Contrasting Behaviors of FA and MA Cations in APbBr3

Sharma, VK and Mukhopadhyay, R and Mohanty, A and Tyagi, M and Embs, JP and Sarma, DD (2020) Contrasting Behaviors of FA and MA Cations in APbBr3. In: Journal of Physical Chemistry Letters, 11 (29). pp. 9669-9679.

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Abstract

It is known that the organic units in hybrid halide perovskites are free to rotate, but it is not clear if this freedom is of any relevance to the structure-property relationship of these compounds. We have employed quasi-elastic neutron scattering using two different spectrometers, thus providing a wide dynamic range to investigate the cation dynamics in methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) over a large temperature range covering all known crystallographic phases of these two compounds. Our results establish a plastic crystal-like phase forming above 30 K within the orthorhombic phase of MAPbBr3 related to 3-fold rotations of MA units around the C-N axis with an activation energy, Ea, of �27 meV, which has no counterpart in the FA compound. MA exhibits an additional 4-fold orientational motion of the whole molecule via rotation of the C-N axis itself with an Ea of �68 meV common for the high-temperature tetragonal and cubic phases. In contrast, the FA compound exhibits only an isotropic orientational motion of the whole FA unit with Ea � 106 meV within the orthorhombic phase and a substantially reduced common Ea of �62 meV for the high-temperature tetragonal and cubic phases. Our results suggest that the rotational dynamics of the organic units, crystallographic phases, and physical properties of these compounds are intimately connected. ©

Item Type:	Journal Article
Publication:	Journal of Physical Chemistry Letters
Publisher:	American Chemical Society
Additional Information:	Copyright to this article belongs to American Chemical Society
Keywords:	Activation energy; Lead metallography; Neutron scattering; Perovskite; Positive ions; Rotation, Crystallographic phasis; Halide perovskites; Orientational motion; Orthorhombic phase; Quasi elastic neutron scattering; Rotational dynamics; Structure property relationships; Wide dynamic range, Lead compounds
Department/Centre:	Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited:	05 Jan 2021 11:38
Last Modified:	05 Jan 2021 11:38
URI:	http://eprints.iisc.ac.in/id/eprint/67176

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