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

All-inorganic lead halide perovskites like CsPbBr, CsPbI, or RbPbI are good replacements for the classical hybrid organic-inorganic perovskites like CHNHPbI, 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 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 crystal structure is characterized by the presence along the direction of the crystal of double rows of edge-sharing PbI octahedra. The lone electron pairs of Pb ions have a strong stereochemical effect on the PbI 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 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 at 460 K.