrium bond distance of 2.073 angstrom, and a harmonic frequency of 516.2 cm(-1); it is followed closely by the states (2)Pi (R(e) = 2.081 angstrom, omega(e) = 639.6 cm(-1)) and (2)Sigma(-) (R(e) = 2.074 angstrom, omega(e) = 536.5 cm(-1)), at 502 and 1976 cm(-1), respectively. The other quartets investigated, A (4)Pi (R(e) = 1.991 angstrom, omega(e) = 555.3 cm(-1)) and B (4)Sigma(-) (R(e) = 2.758 angstrom, omega(e) = 292.2 cm(-1)) lie at 13 291 and 24 394 cm(-1), respectively. The remaining doublets ((2)Delta, (2)Sigma(+)(2) and (2)Pi(3)) all fall below 28 000 cm(-1). Avoided crossings between the (2)Sigma(+) states and between the (2)Pi states add an extra complexity to this manifold of states.

An investigation of the activation process of high temperature shift catalyst

The aim of this work is to address the activation process of a high temperature shift (HTS) catalyst, composed of Fe2O3/Cr2O3/CuO, by analyzing it before activation (HTS-V) and after activation (HTS-A) using complementary characterization techniques. The textural and morphological characterizations were done by transmission electron rnicroscopy (TEM) and nitrogen physisorption at 77 K; crystallographic structure was confirmed by X-ray diffraction (XRD); electronic structure was analyzed by X-ray absorption spectroscopy (XAS) and the chemical composition of the catalyst`s surface was obtained by X-ray photoelectron spectroscopy (XPS). The investigation pointed out that the HTS-V catalyst presents good textural and morphological properties, which are not deeply affected by the activation process (sample HTS-A). The iron oxide phase in the HTS-V catalyst is hematite whereas in HTS-A catalyst is magnetite with Fe2+/Fe3+ ratio close to the expected value (0.5). For both samples, the Cr ions seem to be incorporated in the iron oxide lattice with higher concentration at particle surface. In the HTS-V catalyst, the Cu ions have oxidation number II and occupy in average distorted octahedral sites; after the activation, the Cu ions are partially reduced, suggesting that the reduction of the Cu species is complex. (C) 2007 Elsevier B.V. All rights reserved.