The enormous research efforts dedicated to hybrid organic-inorganic perovskites have led to a deep understanding of these materials; however, the role of entropy and its ramifications for the properties of the materials have been only sparsely explored. In this study, we quantify the phase transition mechanism in the hybrid organic-inorganic perovskite [CH3NH3]PbBr3 by studying low-energy collective phonon modes using a combination of inelastic neutron scattering and ab initio lattice dynamics. We demonstrate that a delicate interplay among hydrogen bonding interactions, lattice vibrational entropy, and configurational disorder determines the thermodynamics and results in the rich phase evolution of [CH3NH3]PbBr3 as a function of temperature. Our results have important implications for the manipulation of macroscopic properties and provide a blueprint for future studies that will focus on unravelling phase transition mechanisms in hybrid perovskites and related materials such as dense and porous coordination polymers.
Bibliographical noteFunding Information:
The authors acknowledge membership of the UK’s HPC Materials Chemistry Consortium (EPSRC EP/L000202) and access to computational resources through PRACE and the STFC's SCARF cluster. A.W. acknowledges support from the Royal Society for a University Research Fellowship, and K.T.B. is funded by EPSRC (EP/M009580/1 and EP/J017361/1). G.K. acknowledges the “Fonds der chemischen Industrie” for support through the Liebig fellowship scheme. The authors thank Prof. A. K. Cheetham for illuminating discussions and H. Boström and E. Reynolds for help with collection of INS data.
© 2018 American Chemical Society.