Thermodynamic characterization of NiAl

Thermodynamic properties of the high-stability intermetallic compound nickel aluminide, NiAl, have been determined from mass-spectrometric, weight-loss effusion, and calorimetric measurements, using samples from a single preparation with a composition determined to be Ni0.986Al1.014. Per mole of NiAl molecules, the specific heat capacity at room temperature of 298 K is 48.54 J · K−1 · mol−1, with a linear temperature dependence of +0.0104 J · K−2 · mol−1. At the same temperature, the enthalpy of formation is −133.7 kJ · mol−1, the entropy is about 53.8 J · K−1 · mol−1 and the enthalpy difference between room temperature and absolute zero is 7.97 kJ · mol−1. The Gibbs free-energy is −130.2 kJ · mol−1 at T = 298 K, with a linear temperature dependence of +5.04 J · K−1 · mol−1. The Debye temperature is 452 K, while the electronic density-of-states at the Fermi-level is about 0.29 states per eV-atom. The NiAl+ ions were observed in the high-temperature mass spectra. Pressures for the gas at these temperatures were estimated and used with the results of quantum-mechanical calculations of total energy, specific heat, and entropy to calculate free-energy functions for the gas. These and additional results are compared with other measurements and discussed in terms of current theories of the electronic and structural properties of the compound.