Impact of Metal Cation on Chiral Properties of 2D Halide Perovskites

Abstract

Chiral two-dimensional (2D) halide perovskites are formed by embedding chiral organic cations in a perovskite crystal structure. The chirality arises from distortions of the 2D metal halide layers induced by the packing of these organic cations. Sn-based octahedra spontaneously distort, but it remains unclear whether this intrinsic structural instability enhances the chirality. We investigate the effect of the metal cation on structural and phonon chirality in MBA$_{2}$Sn$_{\mathrm{x}}$Pb$_{1-\mathrm{x}}$I$_{4}$ (x = 0, 1/2, and 1). Incorporating Sn does distort the metal halide octehedra, yet it only has a minor impact on the structural chirality. In contrast, the phonons in MBA$_{2}$SnI$_{4}$ are substantially more chiral than in MBA$_{2}$PbI$_{4}$, especially the in-plane acoustic modes. However, this enhanced phonon chirality does not lead to a generation of a larger angular momentum under a temperature gradient, because the contributions of different chiral phonons tend to compensate one another.

Figures (6)

• Figure 1: Structural descriptors for octahedral distortions in MBA2Sn_xPb_1-xI4. Unit cells of chiral (a) (S-MBA)2PbI4 and (b) (S-MBA)2SnI4. (c) PbI6 octahedron showing a X$-$M$-$X angle ($\theta_{i}$) used to compute the bond angle distortion $\sigma^{2}$. (d) SnI6 octahedron showing an M$-$X bond length ($d_{i}$) used to compute the bond length distortion $\Delta d$.
• Figure 2: Finite temperature octahedral distortions in (S-MBA)2Sn_xPb_1-xI4. Bond angle variance $\sigma^{2}$ of (a) (S-MBA)2PbI4 (x $=$ 0), (b) ordered mixed and (c) randomly mixed (S-MBA)2Sn_0.5Pb_0.5I4 (x $=$ 1/2), and (d) (S-MBA)2SnI4 (x $=$ 1). Bond length variance $\Delta d$ of (e) (S-MBA)2PbI4 (x $=$ 0), (f) ordered mixed and (g) randomly mixed (S-MBA)2Sn_0.5Pb_0.5I4 (x $=$ 1/2), and (h) (S-MBA)2SnI4 (x $=$ 1). The insets show (a-d) side view and (e-h) top view of the metal halide octahedra.
• Figure 3: Temperature dependence of degree of chirality in (S-MBA)2Sn_xPb_1-xI4. The values of $\chi^{y}$ are shown for (a) $\epsilon_{\ch{A2}}$, (b) $\epsilon_{\ch{MX4}}^{\parallel}$, (c) $\epsilon_{\ch{MX4}}^{\perp}$, and (d) $\Delta r_{\mathrm{HB}}$. The investigated perovskites have different metal cation compositions including (S-MBA)2PbI4 (x $=$ 0), ordered mixed (S-MBA)2Sn_xPb_1-xI4 (x $=$ 1/2), and (S-MBA)2SnI4 (x $=$ 1).
• Figure 4: Phonons in chiral 2D perovskites. (a) Phonon dispersions with Brillouin zone is sampled along high-symmetry paths Γ$-$X, Γ$-$Y, and Γ$-$Z. The dispersion of (S-MBA)2PbI4 is shown in gray and that of (S-MBA)2SnI4 is shown in blue. Phonon density of states (DOS) of the low energy region (0 - 25) of (b) (S-MBA)2PbI4 and (c) (S-MBA)2SnI4. A Gaussian broadening of 0.1 is used for the DOS.
• Figure 5: Circularly polarized phonon dispersions of (a-c) (S-MBA)2PbI4 and (d-f) (S-MBA)2SnI4. Phonon branches are colored according to their circular polarization. Red, blue and gray represent right-handed ($s^{\alpha}_{\mathbf{q},\sigma}$$>$ 0), left-handed ($s^{\alpha}_{\mathbf{q},\sigma}$$<$ 0), and non-polarized ($s^{\alpha}_{\mathbf{q},\sigma}$$=$ 0) phonon modes.