Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite Cs₂NaFeCl₆

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs₂NaFeCl₆ single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA ₁ overtones up to the fifth order. Strong electron-phonon coupling in Cs₂NaFeCl₆ is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs₂NaFeCl₆, important for potential applications of this novel material.