Chapter 3.30. Extreme conditions: high pressure/high temperature

Pressure is a fundamental thermodynamic parameter to alter the physical properties of matter and is a central aspect in Earth and planetary sciences. Pressure is also becoming increasingly important in the synthesis and design of new technological materials. Details of the effect of pressure and its applications in different disciplines can be found in Itié & Pascarelli (2024).

Research at high pressure takes full advantage of all of the characteristics of XAS. In particular, an advantage of XAS with respect to other methods is that, given its chemical selectivity, X-ray absorption spectra solely contain the signal from the absorbing element without any interference from the container or the experimental environment. This is especially convenient when the sample environment is particularly bulky, as is the case in high-pressure experiments. Finally, the possibility of retrieving the coordination geometry of the atom of interest and its speciation provides information about possible chemical reactions that may occur under extreme conditions.

The popularity of studies at high pressure is correlated to the technical development of devices that are able to produce such conditions, with the most popular being diamond anvil cells (DACs) for static pressure generation. For XAS studies at synchrotron facilities such devices may require modifications to cope with the specific needs of the technique. This chapter describes the most common devices used to generate static pressure in combination with high temperature for use in XAS studies. A particular emphasis will be given to the peculiarities, requirements, issues and technical solutions related to the use of high-pressure/high-temperature devices. In a continuous effort to achieve higher pressures and higher temperatures in order to unveil new phenomena, dynamic compression techniques are used. These are briefly described and a few examples of XAS studies employing such techniques are mentioned.

The Paris–Edinburgh cell (PEC) allows high-pressure and high-temperature measurements to be performed in situ on large crystalline, liquid or amorphous samples. At ambient temperature, high pressures of up to 15 GPa can routinely be reached (Klotz et al., 1996). At high temperatures of up to 2300 K simultaneous compression to 10 GPa can be reached (Kono et al., 2014), while at higher pressures the temperature is limited to ∼1000 K (Morard et al., 2007). Using the PEC, large sample volumes can be employed (1–10 mm³). This permits a good control of stoichiometry and favours the study of diluted materials. Importantly, the PEC provides stable pressure conditions and uniform heating of the sample (Rosa et al., 2016). Originally designed for neutron diffraction experiments (Besson et al., 1992), the PEC has been successfully adapted to XAS (Buontempo et al., 1998; Katayama, 2001; Poloni et al., 2005; Principi et al., 2007; Vaccari et al., 2010; Pohlenz et al., 2016; Cochain et al., 2015).

A schematic drawing of the PEC is shown in Fig. 1. The force on the seats of the anvils is generated by a piston–cylinder assembly pressurized by a high-pressure oil circuit. Two opposite anvils concentrate the force onto a small area of about 1/100 of the piston surface, hence providing a magnification of the pressure by a factor of 100. For X-ray applications, the press hosts anvils with truncated conical profiles made either of tungsten carbide or sintered diamond (labelled `B' in Fig. 1). A typical sample assembly is confined between the two anvils. The geometry is such that the X-rays pass through perpendicularly to the load direction. To minimize X-ray absorption all assembly parts should be made of low-Z materials, while maintaining good mechanical and thermal properties. A standard sample assembly for X-rays can be found in Mezouar et al. (1999). Several modifications have been realized with different purposes, such as to increase the mechanical stability at high pressure and temperature or to increase the chemical inertness of the sample container. A review of the evolution of the sample assembly for XAS applications in particular can be found in Rosa et al. (2016). The temperature can be measured by an insulated K-type thermocouple placed in the sample gasket. An additional estimate of the temperature ramp can be obtained from the power delivered to the sample through previously calibrated power curves in pressure–temperature space. The pressure is usually measured by in situ X-ray diffraction following the lattice contraction of internal pressure markers added to the sample, the PVT equation of state of which is known. For high-pressure/high-temperature measurements, both pressure and temperature can be cross-calibrated from the equations of state of two pressure–temperature markers (Crichton & Mezouar, 2002), possibly with a contrasting dependence on their thermoelastic properties (i.e. MgO and NaCl or rhenium and NaCl).

Figure 1 Schematic representation of a Paris–Edinburgh press. Anvils made of tungsten carbide or sintered diamond are marked B. (Courtesy of Nicola Novello.)

The PEC has particularly been used to understand structure and physical properties at high pressure and temperature, in particular for liquids and melts, for which the understanding of the physics remain a challenge, especially at high pressures. As an example, we can report a study of solid and liquid tin in the 0–4 GPa pressure and 300–850 K temperature range (Di Cicco et al., 2006) that was indicated by theoretical studies to be a possible candidate for exhibiting structural changes in its liquid stable or metastable state. The XAS data, analysed by reverse Monte Carlo modelling, indicated the occurrence of an anomalous phase above 2 GPa in liquid tin. This is accompanied by tiny and gradual changes in the local ordering upon pressurisation. The local structure evolves towards an arrangement that has a closer affinity with a close-packed liquid, but without any sharp modification corresponding to a liquid–liquid phase transition.

