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

Using synchrotron angle-dispersive x-ray diffraction and Raman spectroscopy on samples of ${\mathrm{Li}}_2\mathrm{O}$ 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 $50(\pm 5)\mathrm{GPa}$ at ambient temperature. This transition is accompanied by a relatively large volume collapse of $5.4(\pm 0.8)\%$ and large hysteresis upon pressure reversal ($P_{\mathit{\text{down}}}$ at $\sim 25\mathrm{GPa}$). Contrary to a recent study, our data suggest that the high-pressure $\beta$-phase $(B_o=188\pm 12\mathrm{GPa})$ is substantially stiffer than the low-pressure $\alpha$-phase $(B_o=90\pm 1\mathrm{GPa})$. A relatively strong and pressure-dependent preferred orientation in $\beta \text{−}{\mathrm{Li}}_2\mathrm{O}$ is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides.

Figure 1

(Color online) (a) antifluorite $\alpha \text{−}{\mathrm{Li}}_2\mathrm{O}$ structure. (b) Anticotunnite $\beta \text{−}{\mathrm{Li}}_2\mathrm{O}$ structure showing the tri-capped trigonal prismatic coordination. Large atoms represent oxygen and smaller represent lithium.

Figure 2

(Color online) Rietveld refined x-ray diffraction profile of $\alpha$- and $\beta \text{−}{\mathrm{Li}}_2\mathrm{O}$. For the diffraction patterns shown, the final refinement converged to $R\left(F^2\right)=0.1054$ for the $\alpha$ phase and $R\left(F^2\right)=0.1197$ for the $\beta$ phase. In the high pressure phase, only the most intense reflections are labeled. Unit cell parameters for the phase were determined from the positions of the most isolated and/or intense peaks: (002), (011), (111), (211), (013), and (020).

Figure 3

(Color online) ${\mathrm{Li}}_2\mathrm{O}$ ADXD patterns across the phase transition from cubic to orthorhombic, showing the large pressure range of two-phase coexistence.

Figure 4

Powder diffraction rings of $\beta \text{−}{\mathrm{Li}}_2\mathrm{O}$ at $53\mathrm{GPa}$ (a) and $61\mathrm{GPa}$ (b), showing the presence of texture in this phase, and the increase in preferred orientation with pressure. The three most prominent rings shown are the (011), $\left(102\right)+\left(200\right)$, and (111) reflections.

Figure 5

(Color online) EOS for the two ${\mathrm{Li}}_2\mathrm{O}$ phases. In the main plot, solid curves are the Birch-Murnaghan EOS fits to the experimental data (shown as open circles) in this study. Solid squares are the experimental data from Ref. 12 and dotted curves are the theoretically calculated EOS (Ref. 12) for both phases. Inset: Trends in the evolution with pressure of the lattice parameters in the $\beta$ phase. Empty circles are data from this study (error bars shown when they exceed size of data points), and solid squares are experimental data from Ref. 12.

Figure 6

Raman spectra upon increasing (a) and decreasing (b) pressure. Cosmic radiation spikes were removed from two of the spectra.

Figure 7

(Color online) The shift in pressure of ${\mathrm{Li}}_2\mathrm{O}$ Raman bands. Solid lines are fits to the experimental data from this study. Red dotted lines represent the calculated theoretical pressure dependence of the Raman frequencies from Ref. 12. In the cubic phase, the theoretical curve lines up exactly with the experimental result from this study. Vertical dashed lines approximate the phase transition pressure upon increasing and decreasing pressure.

Figure 8

(Color online) (a) $\alpha \text{−}{\mathrm{Li}}_2\mathrm{O}$ along the (111) plane, showing the transition mechanism to $\beta \text{−}{\mathrm{Li}}_2\mathrm{O}$ (b). For the cubic structure shown in (a), all oxygen ions are coplanar, located midway between planes of lithium ions which are separated by $1.032\mathrm{\AA }$ near $50\mathrm{GPa}$. For the orthorhombic structure shown in (b), half the oxygen ions have moved into the lower plane of Li ions (shown as colored polyhedra) and half into the upper (empty), with the planes separated by $1.402\mathrm{\AA }$ near $50\mathrm{GPa}$.

Figure 9

(Color online) Comparison of ${\mathrm{Li}}_2\mathrm{O}$ pressure behavior with that of the alkali-metal sulfides. ${\mathrm{H}}_2\mathrm{O}$ may transition to a cubic antifluorite-type phase above $170\mathrm{GPa}$, and, in the nonmolecular form, may be expected to follow the same trends as the alkali metal chalcogenides. ▿ represents the high pressure limit of experiments.