Emergence of modern high pressure science

In many respects, the science of materials has only fully utilized two of its three fundamental tools —the variables of temperature and chemical composition. Pressure, the third fundamental variable altering materials, is in many ways the most remarkable, as it spans some 60 orders of magnitude in the universe. Yet, its true potential for exploring the nature of materials was for years unfulfilled for a number of reasons: the accessible pressure-temperature conditions were too modest to cause significant changes in many materials, samples under high pressure could not be subjected to thorough analyses, or theory was not sufficiently well developed to understand or predict the variety of phenomena suggested by experiment or observed in nature. Thus, high pressure research existed as a relatively minor subfield within the traditional disciplines of the physical sciences.

This state of affairs has changed dramatically during the last decade. High pressure science has experienced tremendous growth, particularly in the last few years. With recent developments in static and dynamic compression techniques, extreme pressure and temperature conditions can now be produced and carefully controlled over a wide range. Moreover, a new generation of analytical probes, many based on third-generation synchrotron radiation sources, have been developed and can now be applied for accurate determination of the structural, dynamical, and electronic properties of matter under extreme conditions. Finally, developments in computational techniques and advances in fundamental theory tested against bountiful new experimental results are both deepening our understanding of materials as a whole and guiding subsequent experimental work with new predictions.

It was for this reason that this course on high pressure science was held at the International School of Physics “Enrico Fermi” in July 2001. Though presented in a physics forum, the title “High Pressure Phenomena” was chosen to reflect the broad scope of the field and the diversity of recent findings. Indeed, the field spans fundamental physics and chemistry, materials science and technology, the geosciences, planetary science and astrophysics, as well as biology. The highly interdisciplinary character of the field was central to the organization of the School, though the sheer breadth of the field meant that many topics could be treated in only a cursory fashion while others were examined more in depth. The aim of the School was to present the state of the art in techniques used in modern high pressure research, highlighting those topics where applications of these technique are currently having a major impact. The lectures were therefore divided into two types. The first were pedagogic lectures, in which basic methods (both experimental and theoretical) for investigation of matter at high pressure conditions were presented, together with general overviews of applications. The second type was devoted to examining special topics. The topics were interpersed throughout the 10 days of the School in 47 lectures and seminars; the written contributions (some containing material from multiple lectures) fall naturally into the following categories for the Proceedings.

I. Static Compression: Overview and Techniques

II. Dynamic Compression: Overview and Techniques

III. Theory and Fundamentals

IV. Metals and Superconductors

V. Simple Molecular Systems

VI. Chemistry and Biology

VII. Liquids, Glasses, and Nanostructures

VIII. Earth and Planetary Science

Overview of the volume

Accelerating advances in static compression techniques, specifically, those based on the diamond-anvil cell, have been one of the major reasons for the explosive growth in the high pressure field. The first section begins with an overview of the methods by Hemley and Mao, who briefly review the development of opposed anvil methods as well as the growing array of in situ methods now used, including X-ray, neutron, optical, and transport methods with these devices up to multimegabar pressures (> 3 Mbar or > 300 GPa). They then briefly discuss several applications that complement studies presented later in the volume; these include dense hydrogen, new materials, dense oxides, and microbial activity. These techniques and applications can be compared with those based on so-called large volume high pressure devices. As reviewed by Yagi, these methods in principle permit studies with analytical methods that require substantially larger volumes than are possible with conventional diamond-anvil cells. These include, for example, the piston cylinder apparatus, multianvil DIA devices, and double-stage apparatus. As for diamond-anvil cells, an important recent development is the routine use of various large volume devices for in situ studies with synchrotron radiation. Though the pressure range of the “large volume” devices (multianvil presses) is significantly lower than that of conventional diamond-anvil cells, pressures as high as 50 GPa have been reached with sintered diamond. As discussed by Hemley and Mao, a particularly exciting current development is the marriage of conventional large volume and diamond-anvil cell techniques based on the creation of large diamond anvils by chemical vapor deposition.

An important feature of the diamond-anvil cell is its use in generating very high temperatures (> 5000 K) at high pressure with laser heating. Boehler et al. review the laser-heated diamond cell and its applications to high P-T phase diagrams. Introducing an example of the technique (see also Hemley and Mao), the lecture reviews melting experiments and phase diagrams and the use of various criteria to identify melting. Examples of materials studied include alkali halides, simple metals, transition metals, alkaline earths, rare earths, and noble gases. The determination of crystal structures is central to static high pressure research, and underlies the topics discussed throughout the volume. Loveday summarizes the critically important topic of high pressure crystallography, and introduces the principal types of radiation used, X-rays and neutrons, comparing and contrasting the complementary nature of the two. He also provides a brief overview of powder diffraction versus single-crystal diffraction, current efforts to expand both the accuracy and P-T ranges of these techniques, and widely used methods of refinement techniques used to determine atomic positions.

