Thermodynamics of MgB2-by calorimetry and Knudsen thermogravimetry

The vapor pressure of MgB₂, corresponding to the reaction, 2MgB₂(cr)→MgB₄(cr)+Mg(g), has been measured by Knudsen effusion vacuum thermogravimetry. The vapor pressure over the range 600°C-850°C can be expressed as: logp_mg(Pa)=-1.129141(10000/T)+7.328161, where T is the temperature in Kelvin. By extrapolation, the 0.1 MPa decomposition temperature of MgB₂ is estimated to be 1268°C. In a separate calorimetric investigation on the same material, using the isoperibol solution method, we measured the enthalpy of formation of MgB₂, Δ_fH_MgB2°(298.15), to be -(155.9±14.2)kJ·mol^-1. By combining the vapor pressure data and the enthalpy data, the Mg-rich part of the Mg-B phase diagram can be calculated. The measured vapor pressures are approximately an order of magnitude higher than those derived from published thermodynamic data. The lower limit of Mg pressures for the deposition of high-purity MgB₂ thin films at any given temperature may therefore be at a higher value than previously thought.