A review of the oxidation–pressure concept (OPC) and extended Zintl–Klemm concept (EZKC), and the emergence of the high‐pressure Ni 2 In‐type phase of lithium sulfide (Li 2 S) rationalized by reference to a newly defined stability enhancement ratio ( S )

Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61 , 220–224] on the pressure‐induced Ni 2 In phase of Li 2 S, at 30 GPa, three concepts related to high‐pressure phase transitions are reviewed here. This paper firstly reviews evidence that chemical oxidation (by inclusion of oxygen atoms) can produce a similar effect to the application of physical high pressure and temperature, in an effect labelled as the oxidation–pressure concept. Secondly, the pressure‐induced Ni 2 In phase of Li 2 S is the final phase in the double transition antifluorite → anticotunnite → Ni 2 In, as is observed in other alkali metal sulfides. This new phase for Li 2 S could be expected after knowledge of the high‐pressure Cmcm phase of Li 2 SO 4 , which is a distortion of the hexagonal I‐Na 2 SO 4 phase, both having M 2 S subarrays of the Ni 2 In‐type. Thirdly, in order to clarify these links, a simple methodology is proposed for gauging the level of increased stability (by defining a stability enhancement ratio, S ) when the extended Zintl–Klemm concept (EZKC) has been applied. The method uses relative values of the lattice potential energies estimated for Li 2 S and for the pseudo‐lattice Ψ‐BeS derived by applying the EZKC to Li 2 S, after which, Li 2 S can be reformulated as Li + [LiS] − ≡ Li + [Ψ‐BeS].