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We have developed unique x-ray focusing optics by using compound x-ray refractive lens (CRL) designated for the high-pressure XRD beamline BL10XU [1] of SPring-8. The CRL is a set of planar cross-lenses with a quasi-parabolic profile for focusing in two directions, and are fabricated from the SU-8 polymer by deep x-ray lithography at the IMT in Germany [2]. The focusing optics consisted by two sets of CRL and a pinhole in tandem. The set of the upstream CRL and the pinhole as a virtual source are used for generation of homogeneously divergent beam. The downstream CRL, which is placed between a pinhole and a focal point (sample position), is utilized for focusing the x-ray towards the micron size. The focused beam size is defined by the relationship between a pinhole diameter and the ratio of source-lens versus lens-focal point distances. We have succeeded in focusing the x-ray beam down to 2.4 microns in vertical and 2.1 microns in horizontal, as we had designed, under the following conditions: 30 keV x-ray, 10 micron pinhole in diameter and 11:1 ratio of source-lens:lens-sample distances. The beam flux, which was converted to flux density as about 1015 cps/mm2, was enough strong to obtain XRD profiles in sub-second exposure, even at the sample condition under few hundred GPa. Using this system, we have obtained clearly districted XRD profiles from sub-structures of microns size in DAC, such as electrodes for electrical resistance measurement, micro-anvils settled in the sample chamber, and so on. Micro-beam technique will bring us new applications for high-pressure experiments, and become more important in order to enhance precise analysis, such as grains/elements distribution, partial melting, solution and/or chemical reactions in a sample chamber of DAC, as well as in order to collect high-quality XRD under above 500 GPa. Furtehrmore, the sub-micron beam focusing is now achievable and will be a key-technique for a probe of a scanning XRD microscope.
abstracts
Following the development of high brilliance synchrotron x-ray sources, high density crystalline structures of elemental solids have been vastly studied at room temperature and elevated pressures. In the last decades, experimental and computational results have unveiled a vast diversity of crystalline structures adopted by many dense elements. Both complex modulated and exotic structures have been observed [1] and predicted [2]. In this communication, we report results of systematic searches for structural modifications taking place at very low temperature (T>10 K) and high pressure (P<50 GPa) in selected elementals solids. Results for cesium, calcium, barium, and selenium are presented. An extension of the known P-T phase diagram to lower temperature for cesium and selenium indicates that both elements do not adopt crystalline structures different that those already known and documented. We show that calcium at low temperature and high pressure, however, exhibits unusual and large dynamical fluctuations leading to a tetragonal distortion of the simple cubic structure known to exist at room temperature and about 30 GPa. The large amplitude fluxional behaviour leads to the appearance of a new phase, nested at T<30K between 40 and 45 GPa. Finally, barium when compressed at low temperature, transforms into a crystalline structure unobserved at high pressure and room temperature. It is found that, below 140K and in the pressure range of 13 to 35 GPa, barium does not adopt the phase IV structure, i.e., the modulated incommensurate cell, but undergoes a transition from phase II (P63/mmc) to an orthorhombic (Pmna) cell. This new structure corresponds to phase VI. On the basis of an x-ray diffraction study along quasi-isobaric and isothermal paths, we conclude that Ba-VI is most likely metastable. Our results suggest the need to scrutiny other dense elements at very low temperature. Under those conditions, unusual structural modifications are ought to be observed.
