Optical and structural properties of perovskite-related structures under extreme conditions of pressure and temperature

Abstract

Octahedra blocks are the main geometrical features of perovskite-related structures and can undergo distortions by application of external fields. Many physical properties, such as ferroelectricity, piezoelectricity, multiferroicity and photovoltaic properties can be related to the octahedra in these structures. Changes in these physical properties can be expected at high-pressure and high-temperature conditions once cations displacements and octahedral distortions are temperature and pressure-sensitive. Therefore, high-pressure/temperature experiments are a straightforward and robust way to explore the structural, optical, and ferroic properties of perovskites-related materials. In this thesis, we studied several perovskites-related structures under extreme conditions of pressure and temperature. Two groups of perovskites were studied, multiferroics materials and halide perovskites. In the multiferroic materials group, a four-layered member of the Aurivillius family (Bi5FeTi3O15 (BFTO)), and a quadruple perovskite (CaMn7O12 (CMO)) were considered. Despite the technological interest of multiferroic compounds, studies reporting the structural stability in complex multiferroic perovskites under hydrostatic pressure are scarce. This thesis aims to fill this gap of information by investigating the pressure-induced phase transitions of BFTO and CMO. For BFTO, a rich sequence of phase transitions was identified by combining Raman spectroscopy with synchrotron powder x-ray diffraction. Both techniques confirm the existence of three phase transition, and the analysis of the strain induced by the orthorhombic distortion allowed us to infer the order of two of them. In turn, we showed that CMO undergoes at least two structural phase transitions up to 19 GPa, which is unusual for quadruple perovskites. In the halide perovskites group, the Ruddlesden-Popper compound Cs2PbI2Cl2, and the 0-D perovskite Cs4PbBr6 were considered. Metal halide structures have emerged as a state-of-the-art photovoltaic material owing to their extraordinary optoelectrical properties, low cost, and simple solution-based fabrication methods. Temperature-dependent photoluminesce (PL), and Stokes-Anti-Stokes Raman measurements in the range (300 – 16 K) were performed for the Cs2PbI2Cl2 compound. The Raman spectra revealed no evidence of structural phase transitions; however, we observed second-order Raman features, and a new PL band arising at low temperatures. Analyses of the temperature-dependent emission linewidth allowed us to establish that a strong electron-phonon coupling not yet reported for this kind of halide perovskites is present in the Cs2PbI2Cl2 phase. Finally, we discuss the changes in the structure and the PL behavior of luminescent Cs4PbBr6 single crystals (SCs) under high-pressure conditions. The structural analysis demonstrated that its structure undergoes two phase transitions around 3.2 and 4.5 GPa. The first phase transition was also observed in nanocrystals, but the second seems to be characteristic of bulk crystals. In our Cs4PbBr6 bulk SCs, the PL emission is completely suppressed at 3.5 GPa, indicating that in this structure, the high-pressure (monoclinic) phase does not produce a favorable condition for the PL phenomena. In line with the other 0D perovskites systems studies under high-pressure conditions, the PL of our system is very similar to the one reported for Cs3Bi2I9. We proposed that the PL of the luminescent Cs4PbBr6 SCs can be associated with the different size distribution of quantum dots or NCs of CsPbBr3 embedded in Cs4PbBr6. Our findings provide valuable insight into the luminescence mechanism making significant inroads into green photoluminescence origin understanding and shedding light on the structural characteristics and PL properties of luminescent Cs4PbBr6 single crystals under extremes conditions.