G. Irmer

Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

SummaryWe report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 μm3 and 80 μm3, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 μm. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral “age distribution” within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 μm, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.ZusammenfassungWir präsentieren die Ergebnisse einer Untersuchung des Zussammenhangs von Metamiktisierung und U-Pb Isotopendiskordanz in Zirkonen mittles kombinierter SHRIMP-Ionenmikrosonden- und Ramanmikrosondenanalysen. Die räumliche Auflösung der verwendeten Analysensysteme, experimentell mit 200 μm3 (SHRIMP) und 80 μm3 (Ramansonde) bestimmt, gestattet den direkten Vergleich der in Mikrobereichen innerhalb von Zirkon-Einkristallen gemessenen Isotopendiskordanzen und Metamiktisierungsgrade. Wir zeigen, daß der während der SHRIMP-Analyse auf den Zirkon einwirkende Sauerstoffionenstrahl weder signifikante Amorphisierung noch Rekristallisation des benachbarten Zirkonmaterials im 1 μm-Bereich hervorruft. Es ist deshalb möglich, den Grad der Metamiktisierung eines Zirkon-Mikrobereiches auch in den SHRIMP-Analysengrübchen, d.h. erst nach dem Analysieren dieses Bereiches mit der Ionensonde, mittels hochauflösender Ramanmessungen unverfälscht zu bestimmen. Der Einsatz beider Mikromethoden liefert kombinierte Informationen zu den Gehalten und Verteilungen radioaktiver Elemente, zum Grad der Metamiktisierung und seiner Heterogenität und zur lateralen „Altersverteilung” innerhalb von Einzelkörnern. Wir weisen eine deutliche Korrelation des Grades der U-Pb-Isotopendiskordanz mit dem Grad der Metamiktisierung im Mikrobereich (15 μm Auflösung) nach. Dies stimmt mit früheren, an Einzelzirkonen und Populationen getätigten Beobachtungen überein, daß die Metamiktisierung von Zirkon dessen Potential zu sekundären Bleiverlusten erhöht.

Raman and Infrared Spectroscopic Investigations on Aqueous Alkali Metal Phosphate Solutions and Density Functional Theory Calculations of Phosphate–Water Clusters

Phosphate (PO43−) solutions in water and heavy water have been studied by Raman and infrared spectroscopy over a broad concentration range (0.0091–5.280 mol/L) including a hydrate melt at 23 °C. In the low wavenumber range, spectra in R-format have been constructed and the R normalization procedure has been briefly discussed. The vibrational modes of the tetrahedral PO43−(aq) (Td symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO43− (Td) and phosphate–water clusters: PO43− · H2O (C2v), PO43− · 2H2O (D2d), PO43− · 4H2O (D2d), PO43− · 6H2O (Td), and PO43− · 12H2O (T), a cluster with a complete first hydration sphere of water molecules. A cluster with a second hydration sphere of 12 water molecules and 6 in the first sphere, PO43− · 18H2O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO43− · 12H2O cluster and the PO43− · 18H2O cluster but far less so in the case of the unhydrated PO43− or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of ν1(a1) PO43− in aqueous solutions has been measured as a function of concentration and the asymmetric and broad band shape was explained. However, the same mode in heavy water has only half the full width at half-height compared to the mode in normal water. The PO43− is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing ν2(H2O), the deformation mode of water, and the stretching modes, the ν1OH and ν3OH of water, in K3PO4 solutions as a function of concentration and comparison with the same modes in pure water. A mode at ∼240 cm−1 (isotropic R spectrum) has been detected and assigned to the restricted translational mode of the strong hydrogen bonds formed between phosphate and water, P–O ··· HOH. In very concentrated K3PO4 solutions (C0 ≥ 3.70 mol/L) and in the hydrate melt, formation of contact ion pairs (CIPs) could be detected. The phosphate in the CIPs shows a symmetry lowering of the Td symmetry to C3v. In the less concentrated solutions, PO43−(aq) solvent separated ion pairs and doubly solvent separated ion pairs exist, while in very dilute solutions fully hydrated ions are present (C0 ≤ 0.005 mol/L). Quantitative Raman measurements have been carried out to follow the hydrolysis of PO43−(aq) over a very broad concentration range. From the hydrolysis data, the pK3 value for H3PO4 has been determined to be 12.45 at 23 °C.

