Structural Features and Optical Properties of All-Inorganic Zero-Dimensional Halides Cs4PbBr6-x I (x) Obtained by Mechanochemistry

Despite the great success of hybrid CH3NH3PbI3 perovskite in photovoltaics, ascribed to itsexcellentoptical absorption properties, its instability toward moisture isstill an insurmountable drawback. All-inorganic perovskites are muchless sensitive to humidity and have potential interest for solar cellapplications. Alternative strategies have been developed to designnovel materials with appealing properties, which include differenttopologies for the octahedral arrangements from three-dimensional(3D, e.g., CsPbBr3 perovskite) or two-dimensional (2D,e.g., CsPb2Br5) to zero-dimensional (0D, i.e.,without connection between octahedra), as the case of Cs4PbX6 (X = Br, I) halides. The crystal structure of thesematerials is complex, and their thermal evolution is unexplored. Inthis work, we describe the synthesis of Cs4PbBr6-x I (x) (x = 0, 2, 4, 6) halides by mechanochemical procedures with green credentials;these specimens display excellent crystallinity enabling a detailedstructural investigation from synchrotron X-ray powder diffraction(SXRD) data, essential to revisit some features in the temperaturerange of 90-298 K. In all this regime, the structure is definedin the trigonal R3 c spacegroup (#167). The presence of Cs and X vacancies suggests some ionicmobility into the crystal structure of these 0D halides. Bond valencemaps (BVMs) are useful in determining isovalent surfaces for bothCs(4)PbBr(6) and Cs4PbI6 phases,unveiling the likely ionic pathways for cesium and bromide ions andshowing a full 3D connection in the bromide phase, in contrast tothe iodide one. On the other hand, the evolution of the anisotropicdisplacement parameters is useful to evaluate the Debye temperatures,confirming that Cs atoms have more freedom to move, while Pb is moreconfined at its site, likely due to a higher covalency degree in Pb-Xbonds than that in Cs-X bonds. Diffuse reflectance ultraviolet-visible(UV-vis) spectroscopy shows that the optical band gap can betuned depending on iodine content (x) in the rangeof 3.6-3.06 eV. From density functional theory (DFT) simulations,the general trend of reducing the band gap when Br is replaced byI is well reproduced.