Based on the high brightness third generation synchrotron sources and development of tunable monochromators with sub-meV resolution, nuclear resonant X-ray spectroscopy has become a relatively new spectroscopic method in recent years and widely used to study materials under extreme conditions. Nuclear resonant X-ray spectroscopy can be divided into two methods: nuclear resonant inelastic X-ray scattering (NRIXS) and nuclear forward scattering (NFS). From NRIXS, density of states can be acquired and important dynamic, thermodynamic, and elastic information derived such as vibrational kinetic energy, vibrational entropy, Debye temperature and sound velocities. From NFS, hyperfine interaction parameters can be acquired giving information on spin state, oxidation state and magnetic ordering.

57Fe nuclear resonant x-ray spectroscopy has attracted particular attention among physicists and geophysicists because Fe is an archetypal transition element and is a dominant component in the cores of the Earth and other terrestrial planets. At HPCAT, a 2-meV high resolution monochromator (HRM) is used for 57Fe and consists of 2 channel cut silicon crystals (Si( 4 4 0) and Si (9 7 5)). We can now routinely measure NRIXS under high pressures using panoramic DACs and two or three APD detectors in close proximity and NFS while under high pressure and low temperature.

During NRS experiments, the bandwidth of the undulator beam is reduced to ~1eV by a liquid N₂-cooled Si (1 1 1) double crystal monochromator (DCM) in 16 ID-A. After going through HRM in IDC, the bandwidth of X-ray beam is further reduced to ~2meV. The beam then is focused by KB mirrors hitting the sample in IDD. NRS signals are collected by one avalanche photodiode detector (APD) at the forward direction for an NFS experiment or multiple APDs  which are put as close as possible to the sample for NRIXS experiments.

In an XRS experiment, the incident beam hits the sample, scattered x-ray is collected by spherically-bent single-crystal analyzers and focused to the solid state detector in a nearly backscattering geometry. The incident x-ray energy is scanned relative to the elastic line to determine the inelastic shift. Using the same experimental setup of XRS, we can do an inelastic x-ray scattering experiment to get electronic band structure, excitions, plasmons, and their dispersions under high pressure.  During IXS experiments, both incident x-ray energy and scattering angle need to be scanned to obtain the dielectric function and the dynamic structure factor.

To improve the collection efficiency, we have designed, tested and finished commissioning a 17-element analyzer array.  The array consists of three columns of 2-inch diameter bent silicon (111) analyzers from NJ-XRSTech in a vertical Rowland circle backscattering geometry. The energy resolution of the instrument is 1.0 eV.To reduce the scattering from the surrounding gasket, at the end of 2016, we have made a comparison of x-ray Raman spectra using post-sample slits versus that of a polycapillary optic.  The advantage of the polycapillary is that its field of view is about 50 um, so it does not ‘see’ as much of the gasket scattering.  The tradeoff is that the polycapillary has about a 20% transmission efficiency but a wider acceptance angle of collected scattering.   The results of our comparison show a significant improvement in the signal/background ratio using the polycapillary.  For the same sample and counting times, the background is reduced by an about a factor of 5 using the polycapillary.  This is essential to be able to measure samples at mega-bar pressures.

X-ray emission spectroscopy (XES) provides a unique probe for the diagnosis of pressure-induced spin transition in materials. HP XES experiments have become a routine technique at 16IDD and have been applied to both 3d transition and rare earth metals and their compounds (Mn, Fe, Co, Ce, Gd, etc.).

Resonant X-ray emission spectroscopy (R-XES) studies of transition-metal and rare-earth systems give us important information on electronic states, such as intra-atomic multiplet coupling, electron correlation, and inter-atomic hybridization. In HP R-XES experiments, we use the same setup for XES, but also scan the incident energy across the absorption edge and for each incident energy, fluorescence spectra are collected by scanning analyzers.

Recently, we developed a new cryostat used to measure Fe K-beta XES spectra under different pressures and low temperatures. New Fe-based superconductors were successfully measured. This new development will allow us to study magnetic properties and spin transitions of many functional materials (colossal magnetoresistance material, high temperature superconductor etc.) under high pressure and low temperature.

During XES experiments, emitted photons are collected at 90 degrees from the incident beam by a 4-inch analyzer, which focuses and energy analyzes the emitted photons.  A range of elements can be studied by selecting from Si(111), Si(220), Si(422), Si (331),Si(620) or Si(400) analyzers. 

To improve collection efficiency of the XES experiments, we have begun to commission a seven-analyzer XES spectrometer in Aug 2015. The spectrometer is shown in Figure below. The spectrometer has seven 4-inch spherical bent analyzers arranged in a hexagonal pattern to make it compact and accommodate the opening of the most used 2-inch symmetric diamond anvil cells. Fluorescence from the sample is spectrally analyzed and reflected to a Pilatus 100K position sensitive detector by these analyzers. Right now, only 6 pre-aligned emission line can use all 7 analyzers. We are in the process to motorize angle and radial motions of all analyzers to use 7 analyzers for every emission line.

Falling sphere viscosity measurement was developed using white X-ray radiography with high-speed camera (>1000 frame/second). The high-speed white x-ray radiography setup enables us to investigate viscosity of a wide range of materials including very low viscos liquids (around 1 mPa s or less). Figure 1 shows an example of X-ray radiography images of falling of WC sphere in liquid sulfur in PE cell at high pressure recorded with 5000 frames/second and 0.2 ms exposure time.   In addition, the viscosity measurement can be combined with liquid structure measurement to investigate in situ correlation between structure and viscosity of liquids at high pressures.