Mixed formamidinium-methylammonium lead iodide perovskite from first-principles: hydrogen-bonding impact on the electronic properties.

Hybrid perovskites with mixed organic cations such as methylammonium (CHNH, MA) and formamidinium (CH(NH), FA) have attracted interest due to their improved stability and capability to tune their properties varying the composition. Theoretical investigations in the whole compositional range for these mixed perovskites are scarce in part due to the limitations of modeling cationic orientation disorder. In this work, we report on the local variation of the structural and electronic properties in mixed A-site cation MA/FA lead iodide perovskites FAMAPbI evaluated from static first-principles calculations in certain structures where the orientations of organic cations result from examining the energy landscape of some compositions.

Origin of the relaxor state in Pb(B{x}B{1-x}{'})O(3) perovskites.

Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.