Thermodynamic properties of uranium compounds. Part 2.—Low-temperature heat capacity and entropy of three uranium nitrides

Abstract

The heat capacities of UN, UN_1.59 and UN_1.73 have been measured from 11 to 320°K. The C_p curves for the three compounds show anomalies at 52, 94 and 33°K respectively, corresponding to magnetic ordering phenomena. The values of the standard thermodynamic functions are tabulated, together with heats and entropies of the magnetic transitions. UN undergoes type 1 antiferro-magnetic ordering at 52°K; the entropy increment associated with this ordering (0.15 cal deg.^–1 mole^–1) is substantially less than the expected magnetic entropy. Comparison is made with magnetic transitions in other uranium compounds, notably UO₂ and US whose magnetic entropies may be calculated with reasonable accuracy. It seems that the entropy of spin ordering represents only a part of the total magnetic entropy. Possible reasons for this are discussed.