Local Structures around Pb(II) and Sn(II) in CH3NH3PbX3 (X = Cl, Br, I) and CH3NH3SnX3 (X = Br, I) Studied by Pb LIII-Edge and Sn K-Edge EXAFS

Abstract

Local structures around Pb(II) and Sn(II) in CH3NH3PbX3 (X = Cl, Br, I) and CH3NH3SnX3 (X = Br, I) were studied by Pb LIII‐edge and Sn K‐edge EXAFS in the temperature range of 10 to 293 K. The M–X distances (M = Pb, Sn; X = Cl, Br, I), the coordination numbers, and the Debye‐Waller factors were determined in three, four, three, five, and two solid phases of CH3NH3PbCl3, CH3NH3PbBr3, CH3NH3PbI3, CH3NH3SnBr3, and CH3NH3SnI3, respectively. Five kinds of deformed octahedra consisting of halogens were observed.

Temperature-Dependent EXAFS Measurements of the Pb L3-Edge Allow Quantification of the Anharmonicity of the Lead–Halide Bond of Chlorine-Substituted Methylammonium (MA) Lead Triiodide

“…The orthorhombic phase observed here (phase O2) has an even smaller averaged lead–chlorine bond length splitting of σ r = 0.05 Å so that it is justified to assume only one averaged distance. These considerations contrast with the investigations of Ishida et al where two lead–iodine distances were assumed for the orthorhombic phase of MAPbI 3 and four lead–chlorine distances for the orthorhombic phase of MAPbCl 3 . However, when we look at the results of this investigation we quickly see that this analysis leads to a lead–halide distance that is way too small (for example, a value of 3.140(2) Å is given for 11 K results of MAPbI 3 ) .…”

“…To this end, the X-ray absorption near edge structure (XANES) region of the Pb L3-edge was mainly investigated, but more and more publications have been investigating the extended X-ray absorption fine structure (EXAFS) region to better understand the lead–halide bonds. , Because the interatomic distance, in our case the lead–halide distance, is the fundamental physical quantity, EXAFS is particularly sensitive (for details on the EXAFS methodology, see Section ). It should be mentioned that Pb L3-edge EXAFS studies were available before interest in hydride perovskites increased in recent years . Although there are some publications dealing with the EXAFS edges of the anion, with the work being confined to bromine, we want to limit ourselves to the Pb L3-edge for better comparability.…”

The underappreciated lone pair in halide perovskites underpins their unusual properties

“…But this is only part of the story. Obscured in this average structure view are local, dynamic, polar distortions of the octahedral cation environments evident from vibrational spectroscopy, 54 x-ray absorption spectroscopy, 55 pair distribution functions (PDFs) from total scattering, 50,56,57 and ab initio calculations. 50,[58][59][60] Raman spectroscopy 61 has indicated the fluid-like dynamical nature of the perovskite lattices of CsPbBr 3 and MAPbBr 3, part of which is accounted for by the lone-pair activity of Pb 2+ .…”

Metal cation s lone-pairs increase octahedral tilting instabilities in halide perovskites

“…[54][55][56][57][58] In HPs, they occupy the M site, and significant M off-centering motions are observed experimentally and computationally. 9,46,[59][60][61][62][63][64][65][66] Importantly, previous studies show in HPs off-centering instabilities induced by lone-pairs and tilting instabilities can coexist and are competing with each other. 67,68 The tendency for lone-pair-driven distortion is stronger for lighter cations.…”