Abstract
The coupling between the organic CH(3)NH(3)(+) cations and inorganic perovskite PbBr(3)(-) framework in a large single crystal of (CH(3)NH(3))PbBr(3) weighting 13 g was studied using neutron diffraction and inelastic neutron scattering. Two lattice incommensurate (ICM) phases were found, one at higher temperatures, marked ICM(HT), which appeared between 147 and 135 K. The second one, marked ICM(LT), developed below 143 K and remained at 75 K. The transition from the ICM(LT) to ICM(HT) phase upon warming gave rise to extremely large lattice shrinking, followed by extremely large lattice expansion of the tetragonal basal plane of the PbBr(3) lattice. There was a progressive decrease in the width of the Bragg peaks from the PbBr(3) lattice upon warming, which can be described using a critical exponent for each type of Bragg peak to show complete ordering of the atoms into a (CH(3)NH(3))PbBr(3) lattice at 194 K. (CH(3)NH(3))PbBr(3) exhibits six definitive acoustic-like phonon branches at 75 K. The six branches renormalizes into two at 200 K, with the frequencies of both the transverse and longitudinal modes greatly enhanced. The asymmetric structure of the CH(3)NH(3) ions helps to understand the observed behaviors.