A review of the oxidation-pressure concept (OPC) and extended Zintl-Klemm concept (EZKC), and the emergence of the high-pressure Ni_2In-type phase of lithium sulfide (Li_2S) rationalized by reference to a newly defined stability enhancement ratio (S)

摘要

Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61, 220-224] on the pressure-induced Ni_2In phase of Li_2S, 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_2In phase of Li_2S is the final phase in the double transition antifluorite →anticotunnite→Ni_2In, as is observed in other alkali metal sulfides. This new phase for Li_2S could be expected after knowledge of the high-pressure Cmcm phase of Li_2SO_4, which is a distortion of the hexagonal I-Na_2SO_4 phase, both having M_2S subarrays of the Ni_2In-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_2S and for the pseudo-lattice ψ-BeS derived by applying the EZKC to Li_2S, after which, Li_2S can be reformulated as Li~+[LiS]~- ≡ Li~+[ψ-BeS].