Among the numerous devices for generating high pressure, diamond anvil cells (DACs) are the most commonly used for XAS measurements (Eremets, 1996). The DAC can generate static pressure conditions. Pressures in excess of 750 GPa have been reported (Dubrovinsky et al., 2015). Despite many designs, a DAC is conceptually a small mechanical press (less than 0.5 kg in total weight) in which two flawless diamonds are forced together on a microgram-sized sample (Fig. 2). The smaller the area A of the anvil faces, the higher the pressure P reached by the DAC for an equivalent applied force F. The shape and dimensions of the surfaces of the diamonds are customized on the basis of the required maximum pressure. The sample is generally contained within a metallic disc (gasket) in which a hole is drilled with dimensions of about one third of the surface of the diamond culet. The gasket prevents the diamonds from touching and consequently breaking during the compression experiment. The sample is mixed with a pressure-transmitting medium, which can be a solid, liquid or gas, so that the compressive force of the diamond culets is distributed evenly and to prevent the gasket hole from collapsing around the sample. The sample volume may also contain a pressure calibrant for in situ pressure measurement. Ruby (Cr³⁺:Al₂O₃) chips or spheres are generally used for this purpose because the sharp lines (the R1–R2 doublet) of its fluorescence, excited with a green or blue laser, shift in wavelength as a function of pressure (Forman et al., 1972). Alternatively, pressure gauges can be compounds such as NaCl or LiF for which the PVT equation of state is known.

Figure 2 Schematic representation of a diamond anvil cell. The load direction is perpendicular to the anvil culet and generally the X-ray beam passes through the diamond anvils. (Courtesy of Nicola Novello.)

In DACs high temperatures are typically obtained through the use of resistive heaters or by directly laser-heating the sample. The latter method has been used to reach temperatures of up to 6700 K (Boehler, 2000). Resistive heaters, on the other hand, have been used to cover the lower temperature range, and most systems reported in the literature are operational in the range from 300 K to about 1200 K (Dubrovinskaia & Dubrovinsky, 2003). The method of resistive heating is able to homogeneously heat large sample areas, if not the entire sample. The resistive elements can be placed externally around the DAC or inside the DAC in the vicinity of the sample. The first technique heats the whole DAC to temperatures of up to 800 K. These heating coils are commercially available and provide stable and fine control of the temperature over a long period of time. In the second design the resistive heater is placed as close as possible to the sample in order to generate temperatures in excess of 1000 K (Pasternak et al., 2008). Different types of materials can be used as heating elements, such as graphite, tungsten, molybdenum and platinum.

In order to obtain higher temperatures, laser-heated DACs (LH-DACs) are employed (Ming & Bassett, 1974). The LH-DAC technique takes advantage of the mechanical and optical properties of diamonds. In fact, the high transparency of diamonds to infrared radiation allows the sample inside a DAC to be heated via the absorption of infrared laser radiation by the sample itself. In LH-DAC experiments the sample must be thermally insulated from the diamonds. Usually, the temperature at the surface of the sample is determined from the spectrum of radiation emitted by the heated sample. The thermal spectrum is recorded over a wide range of the visible and near-infrared spectrum and is fitted to the Planck function assuming the grey-body approximation with wavelength-dependent Planck emissivity. There are a number of examples of the use of laser-heating systems at synchrotron-radiation facilities to probe matter at extreme conditions of P and T by means of DACs. Some examples can be found at ESRF (ID27; Schultz et al., 2005), APS (Shen et al., 2001; Zhao et al., 2004; Meng et al., 2015) and SPring-8 (Watanuki et al., 2001). The only one dedicated to XAS is beamline ID24 at ESRF (Kantor et al., 2018). Other examples of portable laser-heating systems for DACs have also been reported (Boehler et al., 2009). The solid–liquid phase boundary of iron compressed to over 100 GPa in an LH-DAC has been determined by means of energy-dispersive XAS (EDXAS; Aquilanti et al., 2015). In this work the modifications of the onset of the absorption spectra are used as a reliable melting criterion.

