Prediction of Novel Li-AgII-F Compounds using Evolutionary Algorithms

TL;DR

This work addresses the lack of characterization for Li–Ag(II)–F ternaries and their potential magnetic properties by combining an XtalOpt-based evolutionary search with DFT+U calculations to identify low-energy Li–Ag–F structures. The study finds several metastable polymorphs relative to LiF and AgF2 on the convex hull, with entropic stabilization unlikely under typical conditions, yet proposes non-equilibrium synthesis routes involving fluorination with $F_2$ or $F^*$ radicals that could realize these phases. A standout result is LiAgF3_2, which exhibits a record-like AFM superexchange of $J = -359$ meV within $[Ag_2F_7]$ dimers, highlighting exceptionally strong magnetic interactions in Ag(II)-fluoride networks. Overall, the work expands the materials space for highly correlated fluorides and provides actionable guidance for experimental attempts to synthesize and study these metastable, magnetically rich systems.

Abstract

This work provides a theoretical exploration of the thermodynamic stability and magnetic behaviour of previously unknown ternary Li AgII F compounds. Convex-hull analysis shows that all predicted structures lie slightly above the LiF plus AgF2 decomposition line, indicating a natural tendency toward phase separation; nevertheless, their negative formation energies relative to AgF, LiF, and F2 or F suggest that alternative synthetic pathways may be feasible for these compounds. All studied structures show preference for antiferromagnetic ground state. Notably, the triclinic LiAgF3 type2 is predicted to exhibit an exceptionally large superexchange constant, J equal to minus 358 meV, within Ag2F7 dimers, placing it above the strongest known magnetic exchange interactions reported to date.

Figures (4)

• Figure 1: Crystal structures of the investigated LiAgF$_3$ structures obtained from DFT calculations. Large dark-gray spheres - Ag(II), small gray spheres - F, green spheres - Li. (top) Ag -- F sublattice, (middle) Li -- F sublattice, and (bottom) polyhedral coordination spheres are shown.
• Figure 2: Crystal structures of the investigated Li$_2$AgF$_4$ structures obtained from DFT calculations. Large dark-gray spheres - Ag(II), small gray spheres - F, green spheres - Li. (top) Ag -- F sublattice, (middle) Li -- F sublattice, and (bottom) polyhedral coordination spheres are shown.
• Figure 3: a - Local magnetic structure of LiAgF$_3\_1$ showing the orientations of the spin moments on the Ag(II) cations. The solid lines indicate the nearest-neighbor superexchange pathways ($J_1$), and the dashed blue lines the next-nearest-neighbor ones ($J_2$); bond lengths and angles are shown; b - Comparison of different structural motifs containing the $\mathrm{[Ag_2F_7]}$ unit, including chain- and dimer-type arrangements, together with their corresponding magnetic superexchange constants.
• Figure 4: a - Convex hull of the investigated systems as a function of the AgF$_2$ molar fraction. All structures lie slightly above the convex hull, indicating that they are thermodynamically unstable with respect to binary fluorides; b - Schematic energy profiles ($\Delta E$ [kJ/mol]) for the formation of the most stable lithium–silver(II)–fluorine compounds. The left diagram corresponds to LiAgF$_3\_1$, and the right one to Li$_2$AgF$_4\_1$. The figure indicates that the synthesis of the presented structures is feasible, though it may proceed via alternative substrates (i.e., not from AgF$_2$ and LiF).