Influence of point defects on the elastic properties of mantle minerals and superhard materials

Perfect crystals do not exist in nature. Defects in crystals modify their physical and chemical properties. Elastic properties relate stress to reversible strain and reflect the strength of interatomic bonding forces, which may be influenced by defects. This thesis advances our understanding of how defects influence the elastic properties of mantle minerals and superhard materials.;In this study, I focused on defects associated with ferric iron (Fe 3+) and hydrogen (H) substitution in mantle minerals with application to interpreting the water content of the mantle from observed seismic wave speeds. High-pressure, single-crystal X-ray diffraction experiments were carried out to determine the comparative compressibility of hydrous and anhydrous Fo90 wadsleyite, the dominant phase in Earth's mantle transition zone (410-660 km depth). The results show that hydration of wadsleyite with 1 wt.% H2O reduces its bulk modulus by 4.7%, but has no influence on its pressure derivative. Therefore, the reduction in bulk sound velocity of wadsleyite associated with H defects should persist to mantle pressures. In another study, the equation of state and electronic spin state of ferric iron (Fe3+) in Fe-Al-phase D were determined, pertaining to dense hydrous magnesium silicates that could potentially transport water into the lower mantle. The results show that Fe3+ undergoes a gradual spin transition between 40 and 65 GPa, causing pronounced bulk-elastic softening of Fe-Al phase D within the spin transition pressure interval. Results provide an alternative interpretation for small-scale seismic heterogeneities beneath the Pacific rim.;In addition to mantle silicates, I have determined the influence of nitrogen defects on the elastic properties of natural and synthetic diamond. The measurements of elastic moduli of synthetic nano-polycrystalline diamond (NPD) and natural type Ia diamond feature a newly developed optical contact micrometer for ultrasonic sample thickness measurements and are among the highest precision measurements of diamond elastic properties ever made. The elastic moduli of NPD were found to be equivalent to isotropically averaged single-crystal elastic constants of natural diamond. Knowledge of elastic properties of diamond and other superhard materials will illuminate why certain substances are hard, and improve our ability to identify and design them in the future.