Low-Frequency Phonon Modes in Layered Silver-Bismuth Double Perovskites: Symmetry, Polarity, and Relation to Phase Transitions

Metal-halide perovskites (PSKs) are emergent materials for a large range of applications, and the layered double PSK architectures vastly enrich the opportunities to design their composition, structural properties, and optoelectronic behavior. The stability, crystal phase, and electronic bandgap depend strongly on the bonds and distortions of the octahedra lattice that are at the origin of the vibrational spectrum of these materials. This work investigates the structural dynamics of flakes of exfoliated layered Ag-Bi bromide double PSKs by angle-dependent polarized Raman spectroscopy and density functional theory modeling. The well-defined orientation of the inorganic octahedra lattice with respect to the light polarization allows to correlate the angle-dependent intensity of the Raman signal to the directionality and symmetry of the phonon modes. Low-frequency vibrations are revealed for which a detailed microscopic and group theory assignment of the Raman modes is provided. The temperature-dependent measurements across the phase transitions show marked changes in the phonon frequencies, reveal soft modes, and help to distinguish first from second-order transitions as well as to determine their transition temperature. This provides highly valuable insights to improve the properties of this class of Pb-free PSKs for applications in energy harvesting and optoelectronics.