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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 Δf Ho and Δf Go 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 (Δf Ho) 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 Δf So values, and then the Δf Go 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 Δ Go 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. [ABSTRACT FROM AUTHOR]

The study of phase transitions is usually restricted to two to three transformations. Examples of such transitions include the CaF2 → PbCl2 → Ni2In in alloys, the NaCl → CrB → CsCI or the well documented transformation olivine → spinel of the oxides A2XO4. These transitions, traditionally regarded as partial processes, have prevented the construction of wider structure maps. One of the scarce examples of these maps was reported by Léger and Haines (Eur J Solid State Inorg Chem 34:785-796, 1997) concerning the phase transitions of AX2 compounds (dihalides and dioxides), where increasing the coordination number of the A atom is linked to the pressure increase. The structural information, collected in these maps, is always of interest because it limits the number of possible transition paths which may relate a structure-type into another. However, a careful analysis of the partial phase transitions undergone by different com- pounds, at high temperature and high pressure, reveals that the partial transitions are not isolated processes but they overlap, forming a long, rational pathway that connects all the structures in a coherent manner. Alloys and their related oxides show a similar trend along their concurrent pathways which complement each other. In this work, the analysis is restricted to the AX2 alloys and their corresponding oxides AX2O4, and the results demonstrate that there exists a unifying principle that can be inferred through the simultaneous analysis of all the phase transitions involved in the concurrent structural journeys carried out by both types of compounds. The AX2 alloys begin the walk in the fluorite-type structure, ending in the MoSi2-type structure. In the case of the oxides AX2O4, their cation arrays follow a concurrent pathway that, starting at the filled fluorite-type structure, ends in the final Sr2PbO4-type structure. These structural "journeys" also allows for the discovery of several "missing links" (structure types) which fit into the general sequence and help one understand the whole transitions pathway as a rational process, which takes place simultaneously in the alloys as well as in the cation arrays of the oxides. Very recent works show that alkali metals (Na and K) also join the walk. The extended Zintl-Klemm concept (EZKC) and the concept that relates of oxidation-pressure-temperature effects provide a basis for understanding the observed transitions. [ABSTRACT FROM AUTHOR]