Lithium Oxide: A Quantum-corrected and Classical Monte Carlo Study

Extensive Monte Carlo simulations of lithium oxide, Li₂O, an important material
for fusion applications over a wide range of temperatures have been performed. In the low temperature range 1 – 500 K, quantum path-integral corrections to the enthalpy and unit cell size were determined. We show that classical Monte Carlo underestimates both these quantities and the difference between unit cell parameter with and without quantum corrections is large enough that such corrections should be included in any comparison between theory and experiment. Over the range 300 − 1000 K, the formation energy of Schottky and Frenkel defects are calculated and compared with those from direct free energy minimisation in the quasiharmonic approximation, which also includes quantum corrections; the Monte Carlo results highlight the onset of failure of the quasiharmonic approximation even at modest temperatures and suggest only a small variation of the defect enthalpies with
temperature. Several possible diffusion mechanisms are identified. While an
interstitialcy mechanism activates at around 900 – 1000 K, lithium vacancy
migration dominates from 500 K. The estimated migration energy of the Li-vacancy jump (0.28 eV) agrees very well with the most recent NMR study. At temperatures above 1000 K, the superionic phase transition and subsequent melting are simulated and there is good agreement with available experimental data. Our simulations predict a rapid rise in the heat capacity and the thermal expansion coefficient which continues up to the melting point which leaves two interesting questions for future experimental studies: (i) whether above the superionic transition the heat capacity and the thermal expansion coefficient in antifluorite Li₂O rise up to the melting point, as in our simulations, or fall, as observed in several fluorites, and (ii) the subsequent change in the heat capacity during melting.