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

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.