  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.
