Ab initio investigations of native and protonic point defects in Mg2SiO4 polymorphs under high pressure

Density functional theory simulations were carried out to study the formation and migration energetics and geometric structures of the native point defects and protons in Mg2SiO4 polymorphs (namely, forsterite, wadsleyite and ringwoodite) up to 30 GPa pressure. The energetic favorability of the vacancy and interstitial defects was shown to strongly depend on the atomic and electron chemical potentials. Among the charge-balanced defects studied, the Mg2+-Frenkel defects are energetically most favorable in forsterite whereas the MgO pseudo-Schottky defects are energetically most favorable in wadsleyite and ringwoodite. Our results for the ion migration enthalpies calculated using the nudged-elastic-band technique suggest that the Mg migration is easiest in forsterite and ringwoodite whereas Si migration is easiest in wadsleyite. The proton incorporations at the interstitial and vacant cationic sites were investigated. In the extrinsic limit, the proton incorporation is energetically most favorable at VSi⁗ site for up to three protons. Addition of one more proton prefers to go to VMg″ site. In the intrinsic limit, however, the interstitial and Mg-vacancy sites remain the most favorable. The predicted barrier for the interstitial-to-interstitial proton migration is smaller than that for the magnesium vacancy-to-interstitial migration, and among three phases ringwoodite has the lowest barrier. The effects of proton incorporation on the transition pressure and pressure-volume equation of state were shown to be significant.