A miniature multi-anvil apparatus using diamond as anvils-MDAC: Multi-axis diamond anvil cell.

The diamond anvil cell (DAC) has been widely used in high-pressure research. Despite significant progress over the past five decades, the opposed anvil geometry in the DAC inevitably leads to a disk-shaped sample configuration at high pressure. This intrinsic limitation is largely responsible for the large pressure and temperature gradients in the DAC, which often compromise precise experiments and their characterizations. We designed and fabricated a multi-axis diamond anvil cell (MDAC) by adopting the concept of a multi-anvil apparatus but using single crystal diamonds as the anvil material. Preliminary data show that the MDAC can generate extreme pressure conditions above 100 GPa. The advantages of the MDAC over a traditional opposed anvil DAC include thicker, voluminous samples, quasi-hydrostatic, or designed deviatoric stress conditions, and multidirectional access windows for optical applications and x-ray probes. In this article, we present the design and performance of a prototype MDAC, as well as the application prospects in high-pressure research.

In situ x-ray diffraction study of polyamorphism in H2O under isothermal compression and decompression.

Amorphous-amorphous transformations in H2O have been studied under rapid isothermal compression and decompression in a diamond anvil cell together with in situ x-ray diffraction measurements using brilliant synchrotron radiation. The experimental pathways provide a density-driven approach for studying polyamorphic relations among low-, high-, and very high-density amorphs (LDA, HDA, VHDA) in a pressure range of 0-3.5 GPa at temperatures of 145-160 K. Our approach using rapid (de)compression allows for studying the polyamorphic transformations at higher temperatures than the conditions previously studied under slow (de)compression or isobaric annealing. Multiple compression-decompression cycles can be integrated with in situ x-ray measurements, thus facilitating the study of repeatability and reversibility of the polyamorphic transformations. Fast in situ x-ray diffraction measurements allow for obtaining detailed insight into the structural changes across polyamorphic transformations regarding the (dis)continuity, reversibility, and possible intermediate forms. As demonstrated at isothermal conditions of 145 K and 155 K, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA') appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and HDA' and a continuous change within HDA' involving structural reconfigurations and finally converging to VHDA.

Nuclear resonant inelastic X-ray scattering at high pressure and low temperature.

A new synchrotron radiation experimental capability of coupling nuclear resonant inelastic X-ray scattering with the cryogenically cooled high-pressure diamond anvil cell technique is presented. The new technique permits measurements of phonon density of states at low temperature and high pressure simultaneously, and can be applied to studies of phonon contribution to pressure- and temperature-induced magnetic, superconducting and metal-insulator transitions in resonant isotope-bearing materials. In this report, a pnictide sample, EuFe2As2, is used as an example to demonstrate this new capability at beamline 3-ID of the Advanced Photon Source, Argonne National Laboratory. A detailed description of the technical development is given. The Fe-specific phonon density of states and magnetism from the Fe sublattice in Eu(57)Fe2As2 at high pressure and low temperature were derived by using this new capability.