Detailed Structural Features of the Perovskite-Related Halide RbPbI 3 for Solar Cell Applications.

All-inorganic lead halide perovskites like CsPbBr ₃ , CsPbI ₃ , or RbPbI ₃ are good replacements for the classical hybrid organic-inorganic perovskites like CH ₃ NH ₃ PbI ₃ , susceptible to fast degradation in the presence of humid air. They also exhibit outstanding light absorption properties suitable for solar energy applications. Here, we describe the synthesis of RbPbI ₃ by mechanochemical procedures with green credentials, avoiding toxic or expensive organic solvents; this specimen exhibits excellent crystallinity. We report neutron powder diffraction data, essential to revisit some subtle structural features around room temperature (200-400 K). In all these regimes, the orthorhombic Pnma crystal structure is characterized by the presence along the b direction of the crystal of double rows of edge-sharing PbI ₆ octahedra. The lone electron pairs of Pb ²⁺ ions have a strong stereochemical effect on the PbI ₆ octahedral distortion. The relative covalency of Rb-I versus Pb-I bonds shows that the Pb-I-related motions are more rigid than Rb-I-related vibrations, as seen in the Debye temperatures from the evolution of the anisotropic displacements. The optical gap, measured by diffuse reflectance UV-vis spectroscopy, is ∼2.51 eV and agrees well with ab initio calculations. The thermoelectric Seebeck coefficient is 3 orders of magnitude larger than that of other halide perovskites, with a value of ∼117,000 μV·K ^-1 at 460 K.