Influence of the octahedral cation on the evolution of lattice phonons in metal halide double perovskites: Raman spectroscopic investigation of Cs2B′B″Cl6 (B′=Ag1−xNax; B″=Bi1−xInx)

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 ${\mathrm{Cs}}_{2}{B}^{\ensuremath{'}}{B}^{\ensuremath{''}}{\mathrm{Cl}}_{6}$ (${B}^{\ensuremath{'}}={\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}$; ${B}^{\ensuremath{''}}={\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}$) influence lattice vibrations. A significant enhancement in ${F}_{2g}$ mode intensity---a key factor in determining the cation ordering---is observed with ${\mathrm{Na}}^{+}$ substitution. In contrast to a generally observed trend, despite similar ionic sizes of ${\mathrm{Na}}^{+}$ and ${\mathrm{Bi}}^{3+}$, an increase in cationic ordering is observed as ${\mathrm{Na}}^{+}$ substitutes at ${\mathrm{Ag}}^{+}$ site in ${\mathrm{Cs}}_{2}{\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Bi}{\mathrm{Cl}}_{6}$ and ${\mathrm{Cs}}_{2}{\mathrm{Ag}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{In}{\mathrm{Cl}}_{6}$. The ${F}_{2g}$ mode intensity depends on ${B}^{\ensuremath{'}}$-site cationic ordering (${\mathrm{Ag}}^{+}$ or ${\mathrm{Na}}^{+}$), while its vibrational energy is governed by the ${B}^{\ensuremath{''}}$-site cations (${\mathrm{Bi}}^{3+}$ or ${\mathrm{In}}^{3+}$). The symmetric stretching vibrations depicted by ${A}_{1g}$ mode are mainly influenced by $[{B}^{3+}\text{\ensuremath{-}}{X}_{6}]$ octahedra. The reduction in the linewidth of symmetric-stretching LO phonon mode (${A}_{1g}$) and the disappearance/diminishing of asymmetric-stretching vibrations (${E}_{\mathrm{g}}$) further substantiates the improved cationic ordering. The changes in the vibrational mode intensities with ${B}^{\ensuremath{'}}$-site substitution (${\mathrm{Ag}}^{+}, {\mathrm{Na}}^{+}$) and the appearance of distinct octahedral modes with ${B}^{\ensuremath{''}}$-site substitution (${\mathrm{Bi}}^{3+}, {\mathrm{In}}^{3+}$) 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 ${B}^{\ensuremath{'}}$ and ${B}^{\ensuremath{''}}$ cations and $X$ anion reveal the origin of asymmetric stretching (${E}_{\mathrm{g}}$). 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.