Large Uncertainties in the Thermodynamics of Phosphorus (III) Oxide (P₄O₆) Have Significant Implications for Phosphorus Species in Planetary Atmospheres

Phosphorus(III) oxide (P4O6) has been suggestedto be a major component of the gas phase phosphorus chemistry in theatmospheres of gas giant planets and of Venus. However, P4O6's proposed role is based on thermodynamic modeling,itself based on values for the free energy of formation of P4O6 estimated from limited experimental data. Values ofthe standard Gibbs free energy of formation (Delta G (o) ((g))) of P4O6 in theliterature differ by up to similar to 656 kJ/mol, a huge range. Dependingon which value is assumed, P4O6 may either bethe majority phosphorus species present or be completely absent frommodeled atmospheres. Here, we critically review the literature thermodynamicvalues and compare their predictions to observed constraints on P4O6 geochemistry. We conclude that the widely usedvalues from the NIST/JANAF database are almost certainly too low (predictingthat P4O6 is more stable than is plausible).We show that, regardless of the value of Delta G (o) ((g)) for P4O6 assumed,the formation of phosphine from P4O6 in theVenusian atmosphere is thermodynamically unfavorable. We concludethat there is a need for more robust data on both the thermodynamicsof phosphorus chemistry for astronomical and geological modeling ingeneral and for understanding the atmosphere of Venus and the gasgiant planets in particular.