Thermal Behavior of Solid CH3D in the Region T <10°K

Abstract

An attempt was made to determine the heat capacity of isotopically pure CH3D in the temperature regime where a Schottky-type anomaly was known to occur. Before measurements were performed, the calorimeter system was usually kept at about T=20°K for 24 h or so. The results did not agree with extensive ones obtained earlier by Colwell for CH3D of normal isotopic purity nor were they reproducible from day to day. It had been suggested by Colwell that conversion occurs between the nuclear spin species of this methane at low temperatures and that his results corresponded to the equilibrium mixture at each temperature. Some experiments were therefore done with isotopically and chemically pure CH3D to which had been added 0.17% of O2, the object being to try to catalyze the conversion. While the addition of the O2 had a small effect, the heat capacities were still not reproducible. A subsequent detailed study of a specimen of CH3D of normal isotopic purity disclosed a slow evolution of energy (characteristic time ≥30 min) when the specimen was first cooled below T=10°K. If the calorimeter vessel was held at temperatures of 4°K or less for several hours, then the heat capacities became reproducible and similar to but not identical with those obtained by Colwell. The cause of the slow process cannot be established with certainty but the magnitude of the energy released and the fact that the process can be reversed suggest that conversion between the spin species of CH3D is involved. Several experimental tests showed that the process could not be associated with the λ-type anomalies that occur in solid CH3D at T=16.1 and 23.1°K nor with pathological behavior of the calorimeter system.