Vibrational Spectroscopic Studies and Density Functional Theory Calculations of Speciation in the CO2—Water System

Raman spectra of CO2 dissolved in water and heavy water were measured at 22 °C, and the Fermi doublet of CO2, normally at 1285.45 and 1388.15 cm−1 in the gaseous state, revealed differences in normal water and heavy water, although no symmetry lowering of the hydrated CO2 could be detected. Raman spectra of crystalline KHCO3 and KDCO3 were measured at 22 °C and compared with the infrared data from the literature. In these solids, (H(D)CO3)22– dimers exist and the spectra reveal strong intramolecular coupling. The vibrational data of the dimer (C2h symmetry) were compared with the values from density functional theory (DFT) calculations and the agreement is fair. Careful measurements were made of the Raman spectra of aqueous KHCO3, and KDCO3 solutions in D2O down to 50 cm−1 and, in some cases, down to very low concentrations (≥0.0026 mol/kg). In order to complement the spectroscopic assignments, infrared solution spectra were also measured. The vibrational spectra of HCO3−(aq) and DCO3−(D2O) were assigned, and the measured data compared well with data derived from DFT calculations. The symmetry for HCO3−(aq) is C1, while the gas-phase structure of HCO3− possesses Cs symmetry. No dimers could be found in aqueous solutions, but at the highest KHCO3 concentration (3.270 mol/kg) intermolecular coupling between HCO3−(aq) anions could be detected. KHCO3 solutions do not dissolve congruently, and with increasing concentrations of the salt increasing amounts of carbonate could be detected. Raman and infrared spectra of aqueous Na2 –, K2 –, and Cs2CO3 solutions in water and heavy water were measured down to 50 cm−1 and in some cases down to extremely low concentrations (≥0.002 mol/kg) and up to the saturation state. For carbonate in aqueous solution a symmetry breaking of the D3h symmetry could be detected similar to the situation in aqueous nitrate solutions. Strong hydration of carbonate in aqueous solution could be detected by Raman spectroscopy. The hydrogen bonds between carbonate in heavy water are stronger than the ones in normal water. In sodium and potassium carbonate solutions no contact ion pairs could be detected even up to the saturated solutions. However, solvent separated ion pairs were inferred in concentrated solutions in accordance with recent dielectric relaxation spectroscopy (DRS) measurements. Quantitative Raman measurements of the hydrolysis of carbonate in aqueous K2CO3 solutions were carried out and the hydrolysis degree a was determined as a function of concentration at 22 °C. The second dissociation constant, pK2, of the carbonic acid was determined to be equal to 10.38 at 22 °C.

Fröhlich modes in porous III-V semiconductors

Porous GaP, InP and GaAs structures fabricated by MeV ion-implantation-assisted electrochemical etching were investigated by Raman and Fourier transform infrared spectroscopy. Frohlich modes in the frequency gap between the transverse optical and longitudinal optical frequencies were observed and their longitudinal-transverse splitting was established. The frequency-dependent optical properties in the infrared region were calculated using a dielectric function derived on the basis of an appropriate two-dimensional effective-medium theory. The theoretical reflectance spectra are found to be in good agreement with the experimental ones and the predicted coupled Frohlich-plasmon modes for conducting samples were observed experimentally. The wavelength used in Raman measurements did not fulfil the requirements of effective-medium theory, but the resulting spectra could be explained at least qualitatively by taking into account the diffuse scattering.

Raman studies on GaAs1−xBix and InAs1−xBix

The lattice vibrational properties of new semiconductor alloys, GaAs1−xBix and InAs1−xBix, are reported. These alloys, which were grown by metalorganic vapor phase epitaxy technique, contain a small amount (1.2%–3.8%) of Bi. A detail Raman scattering study of these new alloys, which exhibit weak temperature dependence of the band gap with increasing amount of Bi, is reported here. Good crystalline quality and spatial homogeneity was confirmed using micro-Raman technique. The alloys show ternary compound behavior, confirming substitutional incorporation of Bi into the lattice site. New vibrational modes observed were assigned to GaBi-like and InBi-like modes. In addition, phonon-plasmon coupled modes and vibrational modes corresponding to Bi and As materials were also observed. Results are discussed to characterize these new alloys in detail.

Ion implantation as a tool for controlling the morphology of porous gallium phosphide

We investigate the morphology of porous layers obtained by electrochemical anodization of (100)-oriented n-type GaP substrates before and after a preliminary 5-MeV Kr+ implantation. Apart from favoring the observation of a surface-related phonon in the frequency gap between the bulk optical phonons, ion implantation appears to be an effective means of controlling the morphology of porous GaP, irrespective of initial substrate material features.

Raman and infrared spectroscopic investigation of contact ion pair formation in aqueous cadmium sulfate solutions

Raman and IR data for aqueous CdSO4 and (NH4)2SO4 solutions have been recorded over broad concentration and temperature ranges. Whereas the v1-SO42− band profile is symmetrical in (NH4)2SO4 solutions, in CdSO4 solutions a shoulder appears on the high frequency side which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the v1-SO42− band is the same for all forms of sulfate in (NH4)2SO4 and CdSO4 solutions and is independent of temperature up to 99°C. The high frequency shoulder is attributed to the formation of a contact ion pair [Cd2+OSO32−] (1∶1 associate). Also the v3-SO42− antisymmetric stretching mode shows a splitting in the CdSO4 solution. Further spectroscopic evidence for contact ion pair formation is provided by IR spectroscopy. No higher associates or anionic complexes are required to interpret the spectroscopic data. The degree of association has been measured as a function of concentration and temperature. The thermodynamic association constant, KA=0.15±0.05 kg-mol−1 at 25°C is estimated from the Raman data by an extrapolation procedure by taking account of the activity coefficients. Values are reported for the activity coefficient of the ion pair. From the Raman temperature dependence studies, the enthalpy of formation for the contact ion pair is estimated to be 10±1 kJ-mol−1.

Porosity-induced modification of the phonon spectrum of n-GaAs

Porous GaAs layers have been produced by anodic etching of (100)-oriented crystalline substrates in a solution. Scanning electron microscope images showed the formation of submicron pores, the average dimension of the remaining GaAs walls being of about 100 nm. Raman scattering by LO-phonon - plasmon coupled modes, inherent in as-grown crystals, was not observed in the porous layers. Proposed explanations are either the depletion of the GaAs skeleton due to the surface space-charge effect or the decoupling of the LO-phonon and the plasmon modes at the relative large wavevectors transferred in nanostructures. A new Raman scattering peak at , located between the bulk TO and LO frequencies, has been observed in porous layers and attributed to a surface-related phonon.

The characteristics of high-resistance layers produced in n-GaAs using MeV-nitrogen implantation for three-dimensional structuring

The radiation damage introduced in n-GaAs by 4-MeV N+ implantation at a dose of 1×1015 cm−2 has been analyzed using micro-Raman spectroscopy. Implantation followed by annealing at 600 °C was found to produce a strongly compensated near-surface layer possessing a high crystalline quality. At the same time a pronounced disorder was found underneath the high-resistance layer which enables the fabrication of 2.5-μm thick free-standing membranes using selective electrochemical etching techniques.

Crystallisation behaviour and polytype transformation of polymer-derived silicon carbide

Abstract This paper describes the crystallisation behaviour of polysilane-derived amorphous silicon carbide. The polytype formation and the transformation from β-SiC into hexagonal α-polytypes have been more specifically investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), 29 Si Solid State Magic Angle Spinning Nuclear Magnetic Resonance ( 29 Si MAS-NMR) and Raman spectroscopy. The crystallisation of β-SiC starts around 1200 °C. The different polytypes like 3C, 2H, 4H, 6H and 15R have been identified by the various investigation techniques at temperatures higher than 1400 °C. Crystallisation occurs with a high density of stacking faults that cover large areas in the crystallites as found by TEM observation. They promote the formation of temporary existing α-polytypes which are finally reconverted into β-SiC as found by X-ray and electron diffraction.