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

Origin of the color in cobalt-doped quartz.

Synthetic Co-doped quartz was grown hydrothermally in steel autoclaves at the Technological Center of Minas Gerais (CETEC), Brazil. The quartz samples, originally yellow in the as-grown state acquired blue coloration after prolonged heat treatment times at 500?C near the alpha?beta transition temperature. UV?VIS?NIR absorption spectroscopy shows the characteristic spectra of Co3+ before heat treatment. After heat treatment, the optical absorption spectrum is dominated by two split-triplet bands the Wrst in the near infrared region centered at about 6,700 cm?1 (1,490 nm) and the second in the visible spectral range at about 16,900 cm?1 (590 nm). Both split-triplet bands are typical for Co2+ ions in tetrahedral coordination environments. From the absence of electron paramagnetic resonance (EPR) spectra, we conclude that the Co2+ found in the optical absorption spectra of the blue quartz is not due to an isolated structural site in the quartz lattice. Instead, the blue color is associated with electronic transitions of Co2+ in small inclusions in which the Co site has tetrahedral symmetry. The non-observation of polarizationdepend optical absorption spectra is also in agreement with this model. The results for Co2+ in quartz are diVerent from Co-bearing spinel and staurolite and other silicates like orthopyroxene, olivine, and beryls. The formation process of the color center is discussed

Spectroscopic characterization of transition metal impurities in natural montebrasite/amblygonite.

Natural single-crystal specimens of the montebrasite/amblygonite series from Brazil, with general formula LiAlPO4(F,OH), were investigated by electron microprobe, Raman spectroscopy, X-ray diffraction, and infrared absorption. Since little is known about impurities and their local symmetries, electron paramagnetic resonance (EPR) was applied. Six different paramagnetic impurities and radiation defects were detected by EPR. Three of them, all substituting for Al3+ ions, namely, iron (Fe3+), vanadium (V4+), and niobium (Nb4+) impurities were characterized in this work. The Fe3+ (3d5)-related EPR spectra and angular dependencies show occupation of low-symmetry sites that are revealed in the high asymmetry parameter of the electronic fine structure, E/D = 0.27. Vanadium and niobium impurities are identified through their typical strong hyperfine interactions. Both form interesting examples for which the properties of 3d1 ion (V4+) and 4d1 ion (Nb4+) in the same host matrix can be compared. It is shown that both ions form complex defects of type VO2+ (vanadyl) and NbO2+ (niobyl), showing superhyperfine interaction with two equivalent hydrogen ions and not to fluorine. The EPR rotation patterns are analyzed in detail for three mutually perpendicular crystal planes. Spin Hamiltonian parameters are calculated and discussed

Metavivianite, Fe2+Fe3+2(PO4)2(OH)2?6H2O : new data and formula revision.

The composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mo?ssbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galile?ia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with a = 1.600(3), b = 1.640(3), g = 1.685(3) and 2Vmeas = 85(5)?. The measured and calculated densities are Dmeas = 2.56(2) and Dcalc = 2.579 g cm 3. The chemical composition, based on electronmicroprobe analyses, Mo?ssbauer spectroscopy (to determine the Fe2+:Fe3+ ratio) and gas chromatography (to determine H2O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe2O3 26.60, P2O5 28.62, H2O 26.5; total 101.32 wt.%. The empirical formula is (Fe3+ 1.64Fe2+ 1.23Mg0.085Mn0.06)S3.015(PO4)1.98(OH)1.72?6.36H2O. Metavivianite is triclinic, P1? , a = 7.989(1), b = 9.321(2), c = 4.629(1) A ? , a = 97.34(1), b = 95.96(1), g = 108.59(2)?, V = 320.18(11) A ? 3 and Z = 1. The crystal structure was solved using a single-crystal techniques to an agreement index R = 6.0%. The dominant cations in the independent sites are Fe2+ and Fe3+, with multiplicities of 1 and 2, respectively. The simplified crystal-chemical formula for metavivianite is Fe2+(Fe3+,Fe2+)2(PO4)2(OH,H2O)2?6H2O; the endmember formula is Fe2+Fe3+ 2 (PO4)2(OH)2?6H2O, which is dimorphous with ferrostrunzite

Influence of the matrix on the red emission in europium self-activated orthoceramics.

Different oxide host matrices of ABO4 with A3+ (Eu) and B5+ (Nb, Ta, and Sb) were prepared to investigate the solid state luminescence behavior of Eu3+ as a self-activated emitter in orthoceramics. Crystal structures, phonon modes, metal valence states, optical excitation, and emission luminescence properties including emission decay curves, colorimetry, and nonstoichiometry defects were studied using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Classical intraconfigurational (f?f transitions), interconfigurational (4fn?15d ? 4fn), and charge transfer bands (X5+?O2?) were observed, besides split Stark levels indicating low local symmetry. The influence of host matrices on the optical behavior of orthocompounds was noticed by changes in the energy of the typical Eu3+ transitions. In this case, the larger blue-shifted peaks were observed for EuSbO4 and the larger red-shifted positions for EuNbO4, with respect to emission spectra of orthocompounds studied here. These results were correlated to the respective fifth ionization energies and covalent fraction as well as to the crystal structures exhibited by the different host matrices (I2/a: Nb, Ta; P21/c: Sb). The presence of more than one component for the 5D0 ? 7F0 transition in each compound was observed at low temperature, and it could be justified by nonstoichiometric defects like the oxygen vacancy and Eu2+. These typical defects are observed in nonstoichiometric oxide materials, and they were analyzed in the orthoceramics by EPR and XPS spectroscopy. Finally, the lifetime of the 5D0 state and chromaticity diagrams confirmed our orthoceramics as good optical emitters in the red-end region

A new appraisal of sri lankan bb zircon as a reference material for LA-ICP-MS U-Pb geochronology and Lu-Hf isotope tracing.

A potential zircon reference material (BB zircon) for laser abla tion-in ductively coupled plasma-mass spectrometry (LA-ICP-M S)U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5?1.5 cm3) from SriLanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with somevariation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains,the variation in U content for all analysed crystals ranged from 227 to 368 lgg-1and the average Th/U ratios werebetween 0.20 and 0.47. The Hf isotope composition (0.56?0.84 g/100 g Hf) is homogeneous within and between thegrains ? mean176Hf/177Hf of 0.281674 ? 0.000018 (2s). The calculated alpha dose of 0.59 3 1018g-1for a numberof BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the samecrystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurementreproducibility (0.1?0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that arewithin uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of theoxygen isotope systematic of four BB crystals (13.16? VSMOW)