Frenkel Defects and Interstitial Atoms in Periclase (MgO) at High Temperature by Molecular Dynamics Simulation

Abstract

An analysis program was newly developed to explore the vacancies and interstitial atoms and to determine atomic migration, i.e., atomic diffusion. We applied this program to the results calculated by the molecular dynamics (MD) simulation of periclase (MgO) in which Schottky defects (vacancies) were not initially introduced. Generation, migration and extinction of Frenkel defects and interstitial atoms for only magnesium ion were first observed at high temperature in this MD system and they strongly corresponded to the change of the mean square distance (MSD) of magnesium ion in MD system. On the other hand, we could not observe Frenkel defects and interstitial atoms for oxygen ion and MSD value of oxygen ion had almost constant value. Generation, migration and extinction of Frenkel defects and interstitial atoms cannot be ignored for the diffusion process at high temperature.

Figures

Figure 1.

 Schematic illustration of (a) MgO crystal, (b) “room” of O ion and (c) “room” of magnesium ion, defined in this study. magnesium ions are yellow and O ions are green.

Figure 2.

 Schematic illustration of (a) one snapshot in MD simulation results and (b) the “room” of O ion. Mg ions are yellow and O ions are green. The “room” with Frenkel defect has no O ion in the "room." The “room” with two O ions has the interstitial atoms.

Figure 3.

 Schematic illustration of the correction method of determination of atomic "room."

Figure 4.

 (a) Thermal expansions of molar volumes at 0 GPa. (b) Compressions of molar volumes at 2000 K.

Figure 5.

 A sequence of snapshots from (a) to (e) of simulated MgO structure at 0 GPa and 3000 K. Mg ions are yellow and O ions are green. The interval between snapshots is each 80 MD steps. The arrows indicate the motion of the Mg ion and the dotted circles approximately show the Frenkel defect.

Figure 6.

 Time dependences of MSD of Mg and O ions at 0 GPa and 3000 K. Mg ions are yellow and O ions are green. The indicated areas in green show the MD step ranges of the appearance of Frenkel defect for Mg ion.

Figure 7.

 Temperature dependences of MSD at 0 GPa.

Figure 8.

 Temperature dependences of the appearance frequency of Frenkel defect for Mg ion at 0 GPa.

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