Stereo-Active Lone Pairs Induced Giant Polarization in a 2D Ge-Based Halide Perovskite Antiferroelectric

Antiferroelectrics, characterized by electrically controlled antipolar-polar phase transformation, have attracted tremendous attention as a class of promising electroactive materials for assembling electronic devices. The emerging two-dimensional (2D) halide perovskites with superior compositional diversity offer an ideal platform for exploring electroactive materials, whereas lead-free antiferroelectric counterparts are still scarcely reported. Herein, for the first time, a new lead-free 2D germanium iodide perovskite antiferroelectric (i-BA)2CsGe2I7 (1, i-BA is iso-butylammonium) has been presented, which exhibits a high Curie temperature (Tc) up to 403 K. Remarkably, benefiting from the lone pair stereochemical activity in Ge2+ induced large structural distortion and Cs+ ion off-center displacement, 1 shows well-defined double P-E hysteresis loops in a wide temperature range with a giant maximum polarization up to 18.8 mu C cm-2, which achieves a new high record among molecular antiferroelectrics. Moreover, under a low external electric field of 22.5 kV cm-1, the antipolar-polar phase transformation in 1 affords a recoverable energy storage density Wrec of 0.27 J cm-3 and high storage efficiency up to 79.76%. Such lead-free halide perovskite antiferroelectric with intriguing antiferroelectric behaviors, including high Tc, large polarization and remarkable energy storage properties, is exciting, which provides an alternative candidate for high-performance antiferroelectrics for environmentally friendly electronic devices. An unprecedented 2D germanium iodide perovskite antiferroelectric (i-BA)2CsGe2I7 is presented, which exhibits a high Curie temperature up to 403 K. Remarkably, benefiting from the lone pair stereochemical activity in Ge2+ induced large structural distortion and Cs+ ion off-centre displacement, (i-BA)2CsGe2I7 shows a giant saturated polarization up to 18.8 mu C cm-2 around room temperature and excellent energy storage performance.image