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). Issue 6 (27th October 2017)

• Record Type: Journal Article
• Authors: Vegas, Angel; Jenkins, H. Donald Brooke
• Is Part Of: Acta crystallographica. Volume 73:Issue 6(2017:Dec.)
• Page Start: 1043
• Page End: 1050
• Publication Date: 2017-10-27
• Subjects: lithium sulfide -- high pressure -- oxidation–pressure concept -- alkali metal sulfates -- crystal chemistry -- stability enhancement ratio (S) -- extended Zintl–Klemm concept (EZKC)
• DOI: 10.1107/S2052520617011131
• Languages: English
• ISSNs: 2052-5206

Abstract

Abstract : The Ni2 In phase of Li2 S ( P 63 / mmc ), recently observed at 30 GPa as the final step in the transition antifluorite→anticotunnite→Ni2 In, provides evidence of the oxidation–pressure concept, since the [Li2 S] subarray of the high‐pressure phase of Li2 SO4 ( Cmcm ) is a distortion of the new Li2 S phase. Following the extended Zintl–Klemm concept (EZKC), Li2 S can be formulated as Li + [LiS] – ≡ Li + [Ψ‐BeS] and its usefulness has been enhanced by calculating the values of the ionic strength I . The enhancement factor, S = I [Ψ‐BeS]/ I [Li2 S] = 1.16, denotes increased stability in [Ψ‐BeS] as brought about by the EKZC. Abstract : Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61, 220–224] on the pressure‐induced Ni2 In phase of Li2 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 Ni2 In phase of Li2 S is the final phase in the double transition antifluorite → anticotunnite → Ni2 In, as is observed in other alkali metal sulfides. This new phase for Li2 S could be expected after knowledge of the high‐pressure Cmcm phase of Li2 SO4, which is a distortion of the hexagonal I‐Na2 SO4 phase, both having M 2 S subarrays of the Ni2 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 Li2 S and for the pseudo‐lattice Ψ‐BeS derived by applying the EZKC to Li2 S, after which, Li2 S can be reformulated as Li + [LiS] − ≡ Li + [Ψ‐BeS].