A new long-range sub-structure found in the tetragonal phase of CH3NH3PbI3 single crystals

Hybrid organic-inorganic perovskites have become one of the most promising low-cost alternatives to traditional semiconductors in the field of photovoltaics and light emitting devices. They combine the attractive features of both organic and inorganic materials within a single composite, for instance with excellent electronic properties. We used x-ray diffraction to reveal a sub-structure within CH3NH3PbI3 single crystals. We could observe the presence of additional peaks with a square symmetry in several monocrystalline samples. We discuss these results in terms of two different models: a super-structure modulated in two in-plane orthogonal directions, and a model with tilted domains with a shallow angle of ~0.6°. In both cases, the modulated or tilted domains appear in regions with small lattice expansion. We show that this last model appears to be the most likely to explain our observations.