Phase transitions in solid oxygen as thermometric fixed points

For more than thirty years behaviour of phase transitions in solid oxygen has been the subject of an interest to thermometry because oxygen is one of few substances having three phase transitions in the low temperature range. The triple point of oxygen is the primary fixed point of the International Temperature Scale of 1990 and beta - gamma and alpha - beta phase transitions are recommended as secondary fixed points. In this paper heat capacity of oxygen in the region of the alpha - beta transition is investigated. The heat capacity peak, which has the total width of about 50 mK, has been observed to be composed, in reality, of two sharper peaks separated by 20 mK. Some preliminary results of the beta - gamma transition investigation are also reported.