Cryogenic heat capacities and Neel transitions of two uranium pnictides, UAs2 and USb2

The heat capacities of uranium diantimonide (USb2) and uranium diarsenide (UAs2) - with anti-Cu2Sb structure - have been measured by adiabatic calorimetry from 5 to above 700 K. Both heat capacity curves show sharp, lambda anomalies indicative of cooperative phenomena. The corresponding heat capacity maxima at 272.2 and 202.5 K for UAs2 and USb2, respectively, are related to maxima in respective magnetic susceptibility curves at 283 and 206 K, occasioned by transitions between antiferro- and paramagnetic states (Neel points). The heat capacities at 300 and 350 K (in cal.K-1.mol-1) are 19.11 and 19.18 for UAs2, 19.16 and 19.33 for USb2 (in comparison with 19.2 and 20.3 for (isostructural) UP2 for which the Neel transition was found by Stalinski, et al. at 203K). The differences in the temperature dependency and excess heat capacity between UP2 and the others beyond the Dulong and Petit limit are especially pronounced even at 300 K. Lack of monotonic progression of Neel temperarures with anionic size may be due to the relatively small radius ratio in UP2. Thermodynamic properties for both pnictides as well as the thermodynamics of the transitions have been evaluated. (author)