DETERMINATION OF THE GIBBS ENERGY OF DIAMOND USING A SOLID-STATE CELL

The chemical potential of carbon in diamond, relative to its value in graphite, has been directly determined using a solid state electrochemical cell incorporating single crystal CaF2 as the solid electrolyte. The cell can be represented as Pt, C(graphite) + CaC2 + CaF2 parallel to CaF2 parallel to CaF2 + CaC2 + C(diamond), Pt. The reversible emf of this cell is directly related by the Nernst equation to the Gibbs free energy change for the conversion of diamond to graphite. The difference in the chemical potential of carbon in the two crystal structures varies linearly with temperature in the range 940 to 1260 K mu(C)(diamond) - mu(C)(graphite) = 1100 + 4.64T(+/-50) J mol(-1). On the average, the values given by the equation are 320 J mol(-1) less positive than the currently accepted ones based on calorimetric studies. The difference is primarily in the enthalpy term.