On the crystal structure thermal evolution of formamidinium lead tribromide, CH(NH2)2PbBr3

Although methylammonium lead triiodide (CH3NH3PbI3) is recognized as the best candidate for photovoltaic applications, unfortunately it undergoes fast degradation when exposed to moisture and mild temperatures. Among hybrid organic-inorganic perovskites, formamidinium lead tribromide (FA: formamidinium, CH(NH2)2+) is an excellent alternative given its long-term stability. Here we present a structural study from synchrotron X-ray and neutron diffraction of an undeuterated FAPbBr3 specimen, carried out to follow its crystallographic behaviour in the 1-298 K temperature range. Two phase transitions are identified; at 256-261 K from the cubic structure observed at RT (Pm3m), to a tetragonal symmetry (P4/mbm), and at 159-160 K to an orthorhombic phase (space group Pnma). Our neutron diffraction data allowed us to unveil the configuration of the organic FA units and their full localization within the mentioned temperature range, thus improving the crystallographic description of this compound. The evolution with temperature of the H-bonds between the organic molecule and the inorganic cage is followed. The UV-Vis diffuse reflectance spectrum shows a band gap of 2.23 eV, making it a suitable material for optoelectronic devices.Fil: Abia, Carmen. Instituto de Ciencia de Materiales de Madrid; España. Institut Laue Langevin; FranciaFil: Lopez, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Investigaciones en Tecnología Química. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Instituto de Investigaciones en Tecnología Química; Argentina. Instituto de Ciencia de Materiales de Madrid; EspañaFil: Álvarez Galván, María Consuelo. Consejo Superior de Investigaciones Científicas; EspañaFil: Canadillas Delgado, Laura. Institut Laue Langevin; FranciaFil: Fernández Díaz, María Teresa. Institut Laue Langevin; FranciaFil: Alonso, José Antonio. Instituto de Ciencia de Materiales de Madrid; Españ