Nanocrystalline transition-metal gallium oxide spinels from acetylacetonate precursors via solvothermal synthesis

The synthesis of mixed-metal spinels based on substituted γ-Ga2O3 is reported using metal acetylacetonate precursors in solvothermal reactions with alcohols as solvents at 240 °C. New oxides of Cr, Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen by high-resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed oxide is thus formed, with composition 0.33Ga1.87Cr0.8O4 ( = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr3+, which in turn implies a mixture of tetrahedral and octahedral Ga3+, and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa2O4 is shown to contain octahedral Mn2+, along with some Mn3+, but a different inversion factor to materials reported by conventional solid-state synthesis in the literature, which are known to have a significant proportion of tetrahedral Mn2+. In the case of iron, higher amounts of the transition metal can be included to give an Fe:Ga ratio of 1:1. Elemental mapping using energy dispersive X-ray spectroscopy on the TEM, however, reveals inhomogeneity in the distribution of the two metals. This is consistent with variable temperature 57Fe Mössbauer spectroscopy that shows the presence of Fe2+ and Fe3+ in more than one phase in the sample. Variable temperature magnetisation and electron paramagnetic resonance (EPR) indicate the presence of superparamagnetism at room temperature in the iron-gallium oxides. View Full-Tex

OCORRÊNCIA, CONTEXTO MINERALÓGICO E QUÍMICA MINERAL DA BRAZILIANITA E SEUS DEPÓSITOS EM MINAS GERAIS

A brazilianita é um fosfato gemológico relativamente raro, amarelo a amarelo-esverdeado, descrito em 1945 em amostras coletadas no Pegmatito Córrego Frio (Divino das Laranjeiras, MG), onde vários depósitos similares foram depois reportados. No mesmo Estado, existe ainda outro pegmatito mineralizado, em Itinga. Tais pegmatitos, ora estudados, originam-se de granitogêneses relacionadas ao Orógeno Araçuaí, constituindo em geral corpos tabulares de pequeno porte, além de pouco diferenciados. Análises químicas com microssonda eletrônica no mineral revelaram valores muito homogêneos para os diversos pegmatitos estudados, semelhantes aos da descrição original; Análises sobre feldspatos associados mostraram que os pegmatitos hospedeiros são pobres em B (não turmaliníferos), Cs, Rb e Zr, mas ricos em F. Um deles, o Pegmatito Telírio, apresenta uma anomalia em Li em relação aos demais. A cor amarela do mineral, estudada por EPR e absorção óptica, deve-se a uma banda de absorção no UV próximo, e sua intensidade está relacionada com um centro O1- fixo, do tipo buraco. Diferentes tipos de centros de elétrons são responsáveis pela ampla faixa de estabilidade térmica da cor do mineral; o componente esverdeado é causado por uma extensa banda de absorção na região vermelha do espectro. Nos depósitos estudados, a brazilianita constitui um produto secundário de alteração da montebrasita, em estágio hidrotermal precoce (350-250°C)

Optical phonon features of triclinic montebrasite : dispersion analysis and non-polar Raman modes.

Polarized infrared and Raman spectra of triclinic LiAl(PO4)(OH) [montebrasite] single crystal were recorded for appropriate optical configurations. Dispersion analysis was applied on the infrared reflectivity spectra taken at low incidence angle (11 ) to determine the oscillator parameters and the dipole directions of the polar phonons. In particular, all the 27 polar phonons, predicted by group theory for triclinic P1 structure,were determined. The obtained dielectric tensor parameters have been checked by comparison between predicted and measured infrared spectra at higher incidence angle (34 ). The azimuth and co-elevation angles obtained from the dispersion analysis showed that the response of several polar phonons is close to that of an orthorhombic system. Polarized Raman spectra obtained in several scattering geometries allowed us to obtain well-defined 24 non-polar modes, also in perfect agreement with group theory. The selection rule between Raman and infrared phonons was respected, confirming the centrosymmetric structure and ruling out any relevant influence of defects. The relatively narrow phonon bands are compatible with a highly ordered structure with fully occupied atomic sites

Polarized Raman scattering and infrared spectroscopy of a naturalmanganocolumbite single crystal.

A well-ordered natural manganocolumbite single crystal of high quality was used as a prototype for the first determination of the polarized optical phonon modes of materials with the columbite structure. Electron microprobe and X-ray diffraction characterizations determined the chemical formula asMn0.60Fe0.40(Nb0.80Ta0.20)2O6, a cationic ordering of 81%, and the crystal structure as belonging to the Pbcn group. Polarized Raman and infrared-reflectivity spectroscopies on oriented samples allowed us to discern 50 of the 54 predicted gerade (Raman) modes and 31 of the 38 predicted ungerade (infrared) modes for the columbite structure. The selection rules were verified, and polarization leaks only due to slight sample misorientation, confirming the high purity, ordering and quality of the material. From the polar phonon spectra, intrinsic dielectric merit factors = 29.2 and of 64 THz were determined, showing adequate values for designing applications in microwave circuitry

Supplementary Material: Visible light sensitive mesoporous nanohybrids of lepidocrocite-like ferrititanate coupled to a charge transfer complex: Synthesis, characterization and photocatalytic degradation of NO

Related to the published version: [https://hdl.handle.net/21.15107/rcub_dais_3692]Supporting information for: [http://dx.doi.org/10.1016/j.jphotochem.2018.07.038]Related to the accepted version: [https://hdl.handle.net/21.15107/rcub_dais_4084