P. Allé

Rare-earth patterns in zircons from the Manaslu granite and Tibetan Slab migmatites (Himalaya) : insights in the origin and evolution of a crustally-derived granite magma

The Tibetan Slab gneisses are currently considered as the source for the High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucogranite (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucidate the zircon growth conditions and, indirectly, the processes involved in incipient anatexis and evolution of granitic magmas. In the migmatites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145–700 ppm) and chondrite-normalized (Yb/Sm)N ratios (≤81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically indistinguishable from those of the gneisses and melanosomes. Zircons from the high-Zr tonalitic leucosomes and granitic leucosomes are euhedral and show higher Yb contents (409–2820 prim) and (Yb/SM)N ratios (≥145) than those of the gneisses and melanosomes. The euhedral shapes and distinctive REE patterns of zircons from the high-Zr leucosomes and granitic leucosomes are consistent with crystallization from a melt, whereas the morphological and chemical similarities of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical change. In the Manaslu granite, zircons have rounded cores with REE patterns distinct from those of the rims (e.g., 250 ppm ≤Yb≤710 ppm in the core, 965 ppm ≤Yb≤ 2775 ppm in the rim) but comparable to those from the Tibetan Slab gneisses suggesting inheritance. The rim compositions, however, are distinct from those of either zircon types of the Tibetan Slab leucosomes, indicating that the leucosomes cannot be the unsegregated equivalent to the Manaslu granite parental magma. Comparison of the rim compositions with fractional crystallization models suggests that the range in zircon Sm and Yb contents are consistent with zircon crystallization from a monazite-saturated, xenotime-undersaturated melt. The Yb contents in the different zircons studied, and their variation within a single zircon, further suggest boundary-layer effects and magma compositional heterogeneity, in agreement with previous models which considered that the Manaslu granite resulted from the aggregation of magma batches.

Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector

A new experimental approach to perform in situ electric field diffraction on single crystals using an on-then-off pump–probe mode in situ (i.e. the field-switching method) with a synchrotron or a laboratory X-ray source is presented. Taking advantage of the fast readout of the XPAD hybrid pixel two-dimensional detector and its programmable functionalities, the operation mode of the detector has been customized to significantly increase the efficiency of the method. The very weak electric field-induced structural response of a piezoelectric crystal can be accurately measured. This allows the piezoelectric tensor to be precisely obtained from Δθ shifts while the structural variations can be modelled using a full set of ΔI/I data. The experimental method and methodology are detailed and tested as a case study on pure piezoelectric compounds belonging to the α-quartz family (SiO2 and GaAsO4 single crystals). Using the two scan modes developed, it is demonstrated that tiny Bragg angle shifts can be measured as well as small field-induced Bragg intensity variations (<1%). The relevance of measurements performed with an X-ray laboratory source is demonstrated: partial data sets collected at synchrotrons can be completed, but more interestingly, a large part of the study can now be realized in the laboratory (medium to strong intensity reflections) in a comparable data collection time.

Linear dichroism of the cornea

The dichroic properties of in vitro sheep corneas were studied with a spectrophotometer in transmission mode for several angles of incidence of light beams. Several models of corneal anisotropy have been presented in the literature. The results presented here allow us to believe that the cornea behaves as a dichroic biaxial crystal. Furthermore, this dichroism is weak when the angle of incidence on the corneal surface stays small. The mathematical model that describes these optical properties of the cornea uses Mueller matrices.

Exploration of the retinal nerve fiber layer thickness by measurement of the linear dichroism

An electro-optic device mounted on a slit lamp to assess the degree of polarization of a light beam that has double passed through the retina about the optic-nerve head in the living human eye is described. The asymmetric structure of the retinal nerves fiber layer possesses a linear-form dichroism and will partially polarize an unpolarized light beam that is scattered at the fundus of the eye and has double passed the ocular media (cornea, lens, retina). This partial polarization is a function of the retinal nerves fiber layer thickness, and its measurement may be used for exploring glaucoma and other retinal neuropathies. Experimental conditions allow us to neglect corneal dichroism. The first clinical measurements show a different degree of polarization between normal and glaucomatous eyes and a good correlation with the results obtained by optical coherence tomography.

Comparison of CCD, CMOS and Hybrid Pixel x-ray detectors: detection principle and data quality

We compare, from a crystallographic point of view, the data quality obtained using laboratory x-ray diffractometers equipped with a Molybdenum micro-source using different detector types: CCD, CMOS and XPAD hybrid pixel. First we give an overview of the working principle of these different detector types with a focus on their principal differences and their impact on the data quality. Then, using the example of an organic crystal, a comparison between the detector systems concerning the raw data statistics, the refinement agreement factors, the deformation electron density maps, and the residual density after multipolar refinement is presented. It is found that the data quality obtained with the XPAD detector is the best, even though the detection efficiency at the Mo energy (17.5 keV) is only 37% due to the Si-sensor layer thickness of 300 μm. Finally, we discuss the latest x-ray detector developments with an emphasis on the sensor material, where replacing Si by another material such as GaAs would yield detection efficiencies close to 100%, up to energies of 40 keV for hybrid pixel detectors.

