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 electrochem ical 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 o f this cell is directly related by the Nernst equation to the Gibbs fr ee energy change for the conversion of diamond to graphite. The differ ence in the chemical potential of carbon in the two crystal structures varies linearly with temperature in the range 940 to 1260 K mu(C)(dia mond) - mu(C)(graphite) = 1100 + 4.64T(+/-50) J mol(-1). On the averag e, the values given by the equation are 320 J mol(-1) less positive th an the currently accepted ones based on calorimetric studies. The diff erence is primarily in the enthalpy term.