Actinoid pnictides--II : Heat capacities of UAs2 and USb2 from 5 to 750 K and antiferromagnetic transitions

Abstract

The heat capacities of uranium diarsenide (UAs2) and uranium diantimonide (USb2), with tetragonal structures of the anti-Cu2Sb-type, have been measured by adiabatic-shield calorimetry from 5 to about 750 K. Lambda-type transitions with maxima at 272.2 and 202.5 K for UAs2 and USb2, respectively, are related to maxima in the magnetic susceptibilities at 277 and 203 K, occasioned by transitions from antiferro- to paramagnetism in the compounds. Values of the heat capacities (Cp), entropies [S[deg](T) - S[deg](0)], and Gibbs energy functions -{[G[deg](T) - H[deg](o)]/T} at 298.15 K in cal K-1 mole-1 are 19.12, 29.41 and 15.05 for UAs2 and 19.16, 33.81 and 18.39 for USb2. Tentative resolutions of the cooperative magnetic heat capacities of UAs2 and USb2 lead to the magnetic entropies [Delta]S(mag) = 0.99 and 1.70 cal K-1 mole-1, respectively. The values for both are significantly lower than the spin-only magnetic entropy value R ln 3 = 2.18 cal K-1 mole-1.