1 Scientific RepoRts | 6:24443 | DOI: 10.1038/srep24443 www.nature.com/scientificreports Spontaneous bidirectional ordering of CH NH 3 3 + in lead iodide perovskites at room temperature: The origins of the tetragonal phase Ioannis Deretzis 1 , Bruno N. Di Mauro 2 , Alessandra Alberti 1 , Giovanna Pellegrino 1 , Emanuele Smecca 1 & Antonino La Magna 1 CH 3 NH 3 PbI 3 is a hybrid organic-inorganic material with a perovskite structure and a temperature- dependent polymorphism whose origins are still unclear. Here we perform ab initio molecular dynamics simulations in order to investigate the structural properties and atom dynamics of CH 3 NH 3 PbI 3 at room temperature. Starting from diferent initial confgurations, we fnd that a single-crystalline system undergoes a spontaneous ordering process which brings the CH NH 3 3 + ions to alternately point towards the center of two out of the six faces of the cubic PbI 3 - framework, i.e. towards the 〈100〉 and 〈010〉 directions. This bidirectional ordering gives rise to a preferential distortion of the inorganic lattice on the a-b plane, shaping the observed tetragonal symmetry of the system. The process requires tens of picoseconds for CH 3 NH 3 PbI 3 supercells with just eight CH NH 3 3 + ions. Methylammonium lead iodide (CH 3 NH 3 PbI 3 ) is an excellent material for light harvesting 1–4 and photonic 5 appli- cations, with a solar cell efciency that is nowadays comparable to that of silicon 6 . Current research is mainly dedicated to the optimization of growth and processing methods in order to enhance device performance and overcome stability issues 7,8 . Notwithstanding this rapid technological growth, many questions regarding the fun- damental properties of this material are still open. One of the most important issues that needs further under- standing is the origin of polymorphism: CH 3 NH 3 PbI 3 belongs to a group of ABX 3 perovskite crystals (A, B: cations, X: anion) that typically have a cubic structure. Unlikely, the cubic symmetry is only verifed for tempera- tures T > 327 °K. At lower temperatures, the system crystallizes with either an orthorombic (T < 162° - 165 °K) or a tetragonal (165° < T < 327 °K) symmetry 9,10 . Moreover, reversible phase transitions can occur by simply var- ying the external temperature 9,10 . We note that the main distinctive characteristic of the three CH 3 NH 3 PbI 3 phases is the mean bond-length along the three axes of the - PbI 3 inorganic framework. With this respect, only the cubic phase is isotropic, whereas the tetragonal phase is partially isotropic (it has two out of three equivalent directions) and the orthorombic phase is totally anisotropic. Te temperature-dependent polymorphism of CH 3 NH 3 PbI 3 brings to attention the unknown origins of phase stability in this material. Previous studies have correlated this issue with the presence of the + CH NH 3 3 cation, the respective distortion that this induces to the inorganic cage and its degree of orientational disorder 10,11 . Indeed, it has been demonstrated from theoretical calculations that the lack of spherical symmetry in the + CH NH 3 3 ion affects also the surrounding inorganic cage (through + CH NH 3 3 -I - interactions) and consequently, the bond-lengths between inorganic ions 12,13 . Although such approach could partially explain the orthorobmic (ordered + CH NH 3 3 ) and cubic (disordered + CH NH 3 3 ) phases, difculty to correlate the tetragonal phase to a scheme of “middle” disorder appears. Within this context, both experimental and theoretical studies have been performed, with experimental results ofen increasing the debate on the argument 10,11,14–17 . It also remains unclear whether scattering interactions during measurements can impact on the rotation of the + CH NH 3 3 ions, as the relative rotation barriers are low 18 . On the other hand, static density functional theory (DFT) calculations usually end up predicting quasi-isoenergetic metastable confgurations, regardless of the + CH NH 3 3 orientation 12 . 1 CNR-IMM, Z.I. VIII strada 5, 95121 Catania, Italy. 2 Distretto Tecnologico Micro e Nano Sistemi S.c.a.r.l., Z.I. VIII strada 5, 95121 Catania, Italy. Correspondence and requests for materials should be addressed to I.D. (email: ioannis. deretzis@imm.cnr.it) received: 22 December 2015 accepted: 17 March 2016 Published: 15 April 2016 OPEN