Phase transitions and orientational order in thin oxygen films

The structure of condensed oxygen films has been studied by the transmission electron diffraction method. Specimens were prepared directly in situ by depositing 99.99% pure gas on amorphous carbon and polycrystalline Al substrates. Measurements were made in the temperature range between 5 K and the sublimation point. It was established that the as-grown oxygen films had a short-lived α′ structure, which was transformed into an α-phase or a β modification depending on the temperature range. The temperature behavior of the structural characteristics of oxygen deposits has been investigated. An unusual character of the α–β transition was manifested in the appearance of α′ reflections on the diffraction pattern during the transformation. A few possible mechanisms of the transition are discussed in the paper. The influence of quenching conditions on crystals’ growth has also been investigated. The new conditions for the formation of a magnetically distorted α′-phase were established. An unknown phase of solid oxygen was observed in samples deposited under certain conditions. One of its possible origins is an altered orientational arrangement of molecules in closely packed layers. The conditions required for the disordered phase of solid oxygen to form have been determined. Amorphous oxygen was also observed as a result of rapid cooling of crystalline samples.