The heat capacities of U3As4 and Th3As4

The heat capacities of the arsenides of uranium (U3As4) and Th (Th3As4) have been measured in the temp. range 5-300 deg K. A sharp gamma -shaped anomaly at 195.8 deg K, associated with the ferromagnetic transition, was observed in U3As4. The smoother specific heat curve Cp(T) for nonmagnetic analog Th3As4 has been used to determine the lattice contribution to the heat capacity of U3As4. The resulting magnetic entropy at 290 deg K has been estimated as 10.67 J/K mole of U atom, which is close to the theoretical R ln 4 value. This fact is interpreted in terms of crystal field interaction in the crystal lattice with S4 point symmetry. Critical entropy and energy parameters are compared with the theoretical predictions for 3d Ising and Heisenberg models. The critical coeficients are also considered.21 refs.--AA