The heat capacity of MgCr₂O₄, FeCr₂O₄, and Cr₂O₃ at low temperatures and derived thermodynamic properties

The heat capacity of synthetic eskolaite, Cr₂O₃, and of the synthetic spinels magnesiochromite, MgCr₂O₄, and chromite, FeCr₂O₄ were measured from 1.5 K to 340 K. For MgCr₂O₄, a substantial magnetic contribution to the entropy is revealed by a sharp peak in the heat capacity curve at 12.55 ± 0.05 K, which indicates the transition to antiferromagnetic long-range order. Integration of the heat capacity curve yields a value of 118.3 ± 1.2 J/(mol·K) for the standard entropy at 298.15 K, which is in excellent agreement with that calculated from phase equilibria studies on the reaction MgCr₂O₄ + SiO₂ = Cr₂O₃ + MgSiO₃. The new calorimetric results for Cr₂O₃ indicate a standard entropy at 298.15 K of 82.8 ± 0.8 J/(mol·K). The measurements for FeCr₂O₄ show three distinct heat capacity anomalies, one of which (peaking at 36.5 ± 0.2 K) was missed by previous low temperature heat capacity measurements, which only extend down to 53 K. Integration of the heat capacity curve yields a value for the standard entropy at 298.15 K of 152.2 ± 3.0 J/(mol·K) for FeCr₂O₄, some 6 J/(mol·K) greater than the previous calorimetric value. These low-temperature heat capacity data were combined with high-temperature heat content measurements from the literature to derive heat capacity equations for all three phases to 1800 K. The resulting heat capacity equations were then used to extract revised recommended values of the standard enthalpies of formation and entropies of MgCr₂O₄ and Cr₂O₃ from phase equilibrium data. For FeCr₂O₄, the phase equilibrium data are dubious accuracy, the enthalpy of formation is only approximate.