Lattice Dynamics and Electron–Phonon Coupling in Double Perovskite Cs₂NaFeCl₆

Abstract

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 1g 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.

In(iii)-dictated formation of double Cs2AgxNa1−xFeyIn1−yCl6 perovskites

“…3b). This position is still quite different from the A 1g band frequency reported for [FeCl 6 ] 3À octahedra in pure CNFC and CANFC single crystals, 294-302 cm À1 , 15,21,22 indicating a strong effect of In III addition on the lattice dynamics, even at the indium content being as small as 1%.…”

Raman Spectroscopy of Formamidinium‐Based Lead Mixed‐Halide Perovskite Bulk Crystals

“…For example, Raman spectroscopy can be used for in situ studies of devices fabricated on glass substrates, i. e ., buried layers. Low‐frequency lattice vibrations can also be detected by low wavenumber Raman spectroscopy [25,26] . It has been shown that due to the Fröhlich interaction, the scattering from longitudinal optical phonons dominates the electron scattering in metal halide double perovskites at room temperature, which results in multi‐phonon processes up to the fourth order in the non‐resonant Raman spectrum [27] .…”

“…Low-frequency lattice vibrations can also be detected by low wavenumber Raman spectroscopy. [25,26] It has been shown that due to the Fröhlich interaction, the scattering from longitudinal optical phonons dominates the electron scattering in metal halide double perovskites at room temperature, which results in multi-phonon processes up to the fourth order in the non-resonant Raman spectrum. [27] Furthermore, Raman spectroscopy proved capable for the characterization of the surface structural changes that go hand in hand with perovskite decomposition upon light irradiation.…”

Photoactivated Second Harmonic Generation in Centrosymmetric Double Perovskites

“…The most common reason for the variation of the SHG response with temperature is an alternation of bulk crystal symmetry due to the crystal structure or electronic phase transitions. ,,, However, previous structural characterization studies of the investigated double perovskite materials using temperature-dependent XRD, neutron diffraction, and Raman measurements ,,,− and also our temperature-dependent XRD and Raman measurements of Cs 2 NaFeCl 6 (see Sections S5 and S6) indicate that there is no structural phase transition in both materials in the vicinity of the critical temperatures. It was also reported that even without the structural phase transition, Cs 2 NaFeCl 6 could experience temperature-dependent lattice disorder, which could, in principle, lead to an enhanced SHG response.…”

“…The investigated double perovskite single crystals of Cs 2 NaFeCl 6 and Cs 2 AgBiBr 6 were synthesized by the hydrothermal method as described in Materials and Methods. The obtained crystals typically exhibit a truncated octahedron morphology consisting of flat top and bottom surfaces and six side facets, all having a { 111 } crystallographic orientation. , For SHG measurements, a transmission geometry was chosen, as schematically illustrated in Figure a. Here, a pulsed fundamental light beam with the frequency ω, which propagated along the direction perpendicular to the sample surface, was focused on the back side of the sample.…”