Phase Diagram and Cation Dynamics of Mixed MA1-xFAxPbBr3 Hybrid Perovskites
Mantas Šimėnas, J. Banys, Šarūnas Svirskas, Martynas Kinka, Maciej Ptak, Vidmantas Kalendra, Sergejus Balčiūnas, Daria Szewczyk, Adam Sieradzki, Robertas Grigalaitis, Aron Walsh, Mirosław Mączka, Anna Gągor
Abstract
Mixing\nof molecular cations enhances the optoelectronic properties\nand stability of hybrid lead halide perovskites. Here, we use a multitechnique\napproach to determine the phase diagram and molecular cation dynamics\nof mixed methylammonium-formamidinium MA<sub>1–<i>x</i></sub>FA<i><sub>x</sub></i>PbBr<sub>3</sub> (0 ≤ <i>x</i> ≤ 1) hybrid perovskites. The calorimetric, ultrasonic,\nand X-ray diffraction experiments show a substantial suppression of\nthe structural phase transitions and stabilization of the cubic phase\nupon mixing. We use the broad-band dielectric and Raman spectroscopies\nto study the MA and FA cation dynamics in these compounds. The broad-band\ndielectric spectroscopy indicates the absence of the MA cation ordering\nand a gradual increase of the rotation barrier upon mixing. The room-temperature\ndielectric permittivity substantially decreases as the fraction of\nthe FA cations is increased. No significant changes of the permittivity\nare detected at temperatures, where the dielectric relaxations are\nabsent. We also observe weak signatures of a dipolar glass phase for\nthe intermediate mixing levels. The Raman spectroscopy supports the\ndielectric results and reveals additional subtle information about\nthe FA cation dynamics.