Quantum chemical and experimental thermodynamic studies of HfO(g)

Hafnium dioxide vaporizes primarily to HfO(g) in a reducing environment. The thermochemistry of HfO(g) is calculated from quantum methods and measured via Knudsen effusion mass spectrometry. For the computations, all-electron and relativistic effective core potential calculations are used. The calculation of an accurate dissociation energy and an entire potential energy curve is reported. These calculations lead to Δ_fH°(298) = 63.19 ± 10 kJ/mol, S°(298) = 235.52 J/mol K, and C_p(298-2500 K) = (2.741 × 10^-9)T³ - (9.853 × 10^-6)T² + (1.295 × 10^-2)T + 2.761 × 10^-1 J/mol K. Experimentally, HfO(g) is generated from the reaction of Hf(s) and HfO₂(s) in a specially made Hf Knudsen cell. A third law treatment of the data leads to Δ_fH°(298) of 58.4 ± 12.3 kJ/mol, in good agreement with the calculated value.