- © 2000 Mineralogical Society of America
Structural studies of molecular materials under conditions of variable pressures and temperatures form an essential part of high-pressure crystallography. The high compressibility of these materials makes them excellent systems for examining in detail the relationship between molecular structure and physical and chemical properties. In general, the more complex molecular materials by their very nature exhibit a variety of interactions among the constituent atoms—from weak intermolecular interactions such as van der Waals or hydrogen bonding combined with stronger interactions of the covalent, ionic, and metallic type. Thus, compression studies provide a means to examine the large changes in these interactions with variations in intermolecular and intramolecular distances. This behavior in turn allows one to probe the wide range of phenomena exhibited by molecular systems under the extreme conditions of pressure and temperature that can now be created in the laboratory.
Molecular systems include key volatile species fundamental to understanding the chemistry of the Earth and planets. Low-Z molecular systems are among the most abundant in the solar system, as represented by planetary gases and ices. Their behavior at high pressures is crucial for modeling the structure, dynamics, and evolution of the large planets. Moreover, compression of molecular systems provides the opportunity of forming new materials, possibly with novel properties such as high non-linear optical reponse, superhardness, and high-temperature superconductivity, as well as new classes of disordered and amorphous materials. In each of these applications, structure measured on all length scales is perhaps the most fundamental. Here we review the way in which the structure of a wide range of molecular materials, from hydrogen to large polyatomics, is affected by pressure and temperature. We focus on diffraction (primarily X-ray) methods although we include complementary (but in general indirect) structural studies by spectroscopic methods. The approach examines the structural basis of physical and …