Hybrid Organic-Inorganic Perovskites (HOIPs) have attracted an unprecedented attention as promising candidates for the next-generation of photovoltaic materials due to their exceptional energy conversion rates. Moreover, a better understanding of their remarkably soft atomic structures and their stabilization mechanisms is still necessary [1,2]. In this contribution we present an extensive study on the structure of the prime example of HOIPs methylammonium lead iodide (MAPbI$_3$) using a wide variety of radiation-scattering techniques validated by first-principles simulations across the temperature and pressure axes. We shall first introduce a model-selection protocol using inelastic-neutron scattering, thermophysical properties like the heat capacity, and density-functional theory simulations . This protocol was successfully implemented for MAPbI$_3$, showing that an alternative P1 structure is statistically sounder than the crystallographic Pnma model. Furthermore, we present the first results of high-pressure radiation-scattering experiments performed on MAPbI$_3$ together with a new set of extensive ab initio molecular dynamics simulations. These findings will pave the way towards a better understanding of the fundamental yet scarcely explored role played by pressure in the stability of MAPbI$_3$.
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