Revealing the Atomic Motion Composing the B1–B2 Structural Transformation of MgO Under High Pressures

Magnesium oxide (MgO) is an abundant material on Earth and plays an important role as as an optical window in high-energy-density experiments. At ambient conditions MgO is stable in the B1 (NaCl) structure. When shock compressed¹ a structural transformation has been observed where it morphs into the B2 (CsCl) structure. Ramp compression and x-ray diffraction experiments² have additionally detected a possible intermediate structure along the B1–B2 phase transition. Our goal is to comprehend the atomic motion during the transformation. Calculations based on density functional theory have been performed throughout a structural coordinate space connecting the B1 and B2 structures. The physical process of the transformation is represented as a set of thermodynamically optimal intermediate structures that form a pathway across the structural coordinate space. The structural transformation based on this prediction will be presented. The predicted atomic motion composing this transformation will be shown and compared with previously unexplained experimental observations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.

¹ R. S. McWilliams et al., Science 338, 1330 (2012).

² F. Coppari et al., Nat. Geosci. 6, 926 (2013).