Thermodynamic properties and superconductivity of natural carrollite (CuCo₂S₄)

The thermodynamic properties of natural carrollite with a formula Cu_0.92Co_2.07S₄ are studied using low temperature heat capacity and high temperature oxidative solution calorimetry. The standard thermodynamic functions at 298.15 K are: C_p,m°, S_m°, Δ₀^T H_m°, and Φ_m° are 158.48 J∙K⁻¹∙mol⁻¹, 176.33 J∙K⁻¹∙mol⁻¹, 28.40 kJ∙mol⁻¹, and 81.07 kJ∙mol⁻¹ with an estimated error of 1%. The enthalpies (Δ_fH), entropies (Δ_fS), and Gibbs energies of formation (Δ_fG) from elements at 298.15 K are: −344.46 ± 12.87 kJ∙mol⁻¹, −45.22 ± 1.81 J∙K⁻¹∙mol⁻¹, and −331.14 ± 1.89 J∙K⁻¹∙mol⁻¹. The stability of carrollite relative to the elements is demonstrated with a negative Gibbs energy (ΔG_r°) of formation for several temperatures between 0 and 300 K. A superconductivity transition, which has been observed previously for synthetic carrollite, is confirmed by both the low temperature heat capacity and magnetization measurements.