Thermodynamic properties and superconductivity of natural carrollite (CuCo2S4)

The thermodynamic properties of natural carrollite with a formula Cu0.92Co2.07S4 are studied using low tem-perature heat capacity and high temperature oxidative solution calorimetry. The standard thermodynamic functions at 298.15 K are: Cp,m degrees, Sm degrees, & UDelta;T0 Hm degrees, and & phi;m degrees are 158.48 J & BULL;K- 1 & BULL;mol-1, 176.33 J & BULL;K- 1 & BULL;mol-1, 28.40 kJ & BULL;mol-1, and 81.07 kJ & BULL;mol-1 with an estimated error of 1%. The enthalpies (& UDelta;fH), entropies (& UDelta;fS), and Gibbs energies of formation (& UDelta;fG) from elements at 298.15 K are:-344.46 & PLUSMN; 12.87 kJ & BULL;mol-1,-45.22 & PLUSMN; 1.81 J & BULL;K- 1 & BULL;mol-1, and-331.14 & PLUSMN; 1.89 J & BULL;K- 1 & BULL;mol-1. The stability of carrollite relative to the elements is demon-strated with a negative Gibbs energy (& UDelta;Gr degrees) 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.