Influence of Chloride Substitution on the Rotational Dynamics of Methylammonium in MAPbI_3–xCl_x Perovskites

Abstract

The hybrid halide perovskites MAPbI 3 , MAPbI 2.94 Cl 0.06 , and MAPbCl 3 (MAmethylammonium) have been investigated using inelastic and quasielastic neutron scattering (QENS) with the aim of elucidating the impact of chloride substitution on the rotational dynamics of MA. In this context, we discuss the influence of the inelastic neutron scattering caused by low-energy phonons on the QENS resulting from the MA rotational dynamics in MAPbI 3-x Cl x . Through a comparative temperature-dependent QENS investigation with different energy resolutions, which allow a wide Fourier time window, we achieved a consistent description of the influence of chlorine substitution in MAPbI 3 on to the MA dynamics. Our results show that chlorine substitution in the low temperature orthorhombic phase leads to a weakening of the hydrogen bridge bonds since the characteristic relaxation times of C 3 rotation at 70 K in MAPbCl 3 (135 ps) and MAPbI 2.94 Cl 0.06 (485 ps) are much shorter than in MAPbI 3 (1635 ps). For the orthorhombic phase, we obtained the activation energies from the temperature-dependent characteristic relaxation times τ C3 by Arrhenius fits indicating lower values of E a for MAPbCl 3 and MAPbI 2.94 Cl 0.06 compared to MAPbI 3 . We also performed QENS analyses at 190 K for all three samples. Here we observed that MAPbCl 3 shows slower MA rotational dynamics than MAPbI 3 in the disordered structure. FIGURE 1Visualization of the orthorhombic crystal structures of CH 3 NH 3 PbI 3 (a, b, and c) at 100K 21 and CH 3 ND 3 PbCl 3 (d, e, f, and g) at 80 K 23 (note that for MAPbCl 3 only partially deuterated structural data is available) as well as the MA molecule C 3 jump rotation movement (c, e, and f). Due to the hydrogen (deuterium) bonds between NH 3 (ND 3 ) and the halide atoms in the orthorhombic crystal structures, the orientation of the C-N axes is fixed and leads to distortions of the crystal lattice. In a), b), d), and g) only the C-N atoms and their orientation are shown together with the respective 12-fold coordination polyhedra. Because of the doubled unit cell of CH 3 ND 3 PbCl 3 (compared to the iodide) two different MA molecules can be identified: MA molecule 1 (e; blue color) and MA molecule 2 (f; red color). In MAPbI 3 , however, there is only one MA molecule (c) that occurs in four different orientations (a, b). Due to the double number of MA molecules in CH 3 ND 3 PbCl 3 there are a total of 8 different MA orientations (d, and g). In CH 3 ND 3 PbCl 3 the short N-Cl bond lengths are different in the two MA molecules: MA 1 (Cl2…N1 = 3.273(7) Å and Cl1-N1 = 3.336(11) Å) and MA 2 (Cl3…N2 = 3.300(6) Å and Cl4-N2 = 3.346(10) Å). Also the polyhedral volumes are different, V MA1 = 155.8 Å 3 , V MA2 = 149.4 Å 3 . In MAPbI 3 we can identify the following short N-I bond lengths: I2-N = 3.6804 Å and I1-N = 3.6113 Å. The polyhedral volume in MAPbI 3 is V MA = 206.9 Å 3 . The figures c, e, and f also show the C 3 three-fold jump rotation movement (blue arrow) of the MA molecules around the C-N axis, which is discussed in ...

Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites

“…It appears that the powder sample spectra of MAPB [52] and MAPI [53] also show sharp phonon modes at the same energy than in our experiments, suggesting once more the dispersionless nature of these phonon modes. The neutron powder sample spectrum observed in MAPbCl 3 [53] does not show any sharp features in clear contrast with MAPI, suggesting again that optical phonons exhibit no dispersion in MA-based HOPs. It should be further stressed that the lowest energy mode at 5 meV in MAPB [52] is found at the same position in protonated and deuterated samples, showing that it is not related to a molecular vibration.…”

Role of the Iodide–Methylammonium Interaction in the Ferroelectricity of CH3NH3PbI3

“…Therefore, more elaborate analyses of the orientation of the molecular cation are better performed using other techniques. [7,20,33,34] As a consequence of Fourier truncation and incomplete absorption correction, one normally observes residual electron density peaks, that is, deviations from the model observed electron density, close to the heaviest atoms (here Pb), as their intensity is roughly linked to the electron count. [35,36] Having them at the iodine position instead is probably due to the missing assignment of some electron density.…”

“…[1][2][3][4] Traditionally, semiconductors were thought to be rigid solids with highly defined atom positions. Hybrid perovskites, however, were shown to have a high defect tolerance [5] and a flexible crystal structure, with remarkable positional freedom of the molecular cation [6,7] and ionic movement. [8,9] This latter feature makes the reliable determination of a crystal structure challenging, as the average long-range order no longer reflects all the properties of the material.…”

Zur Rolle der Iodid‐Methylammonium‐Interaktion in der Ferroelektrizität in CH3NH3PbI3

“…Daher müssen zur genaueren Aufklärung dieser Frage andere Techniken hinzugezogen werden. [7,20,33,34] Durch den Abbruch der Fourier-Entwicklung und die unvollständige Absorptionskorrektur würde man normalerweise die stärksten Restelektronendichtepeaks in der Nähe der schwersten Atome (hier Pb) erwarten, da ihre Stärke grob mit der Elektronenzahl korreliert. [35,36] Die Beobachtung an der…”

“…[1][2][3][4] Klassischerweise wurden Halbleiter als feste Materialien gesehen, in denen die Atompositionen zu einem hohen Maße definiert sind. Dementgegen zeigen Hybridperowskite eine hohe Defekttoleranz [5] und eine flexible Kristallstruktur, in dem das Molekülkation eine hohe positionelle Flexibilität [6,7] und Ionenbeweglichkeit besitzt. [8,9] Gerade das Letztere macht eine verlässliche Kristallstrukturbestimmung schwierig, da die gemittelte Fernordnung nicht notwendigerweise alle Eigenschaften des Materials widerspiegelt.…”