Infrared Spectroscopic Study of Vibrational Modes across the Orthorhombic–Tetragonal Phase Transition in Methylammonium Lead Halide Single Crystals

Abstract

Single crystals of the methylammonium (MA) lead halides MAPbI 3 , MAPbBr 3 , and MAPbCl 3 have been investigated using infrared spectroscopy with the aim of analyzing structural and dynamical aspects of processes that enable the ordering of the MA molecule in the orthorhombic crystal structure of these hybrid perovskites. Our temperature-dependent studies were focused on 2 the analysis of the CH/NH rocking, C-N stretching, and CH/NH bending modes of the MA molecule in the 800-1750 cm -1 frequency range. They deliver a direct comparison of the behavior of the three halides on crossing the orthorhombic-tetragonal phase transition in MA lead halide single crystals. Drastic changes of all vibrational modes close to the phase transition were clearly observed. Additional spectral features that were not discussed previously are pointed out. The transformation of the 2-dimensional orthorhombic hydrogen bond layers into a more 3-dimensional arrangement in the tetragonal phase seems to be an important feature providing deeper insight into the mechanisms that lead to a free-rotating MA molecule in the inorganic host structure. The change of the molecules site symmetry in the tetragonal crystal structure seems to be an important feature of the orthorhombic-tetragonal phase transition. For low temperatures it can be stated that the iodide is stronger influenced by hydrogen bonding than the bromide and the chloride. FIGURE 1 Visualization of MAPbI 3 tetragonal (a, b), MAPbI 3 orthorhombic (c, d) andMAPbBr 3 orthorhombic (e, f) crystal structure. Two hydrogen bond layers, layer A and layer B, can be identified. Layer A is shown in b), d) and f). The only difference between layer A and layer B is the orientation of the MA molecule axis. All N-H...I bond lengths are the same for both layers A and B (Table S5). Hydrogen atoms are not shown in the tetragonal structure. The colors refer to the following elements: purple -iodine, dark blue -bromide, dark grey -lead, brown -carbon, light blue -nitrogen, light orange -hydrogen. 8 X-ray diffractionSample purity was proven by X-ray powder diffraction analysis. The measurement was performed by a Panalytical X'Pert Pro MRD powder diffractometer with sample spinner (Bragg-Brentano geometry, λ = 1.5406 Å Cu Kα with 40kV/30mA, step size 0.0032 and time per step 597.72 sec, 2θ from 11-81°). The analysis was done with HighScore Plus Version 3.05. No impurities were observed. FTIR microscope spectrometryInfrared spectra were recorded on a Bruker Vertex 80v FTIR spectrometer, equipped with a globar light source, a KBr beam-splitter and a Bruker Hyperion 2000 microscope using Cassegrainian objectives and an MCT Hg-Cd-Te detector. A detector nonlinear correction routine was applied using the OPUS software package (Bruker). A Linkam THMS600 stage was used for cooling under Argon atmosphere. The thermocouple was calibrated using melting points of different salts (melting points between 580 K and 1074 K) as described elsewhere. 50 The 0°C point (freezing of H 2 O) was included in the calibration line....

Influence of Chloride Substitution on the Rotational Dynamics of Methylammonium in MAPbI3–xClx Perovskites

“…At 70 K, 2 % chlorine in MAPbI 3 (τ C3 = 485 ps) cause a more than three times faster jump rotation than in pure MAPbI 3 (τ C3 = 1635 ps). Corresponding with our investigations with FTIR, 25…”

“…MAPbI 3 , MAPbCl 3 , and MAPbI 2.94 Cl 0.06 was synthesized using a previously reported reaction which produced polycrystalline samples. 25 Argon atmosphere ( Fig. S1).…”

“…In the orthorhombic crystal structure of MAPbCl 3 , which in comparison has a doubled unit cell size, four layers with short hydrogen bonds and two different MA molecules were identified instead of two layers and one MA molecule in MAPbI 3 . 25 The phase transformations cubic to tetragonal and tetragonal to orthorhombic occur at 330 K (cubic → tetragonal) and 161 K (tetragonal → orthorhombic) for MAPbI 3 and at 177 K and 171 K for MAPbCl 3 . 25 The phase transformation temperatures of MAPbI 2.94 Cl 0.06 (326 K cubic → tetragonal and 155 K tetragonal → orthorhombic) are slightly lower compared to MAPbI 3 (Fig.…”

Cation and anion substitutions in hybrid perovskites: solubility limits and phase stabilizing effects

“…Photoluminescence measurements on MAPbI3 single crystals and an iodine-rich MAPb(I 1-x Cl x ) 3 solid solution (for details on the crystal growth method see [15]) did not show a significant increase in the band gap energy. Probably there is no effect because of the small substitutional amount and a possible band gap bowing behavior.…”

Single Crystals: The Next Big Wave of Perovskite Optoelectronics

“…2a. 26,30 The Methylammonium position is disordered in the tetragonal phase from 160 K to room temperature, whereas it is ordered below 160 K in the orthorhombic phase. 26…”