Raman diagnostics of new types of A 3 B 2 X 9 layered crystals

Extended Antibonding States and Phonon Localization Induce Ultralow Thermal Conductivity in Low Dimensional Metal Halide

“…All the observed modes are well matched with the theoretical calculation where the vibrational modes are primarily resulting from the vibration of [Bi 2 I 9 ] 3− and Cs + ions. [ 21,74 ] We observed low frequency Raman active modes at ≈34 (E 1g ), 42 (A 1g ), 48 (E 1g ), 58 (A 1g ), and 63 cm −1 (E 1g ) for Cs 3 Bi 2 I 9 . [ 74 ] The E 1g mode at 34 cm −1 largely involved the vibration of Cs and I atoms as seen from the atom projected DOS (Figure 4b), whereas the rest of low frequency modes are due to bending and stretching motion of Bi‐I bond in Cs 3 Bi 2 I 9 .…”

“…[ 21,74 ] We observed low frequency Raman active modes at ≈34 (E 1g ), 42 (A 1g ), 48 (E 1g ), 58 (A 1g ), and 63 cm −1 (E 1g ) for Cs 3 Bi 2 I 9 . [ 74 ] The E 1g mode at 34 cm −1 largely involved the vibration of Cs and I atoms as seen from the atom projected DOS (Figure 4b), whereas the rest of low frequency modes are due to bending and stretching motion of Bi‐I bond in Cs 3 Bi 2 I 9 . [ 21,75 ] Furthermore, we have observed two modes at 90 (E 2g ; asymmetric) and 105 cm −1 (A 1g ; symmetric) due to bridged Bi–I stretching, and three modes at 120 (E 1g ; asymmetric), 127 (E 2g ; asymmetric) and 147 cm −1 (A 1g ; symmetric) due to terminal Bi−I stretching in the [Bi 2 I 9 ] 3− anion.…”

Strong Electron–Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb)

“…The major peaks of Cs 3 Bi 2 I 9 are summarized in Table , and those of the layered A 3 M 2 I 9 compounds are listed in Table . The Raman spectra of the Bi-containing compounds are in good agreement with previous reports by Vakulenko et al The major peaks for Rb 3 Bi 2 I 9 are near 145 and 130 cm –1 , and the spectrum of Cs 3 Bi 2 I 9 shows a series of 9 peaks, with the strongest peaks near 150, 105, and 60 cm –1 .…”

“…A detailed analysis of the vibrational modes of the Cs 3 Bi 2 I 9 space group ( P 6 3 / mmc) indicates that they derive mainly from the [Bi 2 I 9 ] 3– anion . The crystal structure consists of molecular [Bi 2 I 9 ] 3– anions bound together by three Cs + cations per unit.…”

Strong Coupling of Self‐Trapped Excitons to Acoustic Phonons in Bismuth Perovskite Cs3Bi2I9

“…[ 11a ] Shown in Figure b are the simulated diffraction changes as a result of select major intracell optical phonon modes that have atomic displacements along the c axis, with $$A_1^{\prime}$$ terminal Bi–I symmetric stretch (in‐phase and out‐of‐phase between the two [Bi 2 I 9 ] 3− units), $$A_2^{^{\prime\prime}}$$ terminal Bi–I symmetric stretch (in‐phase and out‐of‐phase), E ′ and E ′′ terminal Bi–I asymmetric stretch, and in‐phase $$A_1^{\prime}$$ bridge Bi–I symmetric stretch. [ 11a,28 ] A 5% range of the Bi–Bi distance of 4.042 Å is considered for the movement of a single Bi atom, which is ≈3/4 of the root‐mean‐square displacement $$\sqrt {B/8{\pi }^2} $$ deduced from the equilibrium thermal factor B at low temperature. [ 29 ] Hence, to have a negligible effect on the (00 l ) intensities, the increase in the optical phonon populations and consequently the atomic displacements needs to be small.…”