In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector

With the remarkable progress of accelerator-based X-ray sources in terms of intensity and brightness, the investigation of structural dynamics from time-resolved X-ray diffraction methods is becoming widespread in chemistry, biochemistry and materials science applications. Diffraction patterns can now be measured down to the femtosecond time-scale using X-ray free electron lasers or table-top laser plasma X-ray sources. On the other hand, the recent developments in photon counting X-ray area detectors offer new opportunities for time-resolved crystallography. Taking advantage of the fast read-out, the internal stacking of recorded images, and the gating possibilities (electronic shutter) of the XPAD hybrid pixel detector, we implemented a laboratory X-ray diffractometer for time-resolved single-crystal X-ray diffraction after pulsed laser excitation, combined with transient optical absorption measurement. The experimental method and instrumental setup are described in detail, and validated using the photoinduced nitrosyl linkage isomerism of sodium nitroprusside, Na2[Fe(CN)5NO]·2H2O, as proof of principle. Light-induced Bragg intensity relative variations ΔI(hkl)/I(hkl) of the order of 1%, due to the photoswitching of the NO ligand, could be detected with a 6 ms acquisition window. The capabilities of such a laboratory time-resolved experiment are critically evaluated.

X-ray diffraction from α quartz crystals under electric fields

The aim of our work is to analyze by diffraction techniques the correlations between structural and physical properties of crystals onto which an electric field is applied. In the laboratory in Nancy we have, based on the ideas of previous work [1], build a device using a field switching technique. It consists of a high voltage supply, the electronics for switching the field, and synchronous counting on four chains combined with a control for stepscanning the diffraction profiles [2]. By using this stroboscopic technique, it is possible to measure very small changes in the Bragg angles due to the strain resulting from the converse piezoelectric effect, and also to measure minute changes in the Bragg intensities due to polarisations of atomic structure and electron density. A first measure was carried out at LURE with the 4-circle diffractometer WDIF-4C on α-quartz, since perfect crystal samples can easily be obtained. Furthermore, quartz is a well-characterised piezo-electric material. Electrodes were vapour deposited onto the (1 1 0) extended faces of a crystal plate of dimensions 5x5x0.5 mm. Our measurements reproduce the known piezo-electric coefficients of α-quartz. We have measured changes in Bragg intensities for 27 reflections, 21 unique. Different atomic models have been investigated in search for an explanation of the origin of piezoelectric effect. Our conclusions are compared with a previous study with a similar technique [3]. [1]A. Paturle, H. Graafsma, H. -S. Sheu, P. Coppens & P. Becker (1991) Phys. Rev. B43, 14683-14691. [2] R. Guillot, P. Allé, P. Fertey, N. K. Hansen & E. Elkaïm ( 2002) J. Appl. Cryst. in print. [3] U. Pietch, J. Stahn, J. Davaasambuu & A. Pucher (2001) J. of Physics and Chemistry of Solids 62, 2129-2133.

Laboratory data collection using a pixel detector: application to ms time-resolved crystallography

X ray diffraction and scattering techniques are unique tools for studying the structural properties of crystals. Thanks to their unprecedented properties, the new generation of X-ray detectors, the hybrid pixel array detectors or ‘pixel detectors’ based on single-photon counting processes, are the future for accurate and time resolved diffraction experiments: high dynamic range and linearity, no readout noise, high signal-to-noise ratio, fast readout time and high frame rates are among their intrinsic characteristics which render them so attractive. First used on synchrotron beam lines, these detectors are also very promising at laboratory sources, in particular for pump-probe or quasistatic experiments and accurate electron density measurements as explained in this presentation. An original laboratory diffractometer made from a Nonius Mach3 goniometer equipped with an IμS Incoated Mo micro source and an XPAD [1] pixel array detector has been assembled at the CRM2 laboratory. Our XPAD detector has also been installed at the CRISTAL Soleil synchrotron beamline (mounted on a 6 circles goniometer for very precise Bragg peak angle shifts measurements). Among the many possible applications, we first have focused on three applications; diffraction of piezoelectrics under electric field [2] (CRM2 and Soleil), photocrystallography (CRM2) and measurement of accurate structure factors for charge density analysis (CRM2). Here will be presented our new experimental approach to perform in-situ electric field diffraction on single crystals using the field-switching method with a synchrotron or a laboratory X-ray source. Taking advantages of the fast readout of the XPAD hybrid pixel 2D detector and its programmable electronic functionalities, the operation mode of the detector has been customized to significantly increase the efficiency of the method. The very weak electric field induced structural response of a piezoelectric crystal (Δ(I)/I about %) can be accurately measured. This also allows the piezoelectric tensor to be accurately obtained from ∆q shifts whereas the structural variations can be modeled using a full set of ∆I/I data. This experimental method and methodology will be detailed as a case study on piezoelectric single crystals belonging to the a-quartz family. Using the two developed scan modes, we demonstrate that tiny Bragg angle shifts can be measured as well as small field induced Bragg intensities variations (< 1%). The relevance of measurements performed with our X-ray laboratory diffractometer will be evidenced: partial data sets collected at synchrotrons can be completed, but more interestingly, most of the data can now be performed in the laboratory (medium to strong intensity reflections) in a comparable data collection time [3]. To stress the quality of the laboratory data the room temperature Ylid[4] structure will also be discussed compared to that obtained from both Agilent Supernova with Atlas CCD and Bruker-AXS D8 Venture with PHOTON100 CMOS diffractometers.