Liquids (for example supercritical aqueous fluids and melts) under extreme conditions (of temperature, pressure or composition) play an important role in both natural and manmade systems, for example in planetary sciences, economic geology and hydrometallurgy, corrosion in power plants, catalysis, geo-engineering (for example geothermal energy and CO₂ sequestration) and material sciences (Brugger et al., 2010; Darr & Poliakoff, 1999). These applications rely on our understanding of the structure of liquids, of the nature and thermodynamic stability of relevant inorganic and organometallic complexes, and of the mechanisms and kinetics of reactions over a wide range of pressures and temperatures.

The increased availability of XAS combined with developments in spectroscopic cell design have had a profound impact on the study of melts and aqueous fluids at high pressure and temperature (Filipponi, 2001; Brugger et al., 2016), similar to its impact on catalysis (Agistini et al., 2023) and high-pressure research (Aquilanti, 2023). X-rays make it possible to probe the electronic structure of atoms in fluids in situ under extreme conditions. XAS is sensitive to the short-range structure around a specific atom and hence is ideally suited for the study of complex, multi-component liquids, such as those encountered in nature. The capacity of extended X-ray absorption fine structure (EXAFS) to provide direct quantitative information about the hydration and structure of metal complexes (including metal–ligand distances) in dilute solutions motivated much of the early work on hydrothermal solutions (see, for example, Pfund et al., 1994; Seward et al., 1993, 1995, 1996; Wallen et al., 1996). The quantitative nature of EXAFS and X-ray absorption near-edge structure (XANES) (see, for example, Benfatto et al., 2002; Liu et al., 2007) remains one of the main strengths of the technique. A recurrent theme in the field is the drive to fully exploit the unique features offered by X-ray beams at synchrotrons. For example, XAS studies have been combined with quantitative measurements of mineral solubility via measurements of X-ray fluorescence or step height (Fig. 1; see, for example, Daval et al., 2010; Liu et al., 2008; Pokrovski et al., 2005; Sanchez-Valle et al., 2004; Wilke et al., 2012) and with measurements of absolute absorbance used to measure the density of the fluid (see, for example, Etschmann et al., 2010; Pokrovski et al., 2008); such measurements also offer a convenient way to calibrate the temperature and pressure at the beam position by comparison with the known equation of state of a sample (Fig. 1). Time-resolved measurements have been used to investigate the kinetics of redox reactions (Cochain et al., 2013), either beam-induced or biologically induced (Mayanovic et al., 2012; Picard et al., 2012), and the kinetics of mineral dissolution (Daval et al., 2010; Testemale, Dufaud et al., 2009). The spatial resolution offered by focused X-ray beams has been used to probe separated phases such as fluid/melt (Louvel et al., 2013) and liquid/vapour (`boiling'; Pokrovski et al., 2008) or to limit the effects of beam-induced photoreduction (see, for example, Tooth et al., 2013).

Figure 1 XAS data for metals in hydrothermal fluids. In this example, aqueous solutions containing 0.045 M Fe2+, 0.35 M NaCl and 0.1 M HCl were measured in an autoclave at 500 bar from 100 to 450°C (Testemale, Brugger et al., 2009). Both transmission (left) and fluorescence (right) spectra were measured concurrently thanks to the geometry of the autoclave (a cross section is shown at the top). This is a typical example of low-concentration measurements where the fluorescence spectra are essential for good spectral data quality (XANES and EXAFS analysis), but the absorbance spectra (transmission) are useful for the quantitative determination of fluid density (the absolute absorbance levels are directly proportional to the fluid density) and of the dissolved metal concentration (which is derived from the variation of absorbance over the absorption edge).

The three main challenges that need to be addressed to measure high-quality XAS data on liquids at elevated temperature and/or pressure are the following. (i) The choice of materials, which need to be X-ray transparent and yet resistant to extremes in pressure and temperature, inert relative to samples that are commonly highly corrosive (for example acidic or strongly oxidizing or reducing), able to contain the liquid and sufficiently pure not to spoil the data with an unwanted contribution to the signal. (ii) A reliable control of reaction parameters, in particular pressure, temperature and sample composition, for long periods of time and with suitable precision. Collection of EXAFS data from dilute samples can take up to >10 h (see, for example, Pokrovski et al., 2009), and for low-density (<0.8 g cm⁻³) aqueous fluids small fluctuations in pressure will cause density fluctuations that are sufficient to affect the quality of the EXAFS data. (iii) Safety in the beamline environment.

