High Ionic Conduction and Polarity-Induced Piezoresponse in Layered Bimetallic Rb₄Ag₂BiBr₉ Single Crystals

Abstract

The emergence of lead-free metal-halide perovskites in modern-day energy research is primarily due to their competent semiconducting and optoelectronic properties, as well as their nontoxicity and improved stability in ambient operating conditions. However, a detailed understanding of ion transport dynamics and the relaxation behavior of these materials is still elusive. In this article, we report on the single-crystal (SC) growth of a bimetallic two-dimensional layered Rb4Ag2BiBr9 and explore its dielectric, piezoelectric, and ferroelectric properties. The dielectric attributes are investigated by studying the temperature-dependent complex impedance, complex electric modulus, and AC conductivity over an extensive frequency range from 4 Hz to 8 MHz. The role of grain and grain boundaries in the effective impedance is established using the Maxwell–Wagner equivalent circuit model from the Nyquist plots. The observed electric modulus spectra are studied using the Havriliak–Nigami and the Kohlrausch–Williams–Watts formalisms to understand the ionic transport and relaxation mechanisms in Rb4Ag2BiBr9. The DC conductivity and relaxation time show Arrhenius-like behavior with the inverse temperature validating the hopping motion of ions. The values of the activation energy required for ion hopping are calculated independently from the relaxation time, hopping frequency, and DC conductivity Arrhenius plots and are in good agreement with a value of 0.47 eV. The scaling of the temperature-dependent conductivity and electric modulus spectra into a single master curve demonstrates the congruence of the ionic conduction and the relaxation phenomena at different temperatures for Rb4Ag2BiBr9. This SC demonstrates decent thermal and ambient stability up to 500 °C and 12 months, respectively. The pristine orthorhombic Rb4Ag2BiBr9 SC shows a piezoelectric amplitude value of ∼564 pm at the maximum applied bias (±10 V), and a saturation polarization of ∼0.14 nC/cm2 estimated from the piezoelectric force microscopy and polarization hysteresis loop measurement, respectively. The ferroelectric and semiconducting attributes of this material can be harnessed for prospective applications as a thin film in designing mechanical energy harvesters as well as functional photoferroelectrics, such as optical switches and ferroelectric photovoltaics.

Preferential perovskite surface-termination induced high piezoresponse in lead-free in situ fabricated Cs3Bi2Br9-PVDF nanocomposites promotes biomechanical energy harvesting

“…5(f ) depicts the calculated piezoresponse hysteresis loop using the equation: PR(E) = A(E) cos [φ(E)]. 61,62 The resultant hysteresis loop is asymmetric, indicating a transition toward positive bias along the electric field axis, which further suggests that the nanocomposite already has a built-in field that encourages polarization in a particular direction. 63 Particularly in the context of designing energy storage devices, the dielectric features of composite films are significant.…”

Halide Tunablility Leads to Enhanced Biomechanical Energy Harvesting in Lead-Free Cs2SnX6-PVDF Composites

“…Halide perovskites are suitable candidates in the aforementioned fields due to their excellent optoelectronic properties, which include low exciton binding energies, strong optical absorption, low trap states, direct radiative recombination, high charge carrier mobility, and a tunable optical band gap in the visible region . Despite such benefits, widespread utility and commercial viability are constrained due to lead toxicity and low operational stability, which demands fabrication and employment of stable and non-toxic perovskites. , …”

“…36 Despite such benefits, widespread utility and commercial viability are constrained due to lead toxicity and low operational stability, which demands fabrication and employment of stable and non-toxic perovskites. 37,38 Replacing lead (Pb) to mitigate environmental toxicity but retaining crucial physico-chemical properties of halide perovskites is non-trivial. Consequently, the development of leadfree perovskites is necessary for a sustainable future.…”