Expansion Coefficients and Transformation Characteristics of Solid Oxygen

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

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