Actinoid pnictides--I : Heat capacities from 5 to 950 K and magnetic transitions of U3As4 and U3Sb4. Ferromagnetic transitions

The heat capacities of triuranium tetraarsenide (U3As4) and triuranium tetraantimonide (U3Sb4), measured by adiabatic calorimetry over the temperature range 5-950 K, show sharp [lambda]-shaped transitions at 196.1 and 147.5 K, respectively. The maxima are related to the appearance of permanent magnetic moments below 198 and 148 K. Excess cooperative entropies associated with ferromagnetic ordering are tentatively estimated as 6.7 for U3As4 and 6.8 cal K-1 mole-1 for U3Sb4. These are larger than the two literature values reported for U3P4 (1.5 and 3.1 cal K-1 mole-1). The fact that these entropy of transition values are much smaller than would be expected from [Delta]St = R In (2J + 1) for the 3H4 ground term (J = 4) and that the observed heat capacities at high temperatures are much larger than would be expected from lattice plus dilational contributions are evidence of crystal field effects. The total electronic entropies to 950 K are estimated as 11.05 and 12.95 cal K-1 mole-1 for U3As4 and U3Sb4, respectively. Thermal functions for both U3As4 and U3Sb4 are integrated from the experimental data up to 950 K. At 298.15 K, the values of Cpo [So(T)-So(0)] and -{[Go(T)-Ho(0)]/T} in cal K-1 mole-1, are 44.82, 73.87 and 38.97, U3As4 and 44.98, 83.60 and 46.89, for U3Sb4.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23080/1/0000655.pd

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

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22763/1/0000318.pd