Crystal structure, lattice dynamics and superexchange in MAgF3 1D antiferromagnets (M = K, Rb, Cs) and Rb3Ag2F7 Ruddlesden-Popper phase

TL;DR

This study uses lattice dynamics calculations to analyze vibrational spectra and predict structural phases of MAgF3 compounds and Rb3Ag2F7, revealing complex phonon behavior and strong superexchange interactions in a novel phase.

Contribution

It provides the first theoretical prediction of an orthorhombic RbAgF3 polymorph and characterizes a new Rb3Ag2F7 Ruddlesden-Popper phase with significant superexchange coupling.

Findings

Lattice dynamics calculations match vibrational spectra of MAgF3 compounds.

Predicted orthorhombic RbAgF3 as a low-temperature ground state.

Superexchange coupling in Rb3Ag2F7 reaches -240.2 meV.

Abstract

With the use of lattice dynamics calculation within hybrid HSE06 framework we were able to understand vibrational spectra of MAgF3 M = K, Rb, Cs compounds. Comparative theoretical study uncovered lack of monotonicity in calculated optical phonons associated with Ag F stretching modes which can be explained through an interplay of Lewis acidity of MI cation and its size. We confirm the tetragonal unit cells of MAgF3 M=Rb, Cs at room temperature. We also theoretically predict an orthorhombic RbAgF3 polymorph as a ground state at low temperature. However, we were not able to detect it by the means of low temperature PXRD at 80 K nor low temperature Raman at 154 K due to a number of constraints. We also describe a novel Ruddlesden Popper phase of Rb3Ag2F7 which can be regarded as quasi 0D system, where superexchange coupling constant between the nearest AgII centres reaches an impressive value of minus 240.2 meV.