A CO2 laser heating system for in situ high pressure-temperature experiments at HPCAT

Stable and metastable structures of tin (IV) oxide at high pressure

We have analysed [Formula: see text] with a combination of synchrotron X-ray diffraction and X-ray absorption spectroscopy across a pressure range of [Formula: see text] GPa with thermal annealing by a [Formula: see text] laser allowing access to all of the known high-density polymorphs of [Formula: see text], and here report their crystallographic information. The metastability of the post-rutile [Formula: see text]-[Formula: see text] and [Formula: see text] structures in [Formula: see text] are investigated by experiment and PW-DFT simulations, revealing a complex energetic landscape and suggesting a significant dependence of the observed phases on the pressure-temperature pathway taken in experiment. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

High-P-T impedance measurements using a laser-heated diamond anvil cell

The electrical conductivity (EC) of minerals found on Earth and throughout the solar system is a fundamental transport property that is used to understand various dynamical phenomena in planetary interiors. High-pressure and high-temperature (P-T) EC measurements are also an important tool for observing phase transitions. Impedance measurements can accurately measure the EC of a nonmetallic sample. In previous measurements under static conditions using a laser-heated diamond-anvil cell (LHDAC), only direct current resistance is measured, but this method overestimates the bulk sample resistance. Moreover, the previous methodology could only be applied to nontransparent samples in an LHDAC using infrared lasers, limiting the range of measurable composition. To the best of our knowledge, no in situ high-P-T EC measurements of transparent materials have been reported using LHDAC techniques. We developed a novel impedance measurement technique under high-P-T conditions in an LHDAC that applies to transparent samples. As a validation, we measured the EC of Mg_0.9Fe_0.1SiO₃ bridgmanite up to 51 GPa and 2000 K and found that the results are consistent with those of previous studies. We also measured the EC values of sodium chloride to compare with those of previous studies, as well as those of cubic boron nitride and zirconia cement to quantify how well they insulate under high P-T conditions. This is the first report of the impedance and EC measurements of transparent minerals in an LHDAC, which allows the measurement of Fe-poor/-free materials, including the major constituents of the interiors of gas giants and icy planets, under extreme conditions.

Overview of HPCAT and capabilities for studying minerals and various other materials at high-pressure conditions

High-Pressure Collaborative Access Team (HPCAT) is a synchrotron-based facility located at the Advanced Photon Source (APS). With four online experimental stations and various offline capabilities, HPCAT is focused on providing synchrotron x-ray capabilities for high pressure and temperature research and supporting a broad user community. Overall, the array of online/offline capabilities is described, including some of the recent developments for remote user support and the concomitant impact of the current pandemic. General overview of work done at HPCAT and with a focus on some of the minerals relevant work and supporting capabilities is also discussed. With the impending APS-Upgrade (APS-U), there is a considerable effort within HPCAT to improve and add capabilities. These are summarized briefly for each of the end-stations.

CO2 laser heating system for in situ radial x-ray absorption at 16-BM-D at the Advanced Photon Source

We present a portable CO₂ laser heating system for in situ x-ray absorption spectroscopy (XAS) studies at 16-BM-D (High Pressure Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory). Back scattering optical measurements are made possible by the implementation of a Ge beamsplitter. Optical pyrometry is conducted in the near-infrared, and our temperature measurements are free of chromatic aberration due to the implementation of the peak-scaling method [A. Kavner and W. R. Panero, Phys. Earth Planet. Inter. 143-144, 527-539 (2004) and A. Kavner and C. Nugent, Rev. Sci. Instrum. 79, 024902 (2008)] and mode scrambling of the input signal. Laser power stabilization is established using electronic feedback, providing a steady power over second timescales [Childs et al., Rev. Sci. Instrum. 91, 103003 (2020)]-crucial for longer XAS collections. Examples of in situ high pressure-temperature extended x-ray absorption fine structure measurements of ZrO₂ are presented to demonstrate this new capability.

Laser heating system at the Extreme Conditions Beamline, P02.2, PETRA III

A laser heating system for samples confined in diamond anvil cells paired with in situ X-ray diffraction measurements at the Extreme Conditions Beamline of PETRA III is presented. The system features two independent laser configurations (on-axis and off-axis of the X-ray path) allowing for a broad range of experiments using different designs of diamond anvil cells. The power of the continuous laser source can be modulated for use in various pulsed laser heating or flash heating applications. An example of such an application is illustrated here on the melting curve of iron at megabar pressures. The optical path of the spectroradiometry measurements is simulated with ray-tracing methods in order to assess the level of present aberrations in the system and the results are compared with other systems, that are using simpler lens optics. Based on the ray-tracing the choice of the first achromatic lens and other aspects for accurate temperature measurements are evaluated.

Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science.In situexperiments and atomistic simulations can provide crucial insight into the nature of liquid-liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.

