Session M20: Matter at Extreme Conditions: Static and Dynamic Experiments

Using high-pressure conditions to access novel structures and engender magnetic control in lanthanide materials

A rational approach to designing the next generation of quantum materials requires hypothesis-driven synthesis. Yet, the complex phase space of solid-state systems and the small energy scales that determine electronic and magnetic order are such that subtle changes in composition can drastically change the structure and properties of the resulting material. In comparison, pressure provides an incrementally tunable vector, serving to increase orbital overlap and leading to more significant covalent interactions and charge delocalization. Thus, the application of static pressure opens up vast regions of phase-space for synthesizing new solid-state materials. However, often times the same attributes that challenge our understanding also lead them to not be recoverable to ambient conditions. It is therefore vital to characterize the structure and properties of these materials in situ in order to gain insight into how these new structural motifs influence the behavior of the materials. In this work, we describe our multimodal approach combining high-pressure synthesis with spectroscopy and calculations to bring new chemical insight into the discovery of materials containing lanthanides primed to exhibit exotic magnetic behaviors. To realize promising synthetic targets, we turned to both traditional solid-state techniques as well as high-pressure experiments in diamond anvil cells. We will discuss our burgeoning chemical intuition for structure formation and metastability in lanthanide intermetallic phase space, as well as present our ongoing efforts towards correlating new structures with magnetic properties both at ambient pressures and under high-pressure synthesis conditions.   

Resonant inelastic x-ray scattering beyond the SASE resolution limit

Resonant inelastic x-ray scattering (RIXS) is a widely used spectroscopic technique, providing access to the electronic structure and dynamics of atoms, molecules, and solids. However, RIXS requires a narrow bandwidth x-ray probe to achieve high spectral resolution. The challenges in delivering an energetic monochromated beam from an x-ray free electron laser (XFEL) thus limit its use in few-shot experiments, including for the study of high energy density systems.

Here we demonstrate that by correlating the measurements of the SASE x-ray spectrum and the RIXS signal, combined with dynamic kernel deconvolution using differentiable programming with a neural surrogate, we can achieve electronic structure resolutions substantially higher than that normally afforded by the bandwidth of the incoming x-ray beam. We further show how this technique allows us to discriminate between the valence structures of Fe and Fe₂O₃, and provides access to temperature measurements and M-shell binding energies estimates in warm-dense Fe compounds.   

Observations of twinning in iron ramp-compressed through the α-ε phase transition

In situ x-ray diffraction experiments of single crystal iron ramp-compressed along the [100] axis at 4 GPa/ns through the α-ε phase transformation at 13 GPa were performed at the Dynamic Compression Sector. Unlike shock experiments, in these ramp compression experiments the bcc lattice is observed to be nearly isotropically relaxed before the phase transition. There was a mixed α/ε phase region starting at ~13 GPa which was observed for 0.75 ns until the peak stress of 18 GPa was reached. In situ x-ray diffraction measurements show the formation of new hcp orientations not reported in shock or quasistatic compression experiments. The new hcp orientations appear to be caused by sequential twins that occur during the phase transition. This twinning mechanism relieves the shear strain caused by the bcc-hcp phase transition from the isotrpically relaxed bcc phase while subject to uniaxial compression. These results show that in iron the induced microstructure through a phase transition and the phase transition mechanism depend on the loading history.   

Laser ablation physics and hydrodynamic code simulation validation from picosecond to nanosecond pulse duration

We report on a series of experiments spanning multiple energy regimes ranging from microJoules to hundreds of Joules and pulse lengths ranging from picosecond to nanosecond. These experiments were conducted to provide experimental data to validate hydrodynamic code simulations and to study ablation physics. The data discussed here includes experiments conducted from 10^10 to 10^13 W/cm2 peak laser intensity, from 1w (infrared) to 2w (green) laser wavelength, and across tamped and untamped samples consisting of tamper materials like sapphire, lithium fluoride, coverslip glass, and Teflon. The target materials consist of either aluminum or silicon and the experiments mentioned here include a small scale tabletop laser experiment, a reanalysis of data taken at LCLS at SLAC, and larger user-facility experiments. With this data, we hope to improve our understanding of laser ablation and how it scales as a function of wavelength and intensity. We further report on efforts to use these data sets to validate two radiation hydrodynamic simulation codes, FLASH and DRACO. LLNL-ABS-855975.   

Phase transitions in aluminum under shock-ramp compression

Aluminum 6061 alloy has been used extensively as an electrode material in dynamic-compression experiments at the Z machine. Such experiments would benefit from improved understanding of aluminum's equation of state in the multi-megabar pressure range. Previous theoretical works suggest that aluminum has three structural phases in the solid with an equilibrium triple-point between fcc (ambient), hcp, and bcc structures in the vicinity of 200-300 GPa and 2000-4000 K, but with significant discrepancies between results. Previous X-ray diffraction measurements have detected the hcp phase above ~200 GPa on the room-temperature isotherm under static compression in a diamond-anvil cell, and both the hcp and bcc phases above ~220 GPa and ~320 GPa, respectively, near the principal isentrope under laser-based dynamic compression. To better constrain the triple-point pressure, a series of shock-ramp experiments is underway at Sandia's Z machine to measure compressibility along elevated-temperature quasi-isentropes. I will review the experimental method and present preliminary results from analyses of velocimetry data.   

