Molecular Packing, Defects, and Transformations in Solid Oxygen

The crystal structure of $\alpha$ oxygen (monoclinic, $\frac{C2\mathrm{}}{m}$, two molecules per cell) determined by Barrett, Meyer, and Wasserman, is consistent with ${\mathrm{O}}_2$ molecules in contact along two rows in the (001) plane, and along two rows lying a plane that is 80.5° from (001); contact is defined as the touching of the contours at the 0.002 level of electron density of the contoured electron-density maps for ${\mathrm{O}}_2$ computed by Bader, Henneker, and Cade. Twinning, with a twinning shear of 19° in the [100] direction on the (001) plane, should be easy; both twins and stacking faults are likely to be common and produce features in the x-ray diffraction patterns. The $\beta$-${\mathrm{O}}_2$ phase can be seen as closely related to the $\alpha$ phase: if the (001) planes shear into an ABCABC... sequence and the configuration in the (001) plane is hexagonalized, the $\beta$-type structure is obtained. In this structure, if the O-O bonds are tipped about 75° to the threefold axis and precess around it, there will be contacting molecules in three directions normal to the threefold axis and also along the triad axis of the rhombohedral cell. Calculated densities of the $\alpha$ and $\beta$ phases are different at the transition temperature; therefore, the transformation is of first order. The order previously has been in doubt because of the sensitivity of the transformation to stress and to inhibiting factors which spread it over a temperature range.