A Practical Review of the Laser-Heated Diamond Anvil Cell for University Laboratories and Synchrotron Applications

In the past couple of decades, the laser-heated diamond anvil cell (combined with in situ techniques) has become an extensively used tool for studying pressure-temperature-induced evolution of various physical (and chemical) properties of materials. In this review, the general challenges associated with the use of the laser-heated diamond anvil cells are discussed together with the recent progress in the use of this tool combined with synchrotron X-ray diffraction and absorption spectroscopy.

A portable on-axis laser-heating system for near-90° X-ray spectroscopy: application to ferropericlase and iron silicide

A portable IR fiber laser-heating system, optimized for X-ray emission spectroscopy (XES) and nuclear inelastic scattering (NIS) spectroscopy with signal collection through the radial opening of diamond anvil cells near 90°with respect to the incident X-ray beam, is presented. The system offers double-sided on-axis heating by a single laser source and zero attenuation of incoming X-rays other than by the high-pressure environment. A description of the system, which has been tested for pressures above 100 GPa and temperatures up to 3000 K, is given. The XES spectra of laser-heated Mg_0.67Fe_0.33O demonstrate the potential to map the iron spin state in the pressure-temperature range of the Earth's lower mantle, and the NIS spectra of laser-heated FeSi give access to the sound velocity of this candidate of a phase inside the Earth's core. This portable system represents one of the few bridges across the gap between laser heating and high-resolution X-ray spectroscopies with signal collection near 90°.

Oxidation of High Yield Strength Metals Tungsten and Rhenium in High-Pressure High-Temperature Experiments of Carbon Dioxide and Carbonates

The laser-heating diamond-anvil cell technique enables direct investigations of materials under high pressures and temperatures, usually confining the samples with high yield strength W and Re gaskets. This work presents experimental data that evidences the chemical reactivity between these refractory metals and CO2 or carbonates at temperatures above 1300 °Ϲ and pressures above 6 GPa. Metal oxides and diamond are identified as reaction products. Recommendations to minimize non-desired chemical reactions in high-pressure high-temperature experiments are given.

From Molecules to Carbon Materials—High Pressure Induced Polymerization and Bonding Mechanisms of Unsaturated Compounds

With the development of high-pressure apparatus, in situ characterization methods and theoretical calculations, high-pressure technology becomes a more and more important method to synthesize new compounds with unusual structures and properties. By compressing compounds containing unsaturated carbon atoms, novel poly-ionic polymers, graphanes and carbon nanothreads were obtained. Their compositions and structures were carefully studied by combining multiple cutting-edge technologies, like the in situ high-pressure X-ray and neutron diffraction, transmission electron microscopy, pair distribution function, solid-state nuclear magnetic resonance and gas chromatography-mass spectroscopy. The reaction mechanisms were investigated based on the crystal structure at the reaction threshold pressure (the pressure just before the reaction taking place), the long-range and short-range structure of the product, molecular structure of the intermediates, as well as the theoretical calculation. In this review, we will summarize the synthesis of carbon materials by compressing the unsaturated compounds and its reaction characteristics under extreme conditions. The topochemical reaction mechanism and related characterization methods of the molecular system will be highlighted. This review will provide a reference for designing chemical reaction and exploring novel carbon materials under high-pressure condition.

Probing disorder in high-pressure cubic tin (IV) oxide: a combined X-ray diffraction and absorption study

The transparent conducting oxide, SnO₂, is a promising optoelectronic material with predicted tailorable properties via pressure-mediated band gap opening. While such electronic properties are typically modeled assuming perfect crystallinity, disordering of the O sublattice under pressure is qualitatively known. Here a quantitative approach is thus employed, combining extended X-ray absorption fine-structure (EXAFS) spectroscopy with X-ray diffraction, to probe the extent of Sn-O bond anharmonicities in the high-pressure cubic (Pa\bar{3}) SnO₂ - formed as a single phase and annealed by CO₂ laser heating to 2648 ± 41 K at 44.5 GPa. This combinational study reveals and quantifies a large degree of disordering in the O sublattice, while the Sn lattice remains ordered. Moreover, this study describes implementation of direct laser heating of non-metallic samples by CO₂ laser alongside EXAFS, and the high quality of data which may be achieved at high pressures in a diamond anvil cell when appropriate thermal annealing is applied.

Pressure-Tuneable Visible-Range Band Gap in the Ionic Spinel Tin Nitride

The application of pressure allows systematic tuning of the charge density of a material cleanly, that is, without changes to the chemical composition via dopants, and exploratory high-pressure experiments can inform the design of bulk syntheses of materials that benefit from their properties under compression. The electronic and structural response of semiconducting tin nitride Sn3 N4 under compression is now reported. A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue-shift spanning the entire visible spectrum. The pressure-mediated band gap opening is general to this material across numerous high-density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure-tuneable electronic properties for future applications.