The α-β phase transition in solid oxygen

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Abstract
Two important factors that determine the mechanism and temperature of the alpha - beta phase transition in solid oxygen are analysed: the magneto-elastic coupling which increases the temperature of the transition, and the antiferromagnetic order in the beta -phase which stabilises this phase at low temperatures, thus lowering the transition point. It has been shown that the competition between magnetic and elastic forces leads to an instability of the crystal and, consequently, to the phase transition. The stability limit of the beta -phase is analysed within the coupled-susceptibility approach, and the stability of the alpha -phase is deduced from measured magnetic susceptibility. It is concluded that the very low temperature of the phase transition (as compared with the Neel temperature of the alpha -phase) is a consequence of the short-range antiferromagnetic order in the beta -phase. Furthermore, it is shown that the alpha - beta transition is clearly of first order.