at the Advanced Photon Source

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Optimizing the signal-to-background ratio is an enormous challenge in the context of static high pressure research.  Weak scattering—from samples with typical dimensions on the order of a few tens of microns or less—can be overcome by a strong background arising from the sample chamber’s surrounding material, often several millimeters thick.  One strategy to suppress the background is to incorporate the use of a multi-channel collimator (also commonly referred to as Soller slits, given their similarity to slit systems used to reduce axial divergence in laboratory diffractometers).  The multi-channel collimator (MCC) consists of two precisely aligned vertical slit assemblies each containing 101 tungsten carbide blades.  The MCC blocks all x-ray scattering from reaching the detector except for the scattering arising from a discrete sample volume defined by the spacing between blades and the distance between the two slit assemblies.  In practice the MCC sampling volume is aligned coincident to the high pressure sample position and rotated about the sample during x-ray exposure.  The scattering from the sample is able to reach the detector whereas the majority of the scattering from the surrounding material and air is blocked by the MCC, significantly increasing the signal-to-background ratio.  HPCAT’s multi-channel collimator is a custom, in-house design with a substantial increase in throughput and angular range (60 degrees in the horizontal plane, 30 degrees in the vertical plane) as compared to other existing assemblies.

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Synthesizing new materials and preserving them at PT conditions where they can be studied in detail can present serous challenge. For example some metal hydrides, which exhibit high temperature superconductivity at high pressure, can be synthesized in Diamond Anvil Cell using laser heating techniques, but often can only be preserved at low temperature through fast quenching (µs time frame) to cryogenic temperatures. To enhance synthesis capability of such materials we have designed the cryostat compatible with HPCAT’s stationary laser heating system located in 16-IDB. The cryostat has a modular design allowing quick modifications for use with different Diamond Anvil Cells and different pressure drives (e.g. double membrane compression / decompression drive, piezo drive, mechanical (screw) drive, etc.). The cryostat features stable sample holder design (no sample motion during cryogenic cooling of a DAC) and flexibility to use various functional DAC attachments such as modulation coils for magnetic susceptibility measurements. The modular flexible design and abundance of electrical feed-throughs (e.g. coaxial connectors reading weak signal from signal pickup coil) allows integration of multiple sample synthesis and analytical techniques, e.g. the sample can be synthesized at high temperature using laser heating setup with in-situ online x-ray diffraction characterization, and then magnetic susceptibility and Raman spectra can be measured after rapid cooling of the sample to cryogenic temperatures. This unique instrument is scheduled to be commissioned in mid-February 2018 and will be first used for synthesis and in-situ characterization of high Tc superconductive metal hydrides.

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