Direct observation of geometric and sliding ferroelectricity in an amphidynamic crystal

TL;DR

This paper reports the first direct measurement of sliding ferroelectricity in an amphidynamic crystal, revealing how geometric and sliding mechanisms generate and control polarization in low-dimensional ferroelectrics.

Contribution

It provides the first direct experimental evidence of sliding ferroelectricity in a high-quality crystal, combining measurements with DFT calculations to elucidate the mechanisms involved.

Findings

Direct measurement of P-E hysteresis in an amphidynamic crystal

Identification of geometric and sliding contributions to polarization

Insight into controlling ferroelectricity via molecular dynamics

Abstract

Sliding ferroelectricity is a recently observed polarity existing in two-dimensional materials. However, due to their weak polarization and poor electrical insulation in these materials, all available experimental evidence till now are indirect, with most based on transport properties in the nanoscale or piezoresponse force microscopy. We report the direct observation of sliding ferroelectricity, using a high-quality amphidynamic single crystal, (15-Crown-5)Cd$_3$Cl$_6$, which possesses a large band-gap and so allows direct measurement of P-E hysteresis. This coordination polymer is a van der Waals material, which is composed of inorganic stators and organic rotators as measured using XRD and NMR characterisation. From DFT calculations, we find that after the freezing of rotators an electric dipole is generated in each layer driven by the geometric mechanism, meanwhile a comparable ferroelectric polarization originates from the interlayer sliding. The net polarization of these two components can be directly measured and manipulated. Our finding provides insight into low-dimensional ferroelectrics, especially the controlling of synchronous dynamics of rotating molecules and sliding layers in solids.