A new high-pressure cell for systematic in situ investigations of micro-scale processes in gas hydrates using confocal micro-Raman spectroscopy.

Natural gas hydrates are ice-like solids composed of gas and water molecules. They are found worldwide at all continental margins as well as in permafrost regions. Depending on the source of the enclathrated gas molecules, natural gas hydrates may occur as coexisting phases with different structures containing predominantly CH4, but also a variety of hydrocarbons, CO2 or H2S. For a better understanding of these complex hydrate formation processes on a micrometer level, an experimental setup with a new high-pressure cell was developed, which can be used in a pressure range between 0.1 MPa and 10.0 MPa. Peltier elements ensure precise cooling of the cell in a temperature range between 243 K and 300 K. The selected temperature and pressure ranges in which the cell can be used make it possible to simulate the formation of gas hydrates in their natural environment, e.g., on continental margins or in permafrost areas at a depth of up to 1000 m. The cell body is made of Hastelloy, which generally also allows the use of corrosive gases, such as H2S, in the experiments. The inner sample space has a volume of about 550 µl. A quartz window allows for microscopic observations and the systematic and continuous in situ Raman spectroscopic investigations of the forming hydrate phase mimicking natural conditions. Single point measurements, line scans, and area maps provide information on spatial heterogeneities regarding compositions and cage occupancies. The pressure cell can be operated as a closed system or as an open system with a defined continuous gas flow. The use of a continuous gas flow also allows for the in situ investigation of transformation processes induced by changes of the feed gas composition. In this paper, all details of the new experimental setup as well as preliminary results of our investigations on the formation of complex mixed hydrate systems both in the open and closed systems as well as the CH4-CO2 transformation process are presented.

The Quantification of Radiation Damage in Orthophosphates Using Confocal µ-Luminescence Spectroscopy of Nd3.

In this study, we present a new concept based on the steady-state, laser-induced photoluminescence of Nd3+, which aims at a direct determination of the amorphous fraction f a in monazite- and xenotime-type orthophosphates on a micrometer scale. Polycrystalline, cold-pressed, sintered LaPO4, and YPO4 ceramics were exposed to quadruple Au-ion irradiation with ion energies 35 MeV (50% of the respective total fluence), 22 MeV (21%), 14 MeV (16%), and 7 MeV (13%). Total irradiation fluences were varied in the range 1.6 × 1013-6.5 × 1013 ions/cm2. Ion-irradiation resulted in amorphization and damage accumulation unto a depth of ~5 µm below the irradiated surfaces. The amorphous fraction created was quantified by means of surface-sensitive grazing-incidence X-ray diffraction and photoluminescence spectroscopy using state-of-the-art confocal spectrometers with spatial resolution in the µm range. Monazite-type LaPO4 was found to be more susceptible to ion-irradiation induced damage accumulation than xenotime-type YPO4. Transmission electron microscopy of lamella cut from irradiated surfaces with the focused-ion beam technique confirmed damage depth-profiles with those obtained from PL hyperspectral mapping. Potential analytical advantages that arise from an improved characterization and quantification of radiation damage (i.e., f a) on the µm-scale are discussed.

Irradiation effects in monazite-(Ce) and zircon: Raman and photoluminescence study of Au-irradiated FIB foils.

Lamellae of 1.5 µm thickness, prepared from well-crystallised monazite-(Ce) and zircon samples using the focused-ion-beam technique, were subjected to triple irradiation with 1 MeV Au+ ions (15.6% of the respective total fluence), 4 MeV Au2+ ions (21.9%) and 10 MeV Au3+ ions (62.5%). Total irradiation fluences were varied in the range 4.5 × 1012 - 1.2 × 1014 ions/cm2. The highest fluence resulted in amorphisation of both minerals; all other irradiations (i.e. up to 4.5 × 1013 ions/cm2) resulted in moderate to severe damage. Lamellae were subjected to Raman and laser-induced photoluminescence analysis, in order to provide a means of quantifying irradiation effects using these two micro-spectroscopy techniques. Based on extensive Monte Carlo calculations and subsequent defect-density estimates, irradiation-induced spectroscopic changes are compared with those of naturally self-irradiated samples. The finding that ion irradiation of monazite-(Ce) may cause severe damage or even amorphisation, is in apparent contrast to the general observation that naturally self-irradiated monazite-(Ce) does not become metamict (i.e. irradiation-amorphised), in spite of high self-irradiation doses. This is predominantly assigned to the continuous low-temperature damage annealing undergone by this mineral; other possible causes are discussed. According to cautious estimates, monazite-(Ce) samples of Mesoproterozoic to Cretaceous ages have stored only about 1% of the total damage experienced. In contrast, damage in ion-irradiated and naturally self-irradiated zircon is on the same order; reasons for the observed slight differences are discussed. We may assess that in zircon, alpha decays create significantly less than 103 Frenkel-type defect pairs per event, which is much lower than previous estimates. Amorphisation occurs at defect densities of about 0.10 dpa (displacements per lattice atom).

Factors affecting the Nd3+ (REE3+) luminescence of minerals.

In this paper, possibilities and limits of the application of REE3+ luminescence (especially the Nd3+4F3/2 → 4I9/2 emission) as structural probe are evaluated. Important factors controlling the Nd3+ luminescence signal are discussed, including effects of the crystal-field, crystal orientation, structural state, and temperature. Particular attention was paid to the study of the accessory minerals zircon (ZrSiO4), xenotime-(Y) (YPO4), monazite-(Ce) (CePO4) and their synthetic analogues. Based on these examples we review in short that (1) REE3+ luminescence can be used as non-destructive phase identification method, (2) the intensities of certain luminescence bands are strongly influenced by crystal orientation effects, and (3) increased widths of REE3+-related emission bands are a strong indicator for structural disorder. We discuss the potential of luminescence spectroscopy, complementary to Raman spectroscopy, for the quantitative estimation of chemical (and potentially also radiation-induced) disorder. For the latter, emissions of Nd3+-related centres are found to be promising candidates.