UC Berkeley

Phase transitions at high pressure are typically expressed as changes in the material’s crystal structure and elastic parameters. This work describes the Fe(II) spin collapse in the ferropericlase system through examining structure factors and through modeling of compressibility data. Detailing electron density distribution changes with compression allows for a comprehensive picture of high-pressure chemistry underpinning the theory in first-principles calculations, giving detailed insight into not only the confidence of measured properties but the electronic landscape leading to essential differences between deep-Earth materials. Experimental charge density changes obtained from single-crystal x-ray diffraction are compared to atomic orbital theory for d-electron spin collapse in high-pressure (Fe,Mg)O, characterized by the spin pairing of the two high energy lone-spin electrons in the Fe valence to a spin-paired state in lower-energy orbitals. Expected changes in bonding can be compared with ratios of local electron density intensities relative to those from atom centers. When normalizing to the oxygen site, there is rough agreement between charge transfer around the cation site (5 percent local charge density changes) and expectation of 6 percent charge transfer for 53 percent Fe stoichiometry (two electrons from Fe divided by the total charge of the cation site). However, the change in charge density in the cation center is roughly twice as large (compared to oxygen) as expected from electron accounting, potentially related to expected electron donation from oxygen. Modeling of compression by way of normalized pressure (F) as a function of finite strain (f) provides an architecture for evaluating the phase transition over a suite of compositions. Through this, the volume collapse of the Fe(II) cation is found and the bulk modulus of both the low- and high- pressure phases is determined. Results are reported as a function of Fe content and display the strengths and limitations of this approach for the data examined.