The heat capacity of iron monoarsenide has been determined by adiabatic calorimetry from 5 to 1030 K and bv drou calorimetrv relative to 298.15 K over the range 875 to 1350 K. A small h-type transition is observed at ?;, = (70.95 kO.02) K. It is related & the disappearance of a doubly helically ordered magnetic-spin structure on heating. The obviously cooperative entropy increment of transition is only A&JR = 0.021. The higher-temperature heat capacity rises considerably above lattice expectations. Part of the rise is ascribed to low-spin electron redistribution in iron, while the further excess above 800 K presumably arises from a beginning low-to high-spin transition, possibly connected with interstitial defect formation in the MnPtype structure. FeAs melts at about 1325 K with A

The heat capacity of chromium arsenide has been measured by adiabatic calorimetry from 5 to 1050 K and enthalpy increments have been taken over the range 875 to 1280 K with respect to 298.15 K by drop calorimetry. The heat capacity shows a distinct bell-shaped transition with a peak at 259.9 K related to the disappearance of antiferromagnetic helical ordering on heating. The enthalpy and entropy of this transition are 177 Cal* mol-' and 0.69 calth K-l mol-I, respectively. At 1170 K another transition is observed related to the phase change from the MnP-to the N&-type structure. The enthalpy and entropy of the latter gradual transition are 280 c& mol-1 and 0.22 cal, K-' mol-r, respectively. Thermodynamic functions have been evaluated and the vahres of C,, {S'(T)-S'(O)>,-{G"(T)-W(O)}/T at 298.15 K are 12.501, 15.40, and 6.990 c& K-l mold1 and 16.09, 32.53, and 19.86 calth K-l mol-r at 1000 K.

The heat capacity of CrSbz has been measured by adiabatic calorimetry from 5 to 991.3 K. At the latter temperature the CrSb,-phase decomposes into the CrSb-phase and an antimonyrich melt. The heat capacity of the two-phase mixture was measured from 991.3 to 1050 K. The heat capacity of CrSb, shows a small sharp h-type transition with a maximum at 274.1 K where the change from the antiferromagnetic to the paramagnetic state occurs. Thelow entropy of the clearly cooperative part of the transition, A& = 0.12 calth K-l mol-I, shows that this contribution is only a small part of the total. From an estimate of the lattice heat capacity of CrSbz outside the h-transition region we find an excess heat capacity amounting to about 1.7 calth K-l mol-l at 300 K, 1.6 calth K-l mol-' at 500 K, and 1.1 calth K-' mol-1 at 800 K, which we attribute to the population of excited electronic states in CrSbz. The total transitional entropy amounts to about 2.6 cal,,, K-l mol-1 at 900 K only slightly more than the R In 3 (= 2.17 calth K-l mol-') expected from randomization of two unpaired spins per chromium atom. The enthalpy of the peritectic decomposition of CrSb, at 991.3 K is (8325120) calth mol-I. The high heat capacity above 991 K is presumably related to the solution of CrSb(s) in the meh. Thermodynamic functions have been evaluated and the values of C,, {S"(T)-s"(O)}, and-{G"(T)-H'(O)]/T at 298.15 K are (19.66*0.02), (27.46*0.03), (14.OO9&0.014) calth K-l mol-I. CrSb, loses antimony on approaching the peritectic temperature and the composition of the decomposing phase is in the range CrSb,,,, to CrSbl.g5. To explore further the homogeneity range of the CrSba-phase some heat-capacity measurements on CrSb1.85 have also been carried out. Combination of the present results with standard Gibbs energies of formation at 850 K from the literature gives for CrSbz.ao: 298.

The heat capacities of triuranium tetraarsenide (U3As4) and triuranium tetraantimonide (U3Sb4) , measured by adiabatic calorimetry over the temperature range 5-950 K, show sharp)t-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 U3As 4 and 6.8 cal K ~ mole-t for U3Sb 4. These are larger than the two literature values reported for U3P 4 (1.5 and 3.1 cal K-j mole-J). The fact that these entropy of transition values are much smaller than would be expected from ASt = R In (2J + 1) for the 3H 4 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 950K are estimated as 11.05 and 12.95 cal K ~mole-~ for U~As4 and 1.13Sb4, respectively. Thermal functions for both U3As 4 and U3Sb 4 are integrated from the experimental data up to 950 K.

Both Zn2+ and Cda+ dopants on the A-sites in FeaOd lead to a loss in the furcation of the lambda anomaly at the Verwey transition but at markedly different dopant levels. Equilibrium adiabatic calorimetry from 5 to 350 K on new compositions of cadmium-and zincdoped magnetites, e.g. Cd0.002Fe2.99804 and Zno.oIFea.ssOr, permits an estimate of the mole ratios of Cd and Zn at which they have equivalent effects on the furcation of the lambda anomaly. These results are consistent with amechanism in which the furcation of the Verweytransition in doped samples depends linearlyon the deviationof the lattice constant from that of pure Fe304. Further studies on a deliberately oxidized sample demonstrate that the furcated lambda anomaly is characteristic of stoichiometric materials; apparently B-site vacancies have a much greater effect on the transition than A-site dopants. Heat capacities and thermodynamic properties of the three samples are summarized.