**Nature of the cubic to tetragonal phase transition in methylammonium
lead iodide perovskite**

WA Saidi and JJ Choi, JOURNAL OF CHEMICAL PHYSICS, 145, 144702 (2016).

DOI: 10.1063/1.4964094

Hybrid organic-inorganic perovskites, as well as the perovskites in general, are known for their phase complexity evidenced by the stabilization of different polymorphs, and thus an understanding of their regions of stability and transitions can be important for their photovoltaic and optoelectronic technologies. Here we use a multiscale approach based on first-principles calculations with van der Waals corrections and classical force-field molecular dynamics to determine the finite-temperature properties of the tetragonal and cubic phases of CH3NH3PbI3. Temperature effects are implicitly included using the quasi- harmonic approximation that can describe anharmonic behavior due to thermal expansion through the dependence of the harmonic frequencies on structural parameters. Our finite-temperature free-energy surfaces predict the lattice and elastic moduli evolution with temperature, and show in particular that the calculated lattice parameters of the cubic and tetragonal phases are to within 1% of experimental values. Further, our results show that the phonons are the major contributing factor for stabilizing the cubic phase at high temperatures mainly due to the low- energy phonon modes that are associated with the inorganic lattice. On the other hand, the configurational entropy due to CH3NH3+ rotational degrees of freedom is slightly more favored in the cubic phase and amounts to less than 0.2% of the T = 0 K free-energy difference between the two phases. Published by AIP Publishing.

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