Emergence of hidden phases of methylammonium lead iodide $\left({\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3\right)$ upon compression

We perform a thorough structural search with the minima hopping method (MHM) to explore low-energy structures of methylammonium lead iodide. By combining the MHM with a forcefield, we efficiently screen vast portions of the configurational space with large simulation cells containing up to 96 atoms. Our search reveals two structures of methylammonium iodide perovskite (MAPI) that are substantially lower in energy than the well-studied experimentally observed low-temperature $Pnma$ orthorhombic phase according to density functional calculations. Both structures have not yet been reported in the literature for MAPI, but our results show that they could emerge as thermodynamically stable phases via compression at low temperatures. In terms of the electronic properties, the two phases exhibit larger band gaps than the standard perovskite-type structures. Hence, the pressure-induced phase selection at technologically achievable pressures (i.e., via thin-film strain) is a viable route towards the synthesis of several MAPI polymorph with variable band gaps.

Figure 1

Energy ranking of various MAPI polymorphs with respect to the orthorhombic ($Pnma$) phase with DFT using the meta-GGA (SCAN + SOC) functional. Structural motifs include the known perovskites (cubic, tetragonal, and orthorhombic) as well as the low-lying polymorphs predicted in this work.

Figure 2

Calculated formation enthalpies per formula unit (12 atoms) of various competing MAPI polymorphs as a function of pressure, computed using the meta-GGA functional (SCAN + SOC). Bottom panel is a magnification of the low-enthalpy region. All enthalpy differences are given with respect to the orthorhombic ($Pnma$) phase.

Figure 3

Pressure-volume dependence for various predicted polymorphs of MAPI and experimental values from Refs. [20, 23].

Figure 4

Electronic band gaps calculated with the hybrid functional PBE0, incorporating spin-orbit effects (SOC) on optimized geometries from meta-GGA for known and predicted phases of MAPI under pressure. Direct band gaps are depicted as solid lines, while dashed lines denote indirect band gaps. The yellow dots represent experimental (photoluminescence) values from Ref. [23].