COMPRESSIONAL BEHAVIOR OF HYDROGEN-BONDED CRYSTALS: ANHYDROUS COMPARISONS AND POLYMORPHISM

Abstract

Hydrogen, despite its ubiquity, remains a little-understood factor in high pressure mineral physics and material science. Hydrogen in the solid state can profoundly affect the structure and behavior of crystal, often as hydrogen bonding interactions. In a geological setting, the addition of hydrogen – typically as water – drastically changes the material properties of mantle minerals, including the rheology, electrical conductivity, elasticity, and phase transition conditions. However, the degree of influence that hydrogen exerts is often variable, which makes the determination of structural and thermoelastic information at high pressures more difficult. To better understand its role in crystal structures, analogue materials have been used to explore the influence of hydrogen in crystal structures at high pressure. In this dissertation, several high pressure crystal structures with relevance to planetary and materials science have been examined in the context of hydrous and anhydrous analogues. Chapter 3 describes the high pressure behavior of the mineral β-behoite (Be(OH)2) as a hydrous analogue structure to α-cristobalite, a high-temperature polymorph of SiO2. The low-pressure structures β-behoite and α-cristobalite are topologically identical, but differ by the added presence of O-H···O hydrogen bonds, which add structural rigidity and resist distortion of the structure. As a result, behoite does not follow cristobalite’s phase transition pathway to higher coordination states. This pathway is discussed in detail within Chapter 4, where the change in Si ion coordination at high pressures creates a polymorph called cristobalite X-I. This phase is the only known octahedrally coordinated phase of SiO2 that cannot be returned to ambient pressures, and represents a bridge in the densification process of SiO2 that occurs within the Earth. Additionally, cristobalite X-I is discussed as an analogue structure for six-coordinated CO2, which may occur in the interiors of giant planets. Lastly, Chapter 5 examines high-pressure hydrogen bonding and polymorphism in melamine, an aromatic organic molecule derived from s-triazine that forms an extensive network of hydrogen bonds. These hydrogen bonds inhibit reactivity and polymorphism when compared to non-hydrogen bonded organic crystals like s-triazine and benzene, and may act as a stabilizer in the formation of co-crystals relevant to molecular materials and organic minerals.