Unique thermodynamic relationships for Δ_fH^o and Δ_fG^o for crystalline inorganic salts. I. Predicting the possible existence and synthesis of Na₂SO₂ and Na₂SeO₂

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M₂S subnets of M₂SO_x (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M₂S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397–398; Vegas (2000), Crystallogr. Rev. 7, 189–286; Vegas et al. (2002), Solid State Sci. 4, 1077–1081]. These structural changes suggest that if M₂SO₂ would exist, its cation array might well have an anti-CaF₂ structure. On the other hand, in an analysis of the existing thermodynamic data for M₂S, M₂SO₃ and M₂SO₄ we have identified, and report, a series of unique linear relationships between the known Δ_fH^o and Δ_fG^o values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M₂SO_x (x = 3 and 4), and the similarly between M₂S₂ disulfide (x = 0) and disulfur oxyanion salts M₂S₂O_x (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δ_fH_o) for as yet unprepared compounds, M₂SO (x = 1) and M₂SO₂ (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609–3620], the values of the absolute entropies were estimated and from them, the standard Δ_fS^o values, and then the Δ_fG^o values of the salts. A tentative proposal is made for the synthesis of Na₂SO₂ which involves bubbling SO₂ through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate ΔG^o to be approximately −500 kJ mol⁻¹. However, examination of the stability of Na₂SO₂ raises doubts and Na₂SeO₂ emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na₂S and Na₂SeO₄. Like Na₂SO₂, this compound is predicted to have an anti-CaF₂ Na₂Se subnet.