Cryocrystal phase transitions applied as temperature standards

Abstract

International temperature scales define several fixed points, the temperature values of which are determined with a very high precision. Some of them, which lie in a low-temperature range, are called gaseous fixed points because the reference substances under normal conditions—in a room-temperature range—are in a gaseous state. When occurring in a solid state at low temperature these reference substances are called cryocrystals or quantum crystals by physicists. These substances play an important role in a standard thermometry. The triple point of argon, oxygen, neon, and hydrogen are primary fixed points of the International Temperature Scale of 1990 (ITS-90) [H. Preston-Thomas, Metrologia 27, 3 (1990)].1 The triple points of nitrogen, methane, and carbon dioxide are secondary fixed points of the scale [R. E. Bedford, G. Bonnier, H. Maas, and F. Pavese, Metrologia 33, 133 (1996)].2 Also, the solid-solid phase transitions in the cryocrystals are secondary fixed points of the scale. The highest-accuracy methods of measurements applied in standard thermometry can be used to determine, with a very small uncertainty, the phase transition temperatures and their reproducibility and stability. The reproducibility and stability of the transition temperature are the most important factors deciding about usefulness of these points in thermometry. Using the highest-accuracy methods, better than 1mK, which at present are used only in metrology, can give new information about the nature and properties of the investigated substances, also of importance for physicists.