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

With the use of lattice dynamics calculation within the hybrid HSE06 framework we were able to understand the vibrational spectra of MAgF3 (M = K, Rb, Cs) compounds. A comparative theoretical study uncovered a lack of monotonicity in calculated optical phonons associated with Ag-F stretching modes, which can be explained through an interplay of the Lewis acidity of the M(i) 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 means of low-temperature powder X-ray diffractometry (at 80 K) nor low-temperature Raman spectroscopy (at 154 K) due to a number of constraints. We also describe a novel Ruddlesden-Popper phase of Rb3Ag2F7 that can be regarded as a quasi-0D system, where the superexchange coupling constant between the nearest Ag(ii) centres reaches an impressive value of -240.2 meV.