Abstract
Vibrational dynamics in halide double perovskites govern several key aspects including carrier recombination and transport properties. Here, we present comprehensive vibrational studies investigated through micro-Raman spectroscopy to understand how octahedral cation substitution in a wide range of metal halide double perovskites (; ) influence lattice vibrations. A significant enhancement in mode intensity—a key factor in determining the cation ordering—is observed with substitution. In contrast to a generally observed trend, despite similar ionic sizes of and , an increase in cationic ordering is observed as substitutes at site in and . The mode intensity depends on -site cationic ordering ( or ), while its vibrational energy is governed by the -site cations ( or ). The symmetric stretching vibrations depicted by mode are mainly influenced by octahedra. The reduction in the linewidth of symmetric-stretching LO phonon mode () and the disappearance/diminishing of asymmetric-stretching vibrations () further substantiates the improved cationic ordering. The changes in the vibrational mode intensities with -site substitution () and the appearance of distinct octahedral modes with -site substitution () allow us to disseminate different octahedral contributions to the vibrational dynamics in the lattice. Further, the vibrational analyses on double perovskites with different choices of and cations and anion reveal the origin of asymmetric stretching (). This mode mainly prevails when sublattice distortions in the lattice exist. Thus, asymmetric-stretching mode can be a measure of sublattice distortion in the double perovskite, and a highly ordered system would exhibit very minimal or no asymmetric vibrations.
1 More- Received 21 January 2023
- Revised 3 April 2023
- Accepted 15 May 2023
DOI:https://doi.org/10.1103/PhysRevMaterials.7.065401
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