The b-g Transition of Oxygen as a Secondary Fixed Point of the ITS-90

Oxygen is a substance in which two solid-to-solid phase transitions occur in the low temperature range, in addition to the triple point at 54 K: the beta–gamma solid-to-solid transition at 43.8 K and the alfa–beta transition at 23.8 K. The triple point of oxygen is a fixed point of the International Temperature Scale of 1990. The solid-to-solid transitions are included in the list of BIPM – CCT secondary fixed points. In the paper, the thermal properties at the beta–gamma transition are presented. The transition is of first order, associated with large structural changes. Measurements were carried out at INTiBS and at INRIM using oxygen sealed cells manufactured and filled at INRIM. Two methods – continuous and pulse heating – were used to determine the oxygen temperature across the transition. A flat plateau was observed for the part of the transition dominated by the phase, while, where the phase becomes predominant, the transition temperature starts to increase substantially along with the sample time constant.