Vohra and Weir present recent developments in CVD-based designer diamond-anvil technology, which has matured in the last few years. New results include the creation of designer eight-probe anvils for electrical conductivity measurements and designer loop anvils for magnetic susceptibility measurements. They also present design concepts for the next generation of designer diamond anvils with multi-tasking capabilities, including joule heating, temperature measurements, diamond strain measurements, and integrated electrical transport and magnetic measurements that complement the large anvil effort for a new generation of “large and smart” anvil high pressure devices (see Hemley and Mao).

Section II provides an overview of dynamic high pressure (e.g., shock-wave) studies of materials. Nellis introduces the topic, focusing principally on the gas-gun results. Some of the unique aspects of shock compression include its use in probing high P-T states, including the Hugoniot equations of state, which have been used in turn for developing static high pressure scales. New techniques allow accurate determination of shock temperatures, shsock profiles, elastic-plastic flow, sound speeds, electrical resistivity, and X-ray diffraction. Recent applications include materials synthesized and recovered from high dynamic pressures, such as nanostructures, films, superconductors, and hard materials. Fortov and Mintsev then discuss more extreme states of matter, the hot dense strongly coupled plasmas at very high temperatures and megabar pressures. Beginning with a brief overview of shock waves and strongly coupled plasmas, including the techniques for producing these states, the authors then summarize properties of plasmas under extreme conditions, including equations of state, optical properties, electrical conductivity, and behavior on adiabatic expansion. Representative examples of recent studies of elemental materials provide a meeting ground for static compression and lower temperature shock studies. A major goal of this work has been to confirm or contradict the hypothesis of a plasma phase transition (e.g., in hydrogen, as discussed in other lectures).

Section III provides an overview of the variety of theoretical approaches used to understand materials at high densities. Ashcroft begins with a thorough overview of fundamental theory, beginning with the formulation of the problem of the behavior of nuclei and electrons with variable volume, the role of pressure in controlling the structure of ions as determined by multicenter potentials, and electrons in static and dynamical lattices. Anticipating the later experimental discussion on pressure effects on the structure of liquids, he introduces the concept of the pair correlation. Specific applications include hydrogen at high pressure, and the possibility of unusual effects of re-entrant melting and liquid-like phases. Many theoretical studies require large scale computational techniques. Scandolo introduces a particularly important method, first-principles molecular dynamics —the Car-Parrinello method, which was first introduced at the 1985 “Enrico Fermi” School. With its beginnings in classical molecular dynamics with variable shape simulation cells and density functional theory, the Car-Parrinello method is well suited for high pressure studies, including predictions of new phases and phase transitions. Applications to elemental materials, including Si, C, H, and O, simple molecular compounds, and metals are presented, followed by perspectives on future directions.

Complementing the above theoretical lectures, Cohen et al. examine new findings regarding the role of magnetism in affecting phase stability, equations of state, and elasticity in materials under pressure. Following a review of the basic theoretical treatment of magnetism, the authors consider Mott insulators (important for a variety of systems considered at the School) and the LDA + U method. Applications include Fe and transition metal oxides (FeO and CoO), especially important for the Earth's interior (as discussed in later lectures) as well as from a fundamental point of view, in view of recent experimental findings (see Goncharov et al.). High pressure studies of rheology, including both elasticity and viscosity, are important for applications from materials science to the geosciences. Poirier reviews fundamental equations, including phenomenological equations of state, viscosity of solids and how this differs from the case of liquids, where the applications to silicate liquids is especially important in Earth science (see Boehler et al.).

The above topics lead naturally to Section IV, which concerns experimental studies of metals and superconductors under pressure. One of the surprises in recent work in this area has been the structural complexity of simple metals at high pressure. As discussed by Syassen, these systems are far from simple: recent examinations (and re-examinations) using new synchrotron X-ray diffraction techniques are finding complex structures, including incommensurate and multiple sublattices in elemental solids. Structure refinements by powder diffraction (supplemented with some single-crystal results, see Loveday), together with first-principles methods discussed above are leading to new insight and systematics. Examples include Cs, Rb, and Li; the latter has been predicted to undergo symmetry breaking transitions with possible parallels to dense hydrogen (see Ashcroft).

