Abstract
The coupling between the organic cations and inorganic perovskite framework in a large single crystal of weighing 13 g was studied using neutron diffraction and inelastic neutron scattering. Two lattice incommensurate (ICM) phases were found; one at higher temperatures, marked , appeared between 150 and 134 K. The second one, marked , developed below 143 K and remained at 75 K. The transition from the to phase upon warming gave rise to extremely large lattice shrinking, followed by extremely large lattice expansion of the tetragonal basal plane of the lattice. There was a progressive decrease in the width of the diffuse peak at the Bragg position from the lattice upon warming, which can be described using a critical exponent to show complete ordering of the atoms into a lattice at 200 K. exhibits six definitive acousticlike phonon branches at 75 K. The six branches reconstruct into two at 200 K, with the frequencies of both the transverse and longitudinal modes greatly enhanced. The asymmetric structure of the ions together with the indirect interactions between the ions mediated through the Br ions are used to understand the observed behaviors.
8 More- Received 7 October 2020
- Revised 22 December 2020
- Accepted 19 January 2021
DOI:https://doi.org/10.1103/PhysRevMaterials.5.025401
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