Ab Initio Thermodynamic Evaluation of Ruthenium Tetroxide (RuO4) Vapor Pressure

In this study, the sublimation and vapor pressure characteristics of RuO4 were systematically investigated using ab initio thermodynamic calculations. Structural optimizations and vibrational frequency analyses were performed for gaseous RuO4 and four candidate solid phases (monoclinic Cm, P21/c, C2/c, and cubic P-43n) within the density functional theory (DFT) framework. Gibbs free energies were evaluated by incorporating electronic energies, zero-point corrections, and entropic contributions from translational, rotational, and vibrational modes. The results identify monoclinic C2/c and cubic P-43n as the most stable solid phases across the studied temperature range. Calculated sublimation temperatures of 322 K at 1 atm and 240 K at 1 × 10−3 atm were obtained in good agreement with experimental melting and boiling points. Calculated vapor pressures show reasonable agreement with experimental measurements below the triple point, with deviations at higher temperatures attributable to approximating liquid-gas behavior using solid-gas sublimation data. These findings provide the first theoretical description of RuO4 vapor pressure and offer a computational framework extendable to other transition-metal ALD precursors.