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Yongtao Liu1 Liam Collins2 Roger Proksch3 Songkil Kim2 Brianna Watson4 Benjamin Doughty5 Tessa Calhoun4 Mahshid Ahmadi1 Anton Ievlev2 Stephen Jesse2 Scott Retterer2 Alex Belianinov2 Kai Xiao2 Jingsong Huang2 Bobby Sumpter2 Sergei Kalinin2 Bin Hu1 Olga Ovchinnikova2

1, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Knoxville, Tennessee, United States
2, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, Asylum Research an Oxford Instruments Company, Santa Barbara, California, United States
4, Department of Chemistry, The University of Tennessee, Knoxville, Knoxville, Tennessee, United States
5, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

Recently, observations of twin domain in methylammonium lead triiodide (MAPbI3) have drawn significant attention. However, whether this twin domain is ferroelectric and/or ferroelastic remains unclear. In addition, previous investigations were limited to the ferroic properties of this twin domain, whereas, the chemical behavior which can correlate with either ferroelectricity or ferroelasticity, has rarely been studied. In this work, we unveil the correlation of ferroelastic domains and chemical variation in the MAPbI3 twin domains using multiple functional imaging techniques. We unambiguously show the mechanical origin of piezoelectric-like contrast by using multiple advanced piezoresponse force microscopy techniques, suggesting the non-ferroelectricity of this twin domain. The combination of helium ion microscopy secondary ion mass spectrometry (HIM-SIMS) and nanoscale infrared spectroscopy (Nano-IR) indicates the ion segregation correlating with the twin domain. Emission excited by polarized light reveals large-scale ordering of crystallographic orientation/chemical make-up correlating with twin domain and its alternative interaction with light. Moreover, density functional theory (DFT) calculations provide a picture describing the interaction of elastic strain, chemical segregation, and ferroelasticity. This work unveils a new structural-chemical interplay in MAPbI3, providing a new line of interpreting and understanding the ferroic, chemical, and optoelectronic behaviors of related HOIPs.

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