Unveiling the Structural Properties, Optical Behavior, and Thermoelectric Performance of 2D CsSn2Br5 Halide Obtained by Mechanochemistry

Abstract

Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn₂Br₅ specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn²⁺ lone electron pair (5s²) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr₄] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately ∼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (∼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 10⁴ μV K^-1 at 350 K, which evolves down to negative values of -3.0 × 10³ μV K^-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m^-1 K^-1.

Figure 1

Le Bail fit for the mechanochemically prepared CsSn₂Br₅ at room temperature from laboratory XRD data from Cu Kα radiation.

Figure 2

Thermogravimetric curve and its respective derivative [d(weight)/dT] showing weight loss evolution of the CsSn₂Br₅ sample (a). DSC curve emphasizing the endothermic and exothermic peaks in the respective heating and cooling runs (b).

Figure 3

FE-SEM images with 8000× (a), 13 454× (b), and 27 000× (c) magnifications.

Figure 4

Rietveld plots for CsSn₂Br₅ at room temperature, where the red crosses represent the observed profile; the full black line is the calculated profile, and the difference is the blue line below. The Bragg positions are displayed as green vertical bars. (a) SXRD and (b) NPD data.

Figure 5

Views along the a-axis (a) and b-axis (b) of the layered crystal structure of CsSn₂Br₅; the [SnBr₄] polyhedra establish layers within the ab plane; Cs atoms are intercalated in between; the distribution of the four Sn–Br bonds in square pyramids is driven by the 5s² lone pair repulsion. The green, gray, and brown atomic representations denote cesium (Cs), lead (Pb), and bromide (Br) atoms, respectively.

Figure 6

(a) [SnBr₄] polyhedron and differences in distances and angles obtained from SXRD and NPD. (b). Distribution of the residual electron density, from X–N techniques, superposed with the [SnBr₄] pyramidal structure.

Figure 7

Thermal evolution of the isotropic displacement parameters (U_iso) for the different atoms within the CsSn₂Br₅ crystal structure.

Figure 8

CsSn₂Br₅ unit cell and the electron density map of [001] (a) and [11̅0] (b) planes. Those planes were chosen to provide a better visualization of the topochemical isolines between Cs–Br and Sn–Br bonds.

Figure 9

(a) Kubelka–Munk transformed diffuse reflectance spectrum of CsSn₂Br₅ (b). Band structure calculated from DFT methods for CsSn₂Br₅ showing an indirect gap transition (Σ→Ζ).

Figure 10

Electrical resistivity ρ (a), Seebeck coefficient S (b), power factor S²σ (c), thermal conductivity κ (d), and thermoelectric figure of merit ZT (e) for the CsSn₂Br₅ sample.