1 SCiENtifiC REPORTS | 6:35685 | DOI: 10.1038/srep35685 www.nature.com/scientificreports Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide P. S. Whitfeld 1 , N. Herron 2 , W. E. Guise 3,4 , K. Page 1 , Y. Q. Cheng 1 , I. Milas 3 & M. K. Crawford 3,5 We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI 3 ) using time-of-fight neutron and synchrotron X-ray powder difraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (T c -T) β , where T c is the critical temperature and the exponent β was close to ¼, as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact frst-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a frst-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI 3 based solar cells. Te origin of the surprisingly high efciencies exhibited by solar cells fabricated with hybrid perovskites remains a subject of widespread interest 1,2 . Photovoltaic (PV) efciencies greater than 20% have been reported 3 , and these values were obtained using materials solution-processed near room temperature, which holds great promise for decreasing the cost of solar energy 4 . To understand the origin of their PV performance, and accelerate the search for new materials, it is essential to frst understand the crystal structures of the hybrid perovskites, characterized by structural phase transitions, considerable static or dynamic disorder, and unknown concentrations of various defects such as halogen anion or organic cation vacancies. Te hybrid perovskites are far more complicated than conventional photovoltaic materials such as Si, CIGS, and CdTe, yet the perovskite structure is the basic framework adopted by a wide vari- ety of functional materials such as ionic conductors, ferroelectrics and superconductors 5 . Te parent perovskite structure ABX 3 is cubic with Pm-3m symmetry but may lower its symmetry by rotating or distorting the ‘BX 6 ’ octahedra and translating the ‘A’ site or ‘B’ site cations. Te structures obtained by simple rotations of the BX 6 octahedra around the axes of the aristotype cubic structure were frst classifed by Glazer 6 , followed by a number of theoretical studies of the possible tilt structures and phase transitions 7–10 . Te hybrid perovskite MAPbI 3 (CH 3 NH 3 PbI 3 ) was frst synthesized and described by Weber in 1978 11 as an analog of CsPbI 3 12 . MAPbI 3 was shown 13 to have three structural phases: a cubic Pm-3m phase above 330 K, a tetragonal I4/mcm phase from 160 to 330 K, and an orthorhombic Pnma phase below 160 K. Te structural phase transitions connecting these phases can be rationalized within the schemes used to describe transitions in per- ovskites with inorganic cations 6–9 , but for MAPbI 3 order-disorder transitions of the MA cations are also involved. To improve our understanding of the crystal structures and phase transitions in MAPbI 3 , we performed detailed structural studies using both time-of-fight neutron and synchrotron X-ray powder difraction. Although 1 Chemical and Engineering Materials Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 2 DuPont Electronics and Communication Technologies, Wilmington, DE 19803, USA. 3 DuPont Central Research & Development, Wilmington, DE 19803, USA. 4 Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL 60439, USA. 5 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA. Correspondence and requests for materials should be addressed to P.S.W. (email: whitfeldps@ornl.gov) or M.K.C. (email: mkcrawford987@gmail.com) Received: 14 June 2016 Accepted: 03 October 2016 Published: 21 October 2016 OPEN