Probing of Pressure-Induced Bonding Transitions in Crystalline and Amorphous Earth Materials: Insights from X-ray Raman Scattering at High Pressure

Knowledge of the electronic structure of crystalline and non-crystalline earth materials at ambient and high pressure are essential in order to understand the atomic origins of electronic, thermodynamic, and mechanical properties of these materials in the Earth’s crust as well as Earth and planetary interiors (Hemley 1998; Laudernet et al. 2004; Stixrude and Karki 2005; Mao and Mao 2007; Price 2007; Stixrude 2007). Pressure-induced changes in the electronic structure of crystalline and amorphous silicates and oxides (glasses and melts) with low- z elements (e.g., Si, O, B, Li, C, etc.) have implications for diverse geophysical and magmatic processes relevant to the evolution and differentiation of the earth (e.g., mantle convection and mantle melting) (Stebbins 1995; Wolf and McMillan 1995; Lee 2005, 2011; Mysen and Richet 2005; Murakami and Bass 2010). Despite this importance, the analysis of the effect of pressure on the electronic structure and the nature of bonding in the crystalline and, particularly, non-crystalline oxides has remained one of the challenging problems in mineral physics and geochemistry, as well as, condensed matter physics. This is mostly because of the lack of suitable experimental probes of electronic bonding around these light elements in the earth materials under pressure. Advances in in situ high pressure technologies, together with progress in X-ray optics in synchrotron radiation and first principle calculations have revealed structural details of bonding transitions of crystalline earth materials at high pressure (Hemley 1998; Mao and Mao 2007; Price 2007; Stixrude 2007). The non-resonant synchrotron inelastic X-ray scattering (NRIXS, also known as X-ray Raman, XRS) is one of the relatively new synchrotron X-ray probes of local structures with element-specificity. It explores the electronic bonding transitions in soft X-ray absorption edges using hard X-rays (e.g., ~ 10 keV) …