RESUMO
Hybrid organic-inorganic perovskites, MAPbX3 (X = halogen), containing methylammonium (MA: CH3-NH3+) in the large voids conformed by the PbX6 octahedral network, are the active absorption materials in the new generation of solar cells. CH3NH3PbBr3 is a promising member with a large band gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure-property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr3 in optoelectronics.
RESUMO
The synthesis and crystal structure evolution of the double perovskite Pb2YSbO6 is reported for the first time. The structure has been analyzed in the temperature range between 100 and 500 K by using a combination of synchrotron and neutron powder diffraction. This compound shows two consecutive first order phase transformations as previously observed for a subgroup of Pb2RSbO6 perovkites (R = rare earths). The thermodynamic parameters associated with the phase transitions were calculated using differential scanning calorimetry (DSC), and the role of the diverse cations of the structure was studied from DFT calculations for the room temperature polymorph. The crystal structure evolves from a C2/c monoclinic structure (a(-)b(-)b(-) tilting system in Glazer's notation) to another monoclinic P2(1)/n (a(-)a(-)b(+)) phase with an incommensurate modulation and finally to a cubic Fm3m perovskite (a(0)a(0)a(0)). The highly distorted nature of the room temperature crystal structure seems to be driven by the polarization of the Pb lone pair which shows a marked local effect in the atomic spatial arrangements. Moreover, the lone pairs have been localized from DFT calculations and show an antiferroelectric ordering along the b monoclinic axis.