Many types of experimental devices have been used for in situ XAS measurements of liquids at high temperature and/or pressure (see the exhaustive review by Filipponi, 2001) and some of them are succinctly presented below to illustrate the various approaches used and to highlight their advantages and limitations.

Cells operating at ambient or low pressure, under a controlled atmosphere or under vacuum have extensively been used to study solids at high temperatures and their temperature-driven structural and redox modifications (phase transitions, melting mechanisms, kinetics of oxidation–reduction reactions etc.). The same cell designs can be adapted for the study of melts and most are based on resistive heating. The graphite crucible-based `L'Aquila–Camerino oven' (Filipponi & Di Cicco, 1994) allows measurements up to 2250°C. The `heating-wire' design (Mysen & Frantz, 1992) has been replicated and optimized (Richet et al., 1993; Farges et al., 1995) and makes EXAFS measurements of melts possible up to 3500°C (under vacuum). These designs benefit from very short heating and cooling times (>100°C s⁻¹). High photon fluxes allow the collection of time-resolved XAS measurements at very high temperatures (Cochain et al., 2013).

The levitation technique is a notable alternative method for high-temperature measurements at low pressures; coupled with laser heating, it offers an efficient, simple and compact containerless method to place a sample in an X-ray or neutron beam (Hennet et al., 2011) and is suitable for XAS fluorescence measurements (Landron et al., 1997).

Fig. 2 shows the different families of cells that are frequently used for XAS measurements at high temperature and moderate to very high pressures.

Figure 2 Summary of the pressure range versus volume of the various families of high-pressure cells used for XAS in situ measurements of liquids. The temperature domains are imposed by the heating technology: <800°C, heating stage for fluid inclusions and capillaries; <1200°C, resistive heating for autoclaves and Paris–Edinburgh cells; <3000°C, laser heating for DAC. The grey band along the volume axis depicts the range of volumes probed with synchrotron beams.

Ionizing radiation induces the radiolysis of water, thus producing radiolytic species such as , H₃O⁺, , , and the hydrated electron, , which upon reaction with one another produce redox-active species such as hydrogen peroxide and dissolved hydrogen [H₂(aq)] and oxygen [O₂(aq)] (Jayanetti et al., 2001; Saffré et al., 2011). As reviewed by Brugger et al. (2016), the effects of radiolytic species on the oxidation states of metal ions in solutions within a spectroscopic cell under X-ray illumination are well established, yet are difficult to predict. The nature of the effect (for example reduction versus oxidation) and the rate of the photoreaction depend upon the solution composition, temperature and the volume of the fluid compared with the volume affected by the beam (Fig. 2). The effects of beam damage need to be carefully considered when interpreting XAS data on hydrothermal reactions. These effects are particularly extreme in the case of small sample volumes, such as fluid inclusions and DAC microbeam experiments (see, for example, Mayanovic et al., 2012), where the sample volume is a few nanolitres and the beam flux is very high, at insertion-device beamlines. In the case of larger sample volumes (relative to the volume of fluid within the beam) and/or lower flux bending-magnet beamlines, beam damage can have beneficial effects, for example by promoting the rate of conversion of the ion in solution to the thermodynamically stable species (see, for example, Fulton et al., 2000) or the rate of mineral dissolution in the case of in situ solubility experiments.

The study of fluids under extreme conditions is a rapidly evolving field. Current trends include (i) co-measurements of EXAFS and other spectroscopies (for example optical and vibrational spectroscopy, small-angle scattering), as well as better control of solution chemistry parameters, in particular pH and redox, (ii) accessing higher pressures with autoclave designs, since deep-earth processes are a hot topic in hydrothermal geochemistry (see, for example, Sverjensky et al., 2014), (iii) lower energy (lower Z elements) spectroscopy enabled by new materials and (iv) adaptation to new detection methods on XAS beamlines, in particular high spectral resolution spectroscopy (Hazemann et al., 2009).