Pressure-Induced Antifluorite-to-Anticotunnite Phase Transition in Lithium Oxide

Using synchrotron angle-dispersive x-ray diffraction (ADXD) and Raman spectroscopy on samples of Li$_2$O pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite ($\alpha$, Fm3m) to orthorhombic anticotunnite ($\beta$, Pnma) phase at 50($\pm$5) GPa at ambient temperature. This transition is accompanied by a moderate volume collapse of 5.4 ($\pm$0.8)\% and large hysteresis upon pressure reversal (P$_{down}$ at $\sim$25 GPa). Contrary to a recent study, our data suggest that the high-pressure $\beta$-phase (B$_o$ = 188 $\pm$12 GPa) is substantially stiffer than the low-pressure $\alpha$-phase (B$_o$ = 90$\pm$1 GPa). A relatively strong and pressure-dependent preferred orientation in $\beta$-Li$_2$O, resulting in changes diffraction intensities, is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides. This work has been supported by LLNL, University of California, under the auspices of the U.S. DOE under Contract No. W-7405- ENG-48 and by the Stockpile Stewardship Academic Alliances Program under grant DOE DE-FG03-03NA00071, and by the NSF(ITR 031339) at UCD.