J. Monecke

Tetrad effect in rare earth element distribution patterns: a method of quantification with application to rock and mineral samples from granite-related rare metal deposits

Abstract In some geological environments, the tetrad effect can be observed as a split of rare earth element (REE) patterns into four rounded segments. A new method is proposed to quantify the sizes of the individual segments, and for the first time, the significance of observed tetrad effects is evaluated by taking analytical errors into account. The outlined method was applied to lanthanide patterns of whole-rock and fluorite samples collected from granite-related rare metal deposits. The REE patterns of the granite and greisen samples investigated exhibit significant tetrad effects that may not be accounted for by analytical uncertainties. It is shown that the study of whole-rock samples is insufficient to determine whether this effect is developed during fractional crystallization or is due to other processes such as fluid-rock interaction. A concave tetrad effect mirroring the pattern of the whole-rock samples was not observed in the REE patterns of related vein fluorite samples. Therefore, it is unlikely that the convex tetrad effect in the samples from the magmatic environment can be explained by removal of a respective complementary REE pattern by a coexisting hydrothermal fluid, as previously suggested. It is proposed that the tetrad effect formed within the magma-fluid system before emplacement in the subvolcanic environment where phase separation caused a split of this system into fluid and magma subsystems. Alternatively, the tetrad effect may also be inherited from an external fluid influencing the system during or after the emplacement of the magma. On the basis of the fluorite data, it is shown that the behavior of Eu in the fluids is not related to the tetrad effect. Consequently, different physico-chemical factors control the occurrence of both phenomena. Y was found to be strongly enriched in samples precipitating from hydrothermal fluids that experienced prolonged interaction with the wall-rocks, whereas the tetrad effect in the fluids vanished with time and increasing distance from the ore-bearing granite. Thus, these different geochemical parameters can be used to reconstruct different aspects of the fluid evolution within this type of deposit.

Semiconductor sieves as nonlinear optical materials

Electrochemical etching techniques were used to fabricate semiconductor sieves of gallium phosphide, i.e., two-dimensionally nanostructured membranes exhibiting an enhanced optical second harmonic generation (SHG) in comparison with the bulk material. The SHG rotational and fundamental polarization dependencies studied under sample excitation by a 1064-nm Nd-YAG laser beam indicate optical homogeneity and uniaxial symmetry of the membranes. The artificial anisotropy and the enhanced nonlinear optical response induced by nanotexturization make semiconductor sieves very promising for use in all-optical devices.

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.

Laser power dependence of the photoluminescence from CdSxSe1-x nanoparticles in glass

The influence of probing laser power density on the band-edge and the trap luminescence from CdSx Se1-x nanoparticles embedded in a glass matrix is reported. Both the position and the strength of the band-edge luminescence are found to be very sensitive to the laser power. It is observed for the first time that the band-edge luminescence shifts initially towards low energy and then towards high energy with increasing laser power. The results are analysed in terms of laser-induced local heating and a band-filling mechanism, respectively, which are found to explain adequately the experimental observations. The shift due to the latter shows up only beyond a threshold value of the probing laser power density. The trap luminescence shows large contributions from surface states and undergoes photodarkening at high laser power, the degree of which depends strongly on the local temperature of the nanoparticles during illumination; this is reported here for the first time. This effect is explained in terms of a permanent trapping of charge carriers due to optical processes and thermally assisted optical processes.

Influence of cage structures on the vibrational modes and Raman activity of methane

Melanophlogite is a naturally occurring SiO2-based clathrate structure which has the same structure of type I gas hydrates. Two types of voids are found in melanophlogite. Furthermore, as in the case of the gas hydrates, melanophlogite traps gas molecules within the voids. In this work we present a joint theoretical and experimental investigation of the CH4 Raman spectra associated with the enclathrated CH4 molecules. We find that the Raman intensities of the totally symmetric hydrogen stretch modes are significantly perturbed by the presence of the clathrate cage and show that the calculated Raman spectra may be used to determine the concentration and location of the enclathrated gas molecules. Relative to the gas-phase structure we identify two effects which are responsible for the change in intensity of the enclathrated molecules relative to the gas phase. The polarizibility of the surrounding cage acts to increase the Raman spectra of the stretch mode in the pentagondodecahedra cage. However, in the lower-symmetry tetrakaidecahedra cage, mixing between the hydrogen stretch mode and other optically silent molecular vibrations counteracts this effect and accounts for the different Raman intensities observed for the two types of voids. We suggest that similar calculations and experiments on the gas hydrates may provide an in situ diagnostic tool for determining the amount of natural gas contained within the gas hydrates on the sea floor.

Bergman spectral representation of a simple expression for the dielectric response of a symmetric two-component composite

A simple expression for the effective dielectric constant of a symmetric two-component composite (aggregate topology) is presented and interpreted in terms of the Bergman spectral representation.

Determination of the hole concentration and mobility of p-GaP by Hall and by Raman measurements

Larger hole concentrations and lower mobilities are measured by Raman scattering of p-GaP in comparison with results of Hall and conductivity investigations. The Hall factor which is responsible for the differences can be obtained by these measurements and is discussed on the basis of a two-band model.

Raman-active modes of porous gallium phosphide at high pressures and low temperatures

Porous gallium phosphide (GaP) with a honeycomb-like morphology and a skeleton relative volume concentration c = 0.7 was investigated by Raman spectroscopy under pressure up to 10 GPa at T = 5 K. The porous samples were prepared by electrochemical etching. The transverse optical (TO) and longitudinal optical (LO) mode frequencies were found to shift with pressure similarly to those of bulk GaP. As in bulk GaP, the TO feature of the porous GaP exhibits a pressure-induced narrowing which is interpreted in terms of a Fermi resonance. The scattering intensity observed on the low-frequency side of the LO mode is attributed to surface-related Frohlich mode scattering. The latter results are interpreted on the basis of an effective medium expression for the dielectric function. The Raman spectra indicate that both the morphology and degree of porosity are unaffected by pressure in the range investigated.

Determination of the charge carrier concentration across growth striations in n‐GaAs by Raman spectroscopy

Using a Raman microprobe, the charge carrier concentration across growth striations in Bridgman n‐GaAs is determined both by the analysis of coupled LO‐phonon‐plasmon modes and their relative intensities compared to that of the uncoupled LO phonon of the depletion layer. It is shown that the charge carrier concentration typically varies by ±10% in the regions both under rough and facetting growth conditions. In the facetting grown region the charge carrier concentration appears to be higher by about 50%.

Observation of raman scattering by localized optical phonons bound to neutral donors in GaP:Si

Abstract Raman scattering by localized optical phonons bound to neutral donors with degenerate ground state has been observed in GaP:Si. The scattering efficiency was measured at T=6.0±0.1 K as a function of the net donor concentration ND-NA ranging from 1.6 ·1017 to 7.1· 1017 cm-3. It is found that reasonable agreement with the theory of weakly coupled electron-phonon modes obtained if a Bohr radius of a 0 =5.4 A is used, whereas a 0 =7.5 A is obtained in the hydrogenic effective-mass approximation using the ionization energy ED=86.7 meV.