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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.

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We report on a series of ambient processed, all-inorganic, leadfree nanocrystalline perovskites, viz. Cs 3 Bi 2 Cl 9−x Br x , which are halide-tunable and demonstrate photocatalytic CO 2 reduction. The conversion yield of CO 2 to CO could be tuned by meticulously regulating the Cl-to-Br ratio within the perovskite material. While all of the samples demonstrated CO 2 reduction purely under photoillumination (i.e., illumination with an applied bias of 0 V vs Ag/AgCl), it was remarkable that the conversion yield almost doubled for all samples on applying a negative potential of −0.5 V vs Ag/AgCl to the photocatalyst in addition to the illumination. This display of synergistic enhancement in yield under simultaneous application of light and electrical stimulus was maximized for the Cs 3 Bi 2 Br 3 Cl 6 nanocrystal variant, which registered a maximum yield of ∼105 μmol/g after 3 h of continuous illumination together with an applied bias of −0.5 V vs Ag/AgCl. The sample demonstrated decent stability over 10 h with a collective CO yield of 239.02 μmol/g. Such a competent CO yield is ascribed to the efficient charge carrier separation and migration to the reactive sites, further corroborated by a quantitative assessment of the electrochemical impedance spectroscopy (EIS) Nyquist plots of the nanocrystal variants. This study strives to provide new avenues and crucial insights into innovative designs, synthesis, and fabrication of competent, lead-free halide perovskite materials for CO 2 reduction using photocatalysis. KEYWORDS: lead-free perovskite, Cs 3 Bi 2 Cl 9−x Br x , photoelectrochemical catalysis, CO 2 reduction, halide tunability