In addition to crystallographic studies of metals, breakthroughs in two additional areas have led to the discovery of new phenomena in metals, particularly at megabar pressures. The first are the numerous new findings in superconductivity, including the creation of new classes of superconductors from large band-gap insulators at ambient pressure. Struzhkin et al. provide an overview of some of these developments, focusing primarily on new magnetic susceptibility techniques that can now be used to multimegabar pressures (e.g., > 200 GPa). The lecture summarizes the evolution of these methods, culminating in the development of the double modulation technique that is currently used at the very highest pressures. Applications to simple metals, chalcogens, and MgB₂ are described. A new development in both the study of metals at very high pressure (> 100 GPa) is the application of highly sensitive Raman scattering techniques. Goncharov et al. review these developments, including details of the experimental technique and applications to Fe and Fe alloys, Re, and MgB₂ (also discussed by Struzhkin et al.).

Historically, simple molecular systems have been a particularly important class of materials for high pressure investigations. With their very high compressibilities, solid state densities can be increased by over an order of magnitude with modern techniques, and the evolution of major changes in physical and chemical properties as a function intermolecular distance can be monitored. Of these systems, hydrogen has been the focus of by far the most interest since the earliest calculations of predicted pressure-induced metallization for the solid. Nellis presents an overview of recent dynamic compression studies of fluid hydrogen and related molecular systems, including the recent observations of minimum metallic conductivity in fluid hydrogen at 140 GPa and high temperature, after passing through an intermediate semiconducting state. The transition is interpreted as a Mott transition in the high density fluid. He goes on to present recent measurements of minimum metallic conductivity in oxygen and nitrogen. Notably, both hydrogen and nitrogen are not metallic at these pressures in the low temperature solid (see Hemley and Mao). Recent evidence is presented for protonic conductivity in water and chemical decomposition of hydrocarbons, both of which have been addressed in static compression experiments.

The following two lectures examine simple molecular systems from the standpoint of static compression techniques. Ulivi reviews selected molecular systems using vibrational and optical spectroscopy. Following a discussion of molecular systems in general, including van der Waals compounds, nitrogen and oxygen are examined in some detail, primarily in the lower pressure range, where the wealth of information that can be obtained from spectroscopic studies has been demonstrated. This includes the magnetism in O₂, which is unique for a simple molecular system, and the proposed pairing of the molecules in high pressure phases. Loveday’s second lecture presents an overview of hydrogen-bonding under pressure. Also discussed in various other lectures, this relatively weak interaction, which controls the behavior of a vast array of materials, including biological systems, undergoes intriguing changes with pressure that reveal a great deal about the nature of the interaction itself. Crucial to the recent progress in this area has been the development of high pressure neutron diffraction techniques, as well as vibrational spectroscopic methods at higher (i.e., megabar) pressures.

Continuing the theme of molecular systems, Section VI focuses on the new insight high pressure studies have provided both for synthetic organic chemistry and the physical properties of biological systems. In his first lecture, Jenner summarizes recent applications of organic synthesis, including pressure effects on rate constants and other aspects of classical physical organic chemistry. Recent applications include investigations of cycoadditions, Michael and related reactions, and ionogenic reactions. Jenner's second lecture then shows how pressure effects on kinetics can be used to identify these and other mechanisms. Winter provides an overview of the high pressure effects in molecular biophysics, which together with other related studies of soft matter constitute another growing research area. After an introduction to lipid mesophases and model biomembrane systems, he summarizes various experimental techniques, including scattering methods, spectroscopy, and high pressure cells (mostly for < 1 GPa). There are surprising effects of these modest pressures on the structure, energetics, and kinetics of transitions in lipid systems; other applications to protein structure (both denaturation and renaturation), and the possible use of pressure to address the protein folding problem. There are implications of these types of investigations in biotechnology and molecular biology, including the behavior of extremophiles (see Hemley and Mao), food science, and understanding fundamental structure-function relationships in biomolecules in general.

The following two chapters in Section VI address new developments in high pressure solid state chemistry. Bini reviews chemical transformations in molecular crystals at pressures in the 10–50 GPa range. Here crystallographic control provided by the solid state, mixing of electronic states under pressure, and a competition between thermal and photochemical reactions distinguish these reactions from the lower pressure solution chemistry described above. Infrared spectroscopy is a particularly useful technique for such study, as shown by examples of chemical reactions involving aromatic molecules, alkenes, and other simple molecules, including their kinetics. McMillan focuses on inorganic solid state chemistry at high pressures and temperatures, beginning with the work of Bridgman, the later synthesis of diamond, and high P-T studies of earth materials. As in the work reviewed by Bini, diamond-anvil cell techniques provide a uniquely powerful window on reacting materials. This is demonstrated by recent studies of molecular materials, including van der Waals compounds, CO₂, and N₂O, nitride spinels, icasohedral B₆O, and LiSi. There are important new opportunities, including the creation of new superhard materials with improved techniques for recovery of materials from high pressure.

