Multiple Ferroelectric Resonance in 2D Organic–Inorganic Halide Perovskite

Dielectric properties of organic–inorganic 2D lead halide perovskite BA2PbBr4 (BA = butyl-ammonium, C4H9NH3) (BAPB) have been investigated as a function of temperature across 9 to 300 K and the frequency range from 100 kHz to 10 MHz. The low-frequency region (100 kHz to 1 MHz) shows nearly constant loss while the high-frequency region (1–10 MHz) exhibits multiple ferroelectric resonance features. The resonance features could originate from electromechanical vibration of ferroelectric domain walls of various sizes with resonance frequency ∼107 rad/s and damping constant ∼107 s–1. The ferroelectricity of the compound is confirmed by the measurement of remanent polarization (∼0.012 μC/cm2 @10 V) at room temperature using the Positive-Up-Negative-Down protocol within the Sawyer–Tower circuit. Additional piezoelectric force microscopy has been done to confirm its ferroelectricity. As structural refinement reveals BAPB crystallizes in the nonpolar pbca space group, the appearance of ferroelectricity in this material seems to be unusual, termed as improper ferroeletrics.