Expansion Coefficients and Transformation Characteristics of Solid Oxygen

As determined by x rays on polycrystalline samples, the rhombohedral $\beta$-oxygen phase (stable from 23.8 to 43.7°K) has a linear expansion coefficient $\frac{\Delta l}{l}=1150\mathrm{}\times {10}^{-6\mathrm{}}$ per °K in the $a$ direction and a very small, but definitely negative expansion coefficient in the $c$ direction. The expansion coefficient of $\gamma$-oxygen (stable from 43.7 to 54.3°K) is also quite high, $\frac{\Delta l}{l}=780\mathrm{}\times {10}^{-6\mathrm{}}$ per °K. Diffraction patterns of the $\alpha$, $\beta$, and $\gamma$ phases, photographed during temperature cycling, showed that a grain of $\alpha$ forms from a grain of $\beta$ with the (001) plane of $\alpha$ forming the (00.1) plane (hexagonal indices) of $\beta$ without changing its orientation, and vice versa, martensitically. However, $\beta$ forms from $\gamma$ with a breakup into near-randomly oriented smaller and/or highly distorted $\beta$ grains.