The main goal of this project is to determine temperature-induced deformations of the inorganic structure and phase transformations in organic-inorganic perovskites MA1-xEAxPbI3 (x from 0 up to 0.5), where MA and EA stand for small organic molecules: methylammonium and ethylammonium, respectively. Cation engineering is one of the routes to control the structure and properties of halide perovskites to enhance optoelectronic performance and stability. MAPbI3 and its modifications are extensively investigated organic-inorganic semiconductors for effective and affordable solar cells. Mixing of molecular cations brings significant changes in phase diagrams. We expect suppression of the phase transitions, enhanced disorder, and stabilization of the high symmetry phases which have the highest technological importance. The research is a part of a larger project that, by using the multi-technique approach, aims to determine (x, T) phase diagram and molecular cation dynamics of MA1-xEAxPbI3.