Heat capacity and thermodynamic properties of CrSb 2 from 5 to 1050 K. Magnetic transition and enthalpy of decomposition

Abstract

The heat capacity of CrSb 2 has been measured by adiabatic calorimetry from 5 to 991.3 K. At the latter temperature the CrSb 2-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 2 shows a small sharp λ-type transition with a maximum at 274.1 K where the change from the antiferromagnetic to the paramagnetic state occurs. The low entropy of the clearly cooperative part of the transition, ΔS t = 0.12 cal th K −1 mol −1, shows that this contribution is only a small part of the total. From an estimate of the lattice heat capacity of CrSb 2 outside the λ-transition region we find an excess heat capacity amounting to about 1.7 cal th K −1 mol −1 at 300 K, 1.6 cal th K −1 mol −1 at 500 K, and 1.1 cal th K −1 mol −1 at 800 K, which we attribute to the population of excited electronic states in CrSb 2. The total transitional entropy amounts to about 2.6 cal th K −1 mol −1 at 900 K only slightly more than the R 1n 3 (= 2.17 cal th K −1 mol −1) expected from randomization of two unpaired spins per chromium atom. The enthalpy of the peritectic decomposition of CrSb 2 at 991.3 K is (8325 ± 20) cal th mol −1. The high heat capacity above 991 K is presumably related to the solution of CrSb(s) in the melt. Thermodynamic functions have been evaluated and the values of C p, { S o( T)− S o(0)}, and −{G o −H o (0)} T at 298.15 K are (19.66±0.02), (27.46±0.03), (14.009±0.014) cal th K −1 mol −1. CrSb 2 loses antimony on approaching the peritectic temperature and the composition of the decomposing phase is in the range CrSb 1.90 to CrSb 1.95. To explore further the homogeneity range of the CrSb 2-phase some heat-capacity measurements on CrSb 1.85 have also been carried out. Combination of the present results with standard Gibbs energies of formation at 850 K from the literature gives for CrSb 2.00: 298.15 K 850 K ΔG inf o/kcal th mol −1 −4.46±0.13 −5.10±0.03 ΔS f o/cal th K −1 mol −1 0.05±0.12 1.59±0.13 ΔH f o/kcal th mol −1 −4.45±0.12 −3.75±0.12