A review of the oxidation–pressure concept (OPC) and extended Zintl–Klemm concept (EZKC), and the emergence of the high-pressure Ni₂In-type phase of lithium sulfide (Li₂S) 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 Ni₂In phase of Li₂S, 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 Ni₂In phase of Li₂S is the final phase in the double transition antifluorite → anticotunnite → Ni₂In, as is observed in other alkali metal sulfides. This new phase for Li₂S could be expected after knowledge of the high-pressure Cmcm phase of Li₂SO₄, which is a distortion of the hexagonal I-Na₂SO₄ phase, both having M₂S subarrays of the Ni₂In-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 Li₂S and for the pseudo-lattice Ψ-BeS derived by applying the EZKC to Li₂S, after which, Li₂S can be reformulated as Li⁺[LiS]⁻ ≡ Li⁺[Ψ-BeS].