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

• Published in: Advanced optical materials 2022-07, Vol.10 (14), p.n/a
• Main Authors: Martín‐García, Beatriz, Spirito, Davide, Lin, Miao‐Ling, Leng, Yu‐Chen, Artyukhin, Sergey, Tan, Ping‐Heng, Krahne, Roman
• Format: Article
• Language: English
• Subjects: anisotropy; Bismuth; Bonding strength; Crystal structure; Density functional theory; double perovskites; Dynamic structural analysis; Energy gap; Energy harvesting; Group theory; Lattice vibration; layered metal‐halide perovskites; Luminous intensity; Materials science; Optics; Optoelectronics; Perovskites; Phase transitions; Phonons; polarized Raman spectroscopy; Raman spectroscopy; Symmetry; Temperature dependence; Transition temperature; vibrational modes; Vibrational spectra
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202200240

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. Single flakes of layered perovskites are mechanically exfoliated such that their octahedra lattice planes are parallel to the substrate surface. This enables a detailed mapping of the directionality of the vibrational modes of the inorganic lattice by polarized Raman spectroscopy. A detailed study of the phonon modes in double perovskites supported by density functional and group theory is presented.