Pressure-induced metallization and isostructural transitions in MoS2

A joint experimental X-ray diffraction, Raman spectroscopy and electrical conductivity measurements, and theoretical study has been performed on molybdenum disulfide (MoS₂) up to 67 GPa. We observe discontinuous changes in Raman spectra and synchrotron x-ray diffraction patterns which provide evidence for isostructural phase transitions. These isostructural phase transitions are accompanied by pressure-induced metallization as evident from the four-probe resistivity measurement. Our theoretical calculations reveal that metallization happens due to overlap of valence and conduction bands as the interlayer spacing reduces upon pressurization. Upon decompression, both isostructural transition and metallization are found to be reversible. Our study on pressure-induced tuning of bandgap/conductivity in MoS₂ will play a vital role in the development of the next generation devices based on transition metal dichalcogenides.   

In-situ high-pressure neutron scattering study of ice phases Ih, II, III and VI

Water ice has very rich P-T phase diagram with known 20 crystalline phases and at least three amorphous states, and their properties are often exhibiting anomalous behavior. To better understand the nature of hydrogen bonds, study of water-water interactions in these phases under equilibrium conditions is very important. Recently, using a piston-cylinder pressure cell (~20 mm3) and a new bellows assembly designed by DAC Tools, LLC, we can load and change the pressure up to 11 kbar at low temperatures. With this high-pressure setup, we measured diffraction and inelastic neutron scattering (INS) in-situ under high pressure for hydrogen-disordered phases ice Ih, III and VI, and for hydrogen-ordered phase ice II. Lattice and molecular dynamics simulations were also performed for these ice phases and compared to the INS spectra. We observed large broadening of the intra-molecular H-O-H bending mode for the hydrogen-ordered and -disordered ice phases, which is explained by the Fermi resonance coupling between the overtone of the librational band and the bending mode of water.   

High stability of Ba3(ZnB5O10)PO4 under pressures

Ba3(ZnB5O10)PO4 (BZBP) stands out as a unique nonlinear optical material, notably as the only one devoid of Beryllium (Be), and it maintains transparency across an extensive range of wavelengths—from thousands of nanometers to the deep-ultraviolet (DUV) region. This distinctive characteristic positions BZBP as a highly promising material for constructing a pivotal component in systems capable of generating DUV lasers. Earlier investigations have highlighted key attributes of BZBP, such as low anisotropic thermal expansion, high specific heat, and superior thermal conductivity, affirming its suitability for DUV laser generation. However, these studies were limited to ambient pressures. Before advancing its practical applications, a comprehensive understanding of BZBP's behavior under both ambient and extreme conditions is imperative. In the current study, BZBP underwent high-pressure conditions, and synchrotron X-ray diffraction was employed to scrutinize its physical properties. X-ray patterns collected at various pressure points, reaching up to 43 GPa, revealed the material's stability without undergoing any phase transformation. The investigation further entailed the determination of its bulk modulus (110 GPa) and linear compressibility along each lattice axis. Additionally, we elucidated the changes in bonding lengths relative to varying pressures, providing valuable insights into BZBP's behavior under high-pressure conditions.   

Progress in toroidal diamond anvil fabrication for use at the upgraded Advanced Photon Source

Applying external pressure in the study of materials allows one to control their electronic and magnetic orders, often resulting in novel emerging phenomena. A path to ultra-high static pressure generation has been opened by the introduction of toroidal diamond anvils. Owing to their reduced culet size and toroidal groove, these anvils have been shown to exceed static pressures of 600 GPa [1, 2]. In this talk, we present the recent developments in the fabrication of toroidal anvils. A versatile, user-friendly fabrication process has been developed. We discuss the reproducibility of the method, as well as preliminary results. Finally, we highlight some of the new possibilities for users coming from combining toroidal anvils with the upgraded Advanced Photon Source.

[1] A. Dewaele et al., Nat Commun 9, 2913 (2018).

[1] Z. Jenei et al., Nat Commun 9, 3563 (2018).   

Pressure-induced structural transitions at low temperature conditions

The researches on high pressure induced phase transitions are long time hot topics in condense matter physics. Many interesting behaviors, such as high Tc superconductivity, were reported recently, but the responsible structures are not well studied at correct P-T conditions and far away from fully understood. In this presentation, several selected systems were in situ studied, including powder and single crystal samples, under high pressure using lab base and synchrotron x-ray diffraction (XRD) techniques in diamond anvil cell at ambient temperature and low temperature conditions, will be presented. Selected cases, such as elements with record high Tc (Ti and Sc), metal hydrides (Ca-H, Lu-H, Hf-H), La-Ni-O system, would be discussed based on the recent XRD results and the first principle calculations, and then propose the possible general trend for their phase diagrams at low temperature and high pressure region.   

Strength and Structure of Ce up to 300GPa

Cerium (Ce) is the most abundant of the rare-earth metals and is of great interest to the high-pressure community due to its unique behavior under pressure; it has a very complex phase diagram, including an isostructural volume collapse unique to the lanthanides and attributed to two minima in its interatomic potential. Above 13 GPa, Ce stabilizes in a body-centered tetragonal (bct, tI2) phase. Using bevelled and toroidal DACs, we have statically compressed Ce to multi-megabar conditions and observe changes in the axial ratio and abnormal compressive behavior. We explore the effect of shear stress on these samples and present computational results in support of the experimental data.   

Vibrational Dynamics of Hydrazine to 50 GPa.

Raman, mid-infrared, and far-infrared spectroscopy have been used to investigate pressure-induced phase transitions, changes in hydrogen bonding, and intermolecular interactions of hydrazine up to 50 GPa. Mid-infrared measurements show splitting in the symmetric and asymmetric N-H stretching vibration bands near a phase transition at 30 GPa, indicating a change in hydrogen bonding. The transition exhibits a large hysteresis loop of ~20 GPa on decompression. Far-infrared measurements reveal splittings and the appearance of new peaks. Raman measurements show lattice vibrational modes splitting and broadening near the phase transitions at 20 and 30 GPa. The results are compared with x-ray diffraction data and first principles simulations.