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

An illustration of the crystallography involved

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'.