Pressure-induced antifluorite-to-anticotunnite phase transition in lithium oxide

Using synchrotron angle-dispersive x-ray diffraction and Raman spectroscopy on samples of Li2O pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite (alpha, Fm-3m) to orthorhombic anticotunnite (beta, Pnma) phase at 50(+/- 5) GPa at ambient temperature. This transition is accompanied by a relatively large volume collapse of 5.4 (+/- 0.8)% and large hysteresis upon pressure reversal (P-down at similar to 25 GPa). Contrary to a recent study, our data suggest that the high-pressure beta-phase (B-o=188 +/- 12 GPa) is substantially stiffer than the low-pressure alpha-phase (B-o=90 +/- 1 GPa). A relatively strong and pressure-dependent preferred orientation in beta-Li2O is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides.