Single-Crystal Structural Characterization of the Metallic Phase of Oxygen

Angle dispersive x-ray diffraction measurements of oxygen single crystals in a helium pressure transmitting medium have been performed up to 133 GPa at room temperature. The crystalline structure of metallic $\zeta$ oxygen, above 96 GPa, is shown to be associated with a continuous displacive structural transformation taking place in the $ab$ plane of the monoclinic $C2/m$ cell of the insulator $\epsilon$ phase. Although of first order, the change of crystalline structure to the metallic $\zeta$ phase from the $\epsilon$ phase is found to be isosymmetric, in agreement with recent calculations. Our Raman spectroscopy measurements on a single crystal of the $\zeta$ phase corroborate the present structure assignment. The interplay between metallization and structural changes is disclosed.

Figure 1

(a) Details of the diffraction image of crystal $B$ of solid oxygen at 133 GPa, overlaid with the indexing of reflections arising from identified twins, indicated by normal and italic indexing, respectively. (b) A magnification of Fig. 1a, illustrating two different reflections belonging to the twins. (c) and (d) Photomicrograph of two oxygen single crystals, surrounded by He in Re gaskets at 125 GPa, with the $c$ axis perpendicular and parallel to the image, respectively. The field of view for both images is approximately $75\times 75\mu {\mathrm{m}}^2$.

Figure 2

(a) Integrated x-ray diffraction pattern of four selected reflections at 133 GPa. (b) Raman spectrum of crystal $B$ of solid oxygen in helium at 118 GPa at 300 K. The stars indicate the positions of the observed Raman active bands. No Raman signal from oxygen is observed between 900 and $1600{\mathrm{cm}}^{-1}$.

Figure 3

Pressure dependence of the lattice constants $a$ ($●$), $b$ (▪), $c$ (▴) and angle $\beta$ of the $C2/m$ monoclinic cell: $a_0=7.970\AA$, $b_0=5.633\AA$, $c_0=3.711\AA$, and $\beta =116.3°$ at 13.2 GPa [5]. The present data are the filled gray (red) symbols. Data plotted with open symbols are taken from Ref. 8. The solid black symbols are the calculation of Ma, Oganov, and Glass [11]. The vertical dashed line indicates the metallic transition pressure.

Figure 4

Equations of state of solid oxygen, in the $\epsilon$ and $\zeta$ phases with a comparison between experiment (EDXD data are taken from Ref. 8) and calculations of Ma, Oganov, and Glass [11] and Kim et al. [12]. The vertical dashed lines indicate the pressure range of the displacive transition.