ABSTRACT
Understanding of the structural properties of hybrid organic-inorganic perovskites (HOIPs) and their behavior is crucial for their use as photovoltaics and for the design and assembly of solar cells. As part of this work, a detailed study was conducted to further understand bismuth iodide perovskites, with a specific focus on the phase transitions of methylammonium and formamidinium analogs. A detailed analysis of the temperature-dependent IR spectra was also performed in order to analyze the structural changes that occur. The presence of five phases in the methylammonium bismuth iodide (MABiI) and four phases in formamidinium bismuth iodide (FABiI) were determined. An additional confirmation of the reported results was obtained from the differential scanning calorimetry. The ambiguities concerning the crystal structure of FABiI were resolved based on the results by X-ray powder diffraction (XRPD).
ABSTRACT
Detailed studies of the Li4GeÐ4-Li2CaGeÐ4 system by solid-phase syntheses of various compositions from pure Li4GeÐ4 to pure Li2CaGeÐ4 in the temperature range from 25 to 1125 °C is investigated for a first time. Solid state synthesis powders are characterized by X-ray and DSC/TG methods. Concentration and temperature two-phase regions of Li4GeÐ4 and Li2CaGeÐ4 as well as two-phase regions of Li2CaGeÐ4 and Li2+2xCa1-xGeO4 are established. Region of pure Li2+2xCa1-xGeO4 solid solution are detected too and it structure is investigated. Being structural analog to Li2+2xZn1-xGeO4 and Li2+2xMg1-xGeO4, Li2+2xCa1-xGeO4 has own specific local environment of the metal ions. The obtained results are compared with those for Li4GeÐ4-Li2ZnGeÐ4 system and for Li2+2xZn1-xGeO4 solid solution. The differences of the phase diagrams and structural features of the solid solutions are discussed.