Anion-polarisation-directed short-range-order in antiperovskite Li2FeSo

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties.  In this paper*, published in Journal of Materials Chemistry A, Professor Saiful Islam and his collaborators characterise the cation short-range order in the antiperovskite cathode material Li₂FeSO, using density functional theory, Monte Carlo simulations and synchrotron X-ray pair-distribution-function data.  

The authors predicted partial short-range cation-ordering, characterised by favourable OLi₄Fe₂ oxygen co-ordination with a preference for polar cis-OL₄Fe₂ over non-polar trans-OLi₄Fe₂ configurations.  this preference for polar cation configurations produced long-range disorder, in agreement with experimental data.

The predicted short-range-order preference contrasted with that for a simple point-charge model, which instead predicted preferential trans-OLi₄Fe₂ oxygen-co-ordination and corresponding long-range crystallographic order.  The absence of long-range order in Li₂FeSO was therefore be attributed to the relative stability of cis-OLi₄Fe₂ and other non-OLi₄Fe₂ oxygen-co-ordination motifs.

The authors explain that this effect is associated with the polarisation of oxide and sulfide anions in polar co-ordination environments, which stabilised the polar short-range cation orderings.  They propose that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges.  Their analysis illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, whereas other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

*'Anion-polarisation-directed short-range-order in antiperovskite Li₂FeSO'.