Atomic-level direct imaging for Cu(I) multiple occupations and migration in 2D ferroelectric CuInP2S6

Abstract

CuInP₂S₆ (CIPS) is an emerging 2D ferroelectric material known for disrupting spatial inversion symmetry due to Cu(I) position switching. Its ferroelectricity strongly relies on the Cu(I) atom/ion occupation ordering and dynamics. Nevertheless, the accurate Cu(I) occupations and correlated migration dynamics under the externally applied energy, which are key to unlocking ferroelectric properties, remain controversial and unresolved. Herein, an atomic-level direct imaging through aberration-corrected scanning transmission electron microscopy is performed to precisely trace the Cu(I) dynamic behaviours under electron-beam irradiation along (100)-CIPS. It clearly demonstrates that Cu(I) possesses multiple occupations, and Cu(I) could migrate to the lattice, vacancy, interstitial and interlayer sites between the InS₆ octahedral skeletons of CIPS to form local Cu_xInP₂S₆ (x = 2-4) structure. Cu(I) multi-occupations induced lattice stress results in a layer sliding along the b-axis direction generating a sliding size of 1/6 b lattice constant. The Cu_xInP₂S₆ (x = 2-4) exists in a type of dynamic structure, only metastable with electron dose over 50 e^- Å^-2, thus generating a dynamic process of $\text{C}{\text{u}}_x\text{In}{\text{P}}_2{\text{S}}_6\left(x=2-4\right)\rightleftharpoons \text{CuIn}{\text{P}}_2{\text{S}}_6$ , a completely unreported phenomenon. These findings shed light on the unveiled mechanism underlying Cu(I) migration in CIPS, providing crucial insights into the fundamental processes that govern its ferroelectric properties.