Dynamic Disorder Restriction of Methylammonium (MA) Groups in Chloride‐Doped MAPbBr₃ Hybrid Perovskites: A Neutron Powder Diffraction Study

Abstract

The hybrid methylammonium (MA) lead halide MAPbX3 perovskites present an appealing optoelectronic behavior with applications in high‐efficiency solar cells. The orientation of the organic MA units may play an important role in the properties, given the degrees of freedom for internal motion of MA groups within the PbX6 network. The present neutron powder diffraction study reveals the dynamic features of the MA units in the hybrid perovskite series MAPb(Br1−xClx)3, with x=0, 0.33, 0.5, 0.67, and 1. From difference Fourier maps, the H and C/N positions were located within the PbX6 lattice; the refinement of the crystal structures unveiled the MA conformations. Three different orientations were found to exist as a function of the chlorine content (x) and, therefore, of the cubic unit‐cell size. These conformations are stabilized by H‐bond interactions with the halide ions, and were found to agree with those reported from theoretical calculations.

Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron and Neutron Diffraction Data

“…At lower temperatures, we unveiled that the halide disorder prevents the cooperative rearrangements needed to drive the octahedral PbX 6 tiltings in intermediate Br/Cl ratios; only CH 3 NH 3 PbCl 3 underwent conspicuous phase transitions (cubic at room temperature, evolving to tetragonal and orthorhombic at 120 K) [33]. H-bond interactions with the halide ions stabilize these conformations, in accordance to reported theoretical calculations [34].…”

“…The crystal growth of CH 3 NH 3 Pb(Br 1−x Cl x ) 3 (x = 0, 0.33, 0.5, 0.67 and 1) [32][33][34] was made from stoichiometric amounts of CH 3 NH 3 X and PbX 2 (X = Cl, Br). Previously, the methyl ammonium bromide and methyl ammonium chloride were synthesized from methyl amine (CH 3 NH 2 ) and the corresponding acid HBr and HCl, respectively, according to the following reaction:…”

“…The neutron powder diffraction (NPD) investigation is essential to obtain a detailed description of these hybrid materials. The NPD data were collected at the D2B diffractometer (ILL, France) [32,34]. In particular, the distribution of MA groups can be elucidated taking advantage of the high contrast in the coherent scattering lengths of Pb, Br, Cl, C, N, and H, of 9.405, 6.795, 9.577, 6.646, 9.36, and −3.739 fm, respectively.…”

“…Looking back at the unit-cell variation, the contraction of the PbX 6 octahedron as Cl is introduced leads to a localization of the MA units, limiting their orientation. This decrease has been deeply studied below room temperature, as the crystal goes through the phase transitions: cubic-tetragonal-orthorhombic [37,39,40]; however, in this case, CH 3 NH 3 Pb(Br 1−x Cl x ) 3 , the behavior is observed in the same crystal system at RT [34]. This finding shows that the MA freedom degree reduction can occur either by a decrease in the symmetry or by size reduction (keeping the symmetry) in the PbX 6 network.…”

“…In this chapter, we review our previous work on the CH 3 NH 3 Pb(Br 1−x Cl x ) 3 system, where we have investigated the crystallographic features in powdered, non-deuterated samples, using the mentioned state of the art techniques, neutron and synchrotron X-ray diffraction [32][33][34]. The crystal structure of MAPbBr 3 has been determined and refined at different temperatures, describing the evolution of the orientation of MA group in the 120-295 K temperature range.…”