Section VII reviews selected recent highlights in the study of liquid, amorphous, and nanostructured materials under pressure. McMillan reviews the topic of liquid state polymorphism —the evidence for transformations in the liquid state analogous to those found in solids. The best examples appear to be from supercooled (i.e., metastable liquids), but recent results point to transitions in liquids within their thermodynamic stability fields. The thermodynamic basis for the effects, including the relationship to melting curve maxima, microsopic two-state models, and the connection to pressure-induced amorphization are reviewed. The last topic is examined in further detail by Arora, beginning with its discovery in ice I and subsequent findings in the silica polymorphs, and other materials. A metastable transition that is clearly affected by the inhibited kinetics of equilibrium phase transitions and therefore temperature, pressure-induced amorphization can occur as a result of pressure-induced decomposition (chemical reactions). Finally, the high pressure properties of carbon nanostructures are of great interest. Venkateswaran and Eklund present high pressure Raman studies of single-walled carbon nanotubes, including both pristine and iodine-doped bundles. They review structure, electronic, and vibrational properties from the standpoint of the unique symmetry of these systems, and present recent studies of the pressure dependence of the Raman spectra of pristine and iodine-doped bundles.

A series of lectures on Earth and planetary interiors are collected in the final section. The section begins with a broad introduction to the field of planetary interiors as a whole by Stevenson. The interior structure, composition, and dynamics of the planets contain a great deal of information about the evolution of our Solar System. In addition, they serve as a testing ground for high pressure theory and as distinctly natural high pressure experiments in which to observe the behavior of materials. The major classes of planetary materials include rocks (minerals), ices (molecular systems), and gases. Approximate methods can be used to determine the pressure as a function of depth within planets (the pressures are known with high accuracy if seismological measurements can be performed). External measurements that reveal information about internal state and past history of the planet include heat flow and the character of magnetic fields. Nellis presents experimental constraints obtained from recent shock wave experiments. An important recent application to the Jovian interiors (of Jupiter and Saturn) is the observation of electrical conductivity in fluid hydrogen discussed above and how convection can give rise to the planet's large and turbulent magnetic field. These interiors may be compared with those of the icy giant planets (Uranus and Neptune), which are composed of water and water-rich molecular mixtures.

The final three lectures summarize the great amount of recent high pressure studies of our planet's interior. Poirier provides an overview of the physics and chemistry of the Earth’s core. He outlines principal current problems, including composition, reactions at the core-mantle boundary, viscosity, energetics and cooling of the outer core, as well as the problem of the phase, crystallization, anisotropy and deformation of the inner core. Boehler et al. examine materials of the Earth’s core and lower mantle, focusing on phase behavior, melting, and chemical reactions for major phases. The review shows how differences in the determinations of melting temperature of iron give rise to the rather different estimated temperatures for the center of the planet (compare Hemley and Mao; Cohen et al.), and the role of additional elements is examined. Measurements of melting of silicates imply a large thermal boundary layer at the core-mantle boundary. The high P-T behavior of deep mantle materials is also reviewed in Yagi's second lecture. Beginning with an overview of the structure of the mantle, he reviews the diversity of techniques including opposed anvil, multianvil, and laser-heated diamond-anvil cells. These are supplemented with in situ X-ray measurements discussed elsewhere as well as analyses of recovered samples by energy-resolved transmission electron microscopy, a technique that complements the X-ray spectroscopy and inelastic scattering discussed earlier. Recent studies of the transformations of upper mantle minerals (olivine, pyroxene, and garnet) to assemblages of phases at lower mantle conditions that are dominated by silicate perovskite —considered the most abundant mineral of the planet— are reviewed. Once thought to be well understood, there is now evidence for surprising effects of Fe²⁺, Fe³⁺, and Al³⁺ partitioning on the physical properties of the silicate perovskite phase that present new questions about the nature of the Earth's mantle.

We are grateful to numerous individuals who made the Summer School a very pleasurable experience and who contributed in important ways to the publication of the Proceedings. First, we thank the President of the Italian Physical Society, Professor Bassani for supporting the course from its inception. We thank B. Alzani and her staff for the masterly practical organization of the School and the great help with logistical details at Villa Monastero. We are grateful to C. Vasini and her staff for their excellent skill and great amount of patience in preparing the volume. We thank S. Gramsch for help in copyediting various lectures. Finally, we thank numerous institutions and agencies for financial support, including the U.S. National Science Foundation (Division of International Programs, Division of Earth Science, and Division of Materials Research), the Italian “Consiglio Nazionale delle Ricerche” and the Venice Office of UNESCO.

R. J. Hemley and G. L. Chiarotti

Directors of the School

M. Bernasconi and L. Ulivi

Scientific Secretaries