Stability of the peroxide group in $\mathrm{Ba}{\mathrm{O}}_2$ under high pressure

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable $\mathrm{Ba}{\mathrm{O}}_2$ phase at high pressure up to 130 GPa found via a first-principles computational structure search and high-pressure experimental investigations. The identified monoclinic structure (space group $C2/m$) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure $Cmmm$ structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, this phase retains the peroxide group and, interestingly, exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron x-ray diffraction (XRD) measurements could not distinguish between $Cmmm$ and $C2/m\mathrm{Ba}{\mathrm{O}}_2$ definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively like that predicted theoretically due to the $Cmmm$ to $C2/m$ phase transition, thus supporting the predicted transformation.

Figure 1

Crystal structures of various $\mathrm{Ba}{\mathrm{O}}_2$ phases: (a) the tetragonal $\mathrm{Ca}{\mathrm{C}}_2$-type $I4/mmm$ structure, (b) the orthorhombic $Cmmm$ structure, and (c) the monoclinic $C2/m$ structure. The large (green) and small (red) spheres correspond to Ba and O atoms, respectively.

Figure 2

(a) Enthalpy differences per formula unit as a function of pressure for the $I4/mmm$, $Cmmm$, $\mathrm{Al}{\mathrm{B}}_2$-type, and predicted $C2/m$ structures, where the enthalpies of the $Cmmm$ structure are chosen as the basis. The phonon dispersion curves of (b) $C2/m$ and (c) $Cmmm$ at 150 GPa. (d) Changes in energy as a function of the atom displacement [corresponding to the soft phonon modes both at $Z$ (0.0, 0.0, 0.5) and $R$ (−0.5, 0.5, 0.5) points] with various amplitude values at 150 GPa for the $Cmmm$ structure. Inset shows the atomic motion in the soft phonon modes. The pink and blue arrows represent the atomic vibrational modes at the $Z$ point and the $R$ point of the $Cmmm$ structure, respectively.

Figure 3

X-ray diffraction (XRD) patterns measured in the diamond anvil cell experiment of this paper. The data are integrated using DIOPTAS [32], and the background is subtracted. The patterns presented by different colors correspond to different phases. The pressure was increased to 130 GPa at room temperature, and then the sample was laser heated. After this, the pressure dropped to 96 GPa, and XRD patterns were measured on the pressure release. Ticks indicate simulated XRD peaks for different phases, and “Re” denotes the diffraction peak of the rhenium gasket.

Figure 4

Raman spectra of $\mathrm{Ba}{\mathrm{O}}_2$ at various pressures. (a) The Raman spectra of the low-pressure phase transition from $I4/mmm$ to $Cmmm$. The laser excitation wavelength is $\lambda =532\mathrm{nm}$. (b) The Raman spectra of $\mathrm{Ba}{\mathrm{O}}_2$ at high-pressure range with laser excitation wavelength $\lambda =632.8\mathrm{nm}$. (c) The comparison of experimental Raman spectrum with theoretical ones for different structures at 120 GPa.

Figure 5

(a) Frequencies of the measured and calculated Raman modes of $\mathrm{Ba}{\mathrm{O}}_2$ as a function of pressure. The solid gray triangles and open gray circles are the data reported in Ref. [15]. Solid green and blue lines are the calculated Raman data of $Cmmm$ and $C2/m$ structures, respectively. Inset shows zoomed-in experimental and theoretical dependencies at high pressures. The error bars in the Jilin experiment are indicated when they are smaller or larger than the size of the symbols, and the error bars on the frequencies are smaller than the size of the symbols. (b) Volume per formula unit for the orthorhombic $Cmmm$ structure of $\mathrm{Ba}{\mathrm{O}}_2$ at high pressure range in comparison with those previously obtained at lower pressure condition. The squares and green solid line are the experimental and calculated data reported in Ref. [15]. The hexagons and solid purple triangles are the calculated volumes of $Cmmm$ and $C2/m$ structures, respectively. Red and blue solid circles correspond to data in this paper measured before and after laser heating, respectively. The error bars at each data point are indicated when they are smaller or larger than the size of the symbols. The solid line is the Vinet fit, as given in the text, with $B_0=253\left(07\right)\mathrm{GPa}$ and $B_0^{\prime }=4.18\left(6\right)$ and fixed $V_0=36{\AA }^3$ ($P=33.6\mathrm{GPa}$).

Figure 6

(a) Electronic band structure (left panel) and projected density of states (right panel) of $C2/m$ structure at 110 GPa. (b) The band gap values of $Cmmm$ and $C2/m$ structures at various pressures.