The principal technical issue related to the use of DACs for XAS is the crystal structure of the diamond anvils. In an XAS experiment the absorption coefficient is measured as a function of several hundreds of electronvolts. In such a large energy range Bragg conditions are satisfied for the crystal structure of the diamond, causing the X-rays to be diffracted from the anvils out of the line of the X-ray beam. This removes photons from the incident beam at precise energy values, leading to large dips in the transmitted X-ray intensity. These will broaden with increasing pressure because of the strain in the diamond lattice. As shown in Fig. 3(a) these dips appear as peaks (commonly called glitches) in the spectra, spoiling the data and severely limiting their exploitation. Several efforts have been made to overcome this dramatic problem in using single-crystal diamonds. The most straightforward is to orient the DAC with respect to the X-ray beam to obtain a relatively large glitch-free energy bandpass (Fig. 3b). This is a viable method whenever the region of interest of the spectra is limited to the X-ray absorption near-edge structure (XANES). With this aim, the use of X-ray absorption spectrometers in dispersive geometry offers the advantage of visualizing the spectrum over the whole energy range in `live' mode, and therefore the glitches can be removed from the energy region of interest in a reasonably short time by following their energy position as a function of the orientation of the diamonds. This is one of the reasons why there are many EDXAS studies at high pressure using DACs (Aquilanti et al., 2009). Nonetheless, there are some reports of high-pressure studies using diamonds at energy-scanning X-ray absorption spectrometers in which the glitches have been eliminated by measuring spectra at several different orientations of the cell. In this way the glitches appear at different energies, and a glitch-free composite spectrum can be produced (Sapelkin & Bayliss, 2001). Practical use of this method, however, seems to be quite tricky and complicated. As an effective approach to overcome the Bragg glitches, the use of polycrystalline B₄C for one or both anvils has been attempted (Freund et al., 1989), although its use limits the maximum achievable pressure. Extended X-ray absorption fine-structure (EXAFS) data have also been obtained at high pressure using DACs by employing a low-Z gasket (such as beryllium) and recording spectra through the gasket rather than the diamonds, but uncertainties in the sample thickness under high pressure limit its application to XANES spectra (Hu et al., 1994). Later, an approach for acquiring high-quality EXAFS spectra with a wide energy range at high pressure using a DAC was proposed (Hong et al., 2009). This is based on an iterative algorithm based on repeated measurements over a small angular range of DAC orientation, for example within ±3° relative to the X-ray beam direction. An invaluable solution to the glitch issue in EXAFS spectra is the use of nano-polycrystalline diamond (NPD) anvils instead of single-crystal diamond anvils (Irifune et al., 2003; Ishimatsu et al., 2012). These consist of randomly oriented diamond grains of several tens of nanometres synthesized from graphite under high pressure and high temperature without a binder, producing high-purity single-phase diamonds. Since the grains are randomly oriented, Bragg's law is always satisfied independently of the energy; therefore, the intensity of the diffracted X-rays changes moderately with energy and consequently the NPD imparts a smooth background to the absorption profile. NPD anvils have successfully been used for a study of vitreous GeO₂ up to 44 GPa (Baldini et al., 2010). In a study of the local structure of rubidium at high pressure it has been shown that it is possible to obtain EXAFS data of outstanding quality that are exploitable to a maximum k-range of 16 Å⁻¹ (De Panfilis et al., 2015). The synthesis and stability of Xe₂O₅ and Xe₃O₂ under pressure has been studied using XAS at the Xe K edge (E = 34 561 eV; Dewaele, Worth et al., 2016). While the occurrence of Bragg peaks from diamond anvils would have been particularly severe at this energy, the use of NPD anvils allowed the researchers to use a full EXAFS refinement to detect a possible direct reaction between xenon and oxygen at high pressure and temperature. Other work showing the use of NPD anvils can be found in Ishimatsu et al. (2016). These examples show that the introduction of NPD anvils has revolutionized XAS measurements under pressure, although the fact the NPD anvils are not yet commercialized remains a strong limitation to their wide use.

Figure 3 (a) X-ray absorption spectrum of arsenic recorded inside a DAC. The peaks that are spread all over the energy range are the Bragg peaks due to the single-crystal diamond anvils. (b) X-ray absorption spectrum of arsenic recorded inside a DAC after rotation with respect to the X-ray beam.

A second issue related to the use of the DAC for XAS concerns the strong absorption of diamond anvils at energies below 7 keV. For instance, the transmission of 4 mm of diamonds, corresponding to the total thickness of a couple of standard anvils, is ∼10⁻⁴. Various solutions have been devised to this problem. One is to use light-element gaskets (beryllium or beryllium–epoxy) and to pass the beam through the sample perpendicular to the anvil faces, as mentioned above. Another popular solution is to reduce the thickness of the diamonds using a combination of fully perforated diamond as a diamond backing plate with a miniature anvil and a partially perforated diamond. In this way, the diamond thickness along the X-ray path can be reduced drastically (to less than 1 mm) while simulteneously maintaining a good mechanical strength to reach pressures in excess of 100 GPa (Dadashev et al., 2001). An example of the use of this configuration of the anvils is the high-pressure XAS study of titanate perovskite at the Ti K edge (E = 4966 eV; Itié et al., 2007).

