Materials Under Extreme Conditions

The following sections are included:

• Preface
• Foreword
• Acknowledgments
• Contents

All physical and chemical transformations are controlled by the thermodynamical variables of temperature, pressure and chemical potential. Acting on these external or internal parameters it is possible to affect the equilibrium composition of the systems and the kinetics of the transformation. The practical and theoretical implications of controlling the physical and chemical processes are evident. In the interest of the present book we shall confine the discussion to the effects of temperature and pressure and, without much loss of generality, we shall refer mainly to chemical processes…

Substantial progress in high-pressure science and in its applications to material science and related fields has only occurred in the last century, and in particular, thanks to the pioneering work of Percy W. Bridgman. The slow but continuous progress in high-pressure technology and in applications for the measurements of a variety of physical properties of materials in the 19th and approximately the first half of the 20th century, have been described in detail by Bridgman, but his historical reconstruction only occasionally referred to specific applications in chemistry…

The equation of state (EOS), in the usually accepted formulation, gives the interconnection between the thermodynamic parameters of pressure P, volume V, temperature T and number of particles n, or of their conjugate variables, in the form,

• The basic thermodynamic relations (3.1 and 3.2)
• The extension to a pressure-temperature regime of atomization or metallization, or even transformation to plasmas
• The connection with microscopic and electronic structures, with intermolecular interactions at work and with the microscopic dynamics of the material
• The onset of phase transitions
• The range of applicability
• The experimental methods for its direct determination
• The characterization with parameters
• The comparison of experiments with theoretical derivations at various levels of approximation
• The theoretical foundation of the semi-empirical formulations of common use

A description of the experimental methods employed to induce and study chemical processes at high pressure, first requires the identification of the pressure range of interest. In fact, the pressure range and the modalities of high-pressure generation (static or dynamic) sharply define the nature of the chemical processes, and especially the related probing techniques. Most of the organic syntheses are performed below 1 Kbar, where a commercial technology offers reliable high-pressure reactors working on large volumes and at variable temperature. Going to the GPa range, the sample size decreases more and more with increasing pressure, whereas the experimental difficulties correspondingly grow. Here, the commercial instrumentation availability is noticeably reduced, and custom apparatus are generally designed and built according to the application. Large volume presses are employed for syntheses where the reactants are mainly inorganic solid materials, offering the advantage of synthesizing mm³ size products using pressures as high as some tens of GPa and temperatures of thousands K. All these methods rarely allow in situ monitoring of the chemical reaction. The third method, in this schematic approach, is represented by the diamond anvil cell (DAC) technique. This method is extremely useful from a fundamental point of view, being the unique tool to understand chemical transformations occurring under higher pressure conditions (up and above the Mbar) such as in the Earth's interior. This technique successfully conjugates the possibility of monitoring the reaction in situ over a wide P-T range, the obvious drawback being represented by the sample dimension (a few hundreds of microns in diameter at 1 GPa, becoming a few microns in the Mbar range), which also defines the amount of recoverable product…

In this chapter the basic concepts regarding the effects of pressure on chemical reactivity will be presented, focusing attention on the aspects of interest for treating reactions occurring at pressures in the order of or greater than 1 GPa, whose description is the main purpose of this book. The peculiarities of these pressure conditions regard the physical state and the electronic properties of the molecular systems, which can greatly differ from ambient conditions. Condensed phases, both liquid and solid, characterize the pressure regime of interest, posing specific additional requirements in both the nucleation and in the propagation steps. Requirements for nucleation are mainly of steric nature and concern the successful realization of the proper relative conformation to match the conditions for achieving the transition state. In the propagation step, the dynamics of the system, mainly related to the transport of reactants to reactive sites, becomes dominant. In addition, pressure can strongly modify the electronic distribution leading to the formation of species capable directing the reactivity along pathways not accessible at ambient conditions. This is actually a relevant aspect that allows a precise distinction with the reactions occurring in the kbar range, which have been extensively reviewed by several authors [715–717] and will be not considered here…

Chemical reactions at high pressure have been studied for a fairly large number of molecular crystals. The high-pressure reaction threshold ranges from approximately 1 GPa to several tens of GPa, depending on the structure and stability of the intramolecular bonding in the molecular units of the system. In several cases, molecular rearrangements or phase transitions occur before reaching the reaction threshold, and these processes are generally of primary preliminary importance to elucidate the reaction mechanisms. Here, attention will be confined to studies under static high pressures. These conditions have the advantage of a fine control of both temperature and pressure of the reaction, thus allowing for a more detailed study of the kinetics and the mechanism of the chemical reactions…

The following sections are included:

• Bibliography
• Appendix A
• Index