Infrared and Raman spectra of the elpasolites Cs2NaSbCl6 and Cs2NaBiCl6: Evidence for a pseudo Jahn-Teller distorted ground state

Giant Electron–Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite

“…The room-temperature non-resonant Raman spectrum measured using 808 nm laser light is presented Figure 3b, with the relative wavelengths used for Raman excitations within this paper indicated in Figure 2b via the red vertical line. Here we see a total of 4 bands below 200 cm −1 , as expected from our group factor and finestructure analysis, with their intensity and frequency in complete agreement with that measured from analogous elpasolite systems [28][29][30] . According to our assignment, the narrow low frequency band at 40 cm −1 is ascribed to the T 2g mode of the Cs + ions, while the 3 other more intense bands at 75, 135 and 175 cm −1 belong respectively to the T 2g (breathing), E g and A 1g (both stretching) modes of the (AgBr 6 ) 5− octahedron, given that the following general relation 30…”

The underappreciated lone pair in halide perovskites underpins their unusual properties

“…But this is only part of the story. Obscured in this average structure view are local, dynamic, polar distortions of the octahedral cation environments evident from vibrational spectroscopy, 54 x-ray absorption spectroscopy, 55 pair distribution functions (PDFs) from total scattering, 50,56,57 and ab initio calculations. 50,[58][59][60] Raman spectroscopy 61 has indicated the fluid-like dynamical nature of the perovskite lattices of CsPbBr 3 and MAPbBr 3, part of which is accounted for by the lone-pair activity of Pb 2+ .…”

Composition design, optical gap and stability investigations of lead-free halide double perovskite Cs2AgInCl6

“…14,15 Among them, Cs 2 NaBiCl 6 is a well-known compound and its ferroelectric properties were reported. [16][17][18][19] Very recently, three research groups evaluated Cs 2 AgBiX 6 (X ¼ Cl À , Br À ) independently in order to discover new solar cell absorbers, and indirect band gaps were determined. [20][21][22] Such a kind of lead-free double perovskite becomes a new host topic for materials discovery, and Cs 2 AgInCl 6 has been predicted 1a and recently reported as a new lead-free halide double perovskite with a direct band gap by Volonakis et al, which shows potential for solar cell absorbers.…”