Given the increased use of the LH-DAC for high-pressure/high-temperature studies, it is worth mentioning that specific experimental requirements must be fulfilled. To reach the required temperatures the laser beams need to be focused to a few tens of micrometres. To minimize the temperature gradients, the samples inside the DACs must be fully irradiated by the laser and therefore the sample must be smaller than the laser spots. The misalignment between the laser and the X-ray beams cannot exceed, and must remain stable within, ∼1 µm. These conditions lead to the need for an X-ray beam spot that is a few micrometres in size and stable. Since molten matter is unstable under in situ laser-heating the acquisition times should be short, ideally of the order of milliseconds. Finally, it has to be taken into account that chemical reactions and decomposition might occur inside a LH-DAC, and therefore particular care has to be taken in the preparation of the sample container.

Over the past 50 years static compression studies have produced remarkable findings, allowing the discovery of new phenomena up to pressures of several hundred gigapascals and temperatures ranging from near-zero to several thousand kelvin. However, the maximum pressure that can be attained with DACs is limited by the strength of diamond and the achievable culet size. Nonetheless, expanding the achievable pressure range, possibly in combination with high temperature, might reveal many novel phenomena. The dynamic compression of matter induced by powerful lasers offers the possibility of exploring extreme states beyond the static limit of the DAC and mimicking the conditions in the interior of the Earth and other planets or producing extreme states of matter such as so-called warm dense matter (WDM; Rosmej et al., 2007). The WDM regime falls between condensed matter and plasmas; this state of matter is very common in astrophysical environments and is believed to exist in the inner core of large planets. Because of its extreme pressures and temperatures WDM tends to be extremely transient, and measurement of the microscopic properties of WDM is a challenging task. Among the possible diagnostics, XAS is a particularly suited to access structural information on a noncrystalline state of matter where short-range order dominates. WDM is generally created by the shock from a high-power laser. Single-shock compression produces large amounts of entropy and corresponding heating as the states are along the Hugoniot. This temperature rise limits the ability to dynamically compress materials to high density, and usually produces melting at very high pressures, so that the liquid phase is probed. Alternatively, ramp or multishock compression achieves quasi-isentropic compression and therefore off-Hugoniot states with lower temperatures. Hence, materials can maintain a solid structure while being compressed into the terapascal regime. Both shock and ramp loading involve strain rates that are many orders of magnitude higher than static compression. The sample is not exposed directly to the shock, but is normally confined to maintain pressure and create a spatially uniform compression state. The compressed matter must be measured using ultrafast probes. By changing the delay between the pump (the shock) and the probe it is possible to measure the sample under different pressure and temperature conditions. The shock strength and the properties of the shocked sample can be evaluated by measuring the shock velocity using a VISAR (velocity interferometer system for any reflector) and comparing it with simulation codes. XAS studies of dynamically compressed materials have been reported. As examples, we mention a study of solid iron up to 560 GPa achieved by multiple shock (Ping et al., 2013). In this work, both the dynamical compression and the X-ray pulse were generated at the OMEGA laser facility. In other work, a pressure of 420 GPa and a temperature of 10 800 K have been reached for iron, thus exceeding the inner core boundary condition of the Earth. XANES spectra were recorded using an X-ray free-electron laser beam at LCLS coupled with a laser-induced dynamic compression technique (Harmand et al., 2015). Using a single 100 ps synchrotron X-ray pulse, nanosecond-lifetime equilibrium states of WDM iron have been measured by K-edge absorption spectroscopy (Torchio et al., 2016).

Dynamic compression also offers the possibility to decipher the kinetics and mechanisms of solid–solid phase transitions. Dynamic compression coupled with EDXAS has been used to characterize a martensitic transformation in zirconium with millisecond timescale kinetics (Dewaele, André et al., 2016). On copper foil, the dynamic change in atomic scale structure under high pressure induced by a laser shock with a nano­second timescale has been studied (Niwa et al., 2016). It is suggested that the uniaxial shock compression of 4 ns caused by laser irradiation resulted in the formation of a more complicated local structure which differs from that obtained through a conventional mechanical deformation with a much slower strain rate. Therefore, this approach can provide new structural information which cannot be obtained by static compression and will help to clarify phenomena in which atomic movement plays an important role, such as dislocations, diffusion, deformation and phase transitions.

Matter under extreme conditions is central to scientific research aimed at discovering new properties of materials. Key to science under extreme conditions is, on one hand, the availability of suitable probes, for which the development of large-scale facilities provides the essential breakthrough. One the other hand, research at high pressure and high temperature relies on the capability to generate such extreme conditions. We have described the most common devices used to produce high pressure and temperature in XAS with an eye to novel techniques based on dynamic compression to explore new states of matter, which is of paramount importance in many fields such as materials science, planetary science and, not least, fusion research.