CONDENSED MATTER Impact of (Zr, Cu) Ion Substitution on the Optical, Dielectric, and Impedance Behavior of BiFeO 3 A. Sathiya Priya 1 & D. Geetha 1 Received: 4 August 2020 /Accepted: 5 November 2020 # Sociedade Brasileira de Física 2020 Abstract In this paper, we report about the enhanced optical and dielectric properties of (Zr, Cu) dual-doped ceramics prepared by cost- effective sol-gel method. The variation in optical and electrical parameters with increased Zr concentration was studied in detail. The UV–visible spectra showed absorption peaks at 350 nm and 440 nm for all the samples. The optical bandgap energy of the Bi 1-x Zr x Fe 0.98 Cu 0.02 O 3 system was found to decrease from 2.41 to 1.78 eV as a measure of Zr concentration. Reduced bandgaps show the potential of these ferrites in visible light photocatalysis. With increase in frequency, the dielectric constant and dielectric loss were found to decrease without any resonance peak while the electrical conductivity increased. At 1 MHz, the dielectric constant (ε′) value as high as 8702 has been achieved for the doped sample. Impedance spectroscopy confirmed the semicon- ductor behavior of the ferrites. The Nyquist plots favor the non-Debye relaxation process. A remarkable improvement in the dielectric response of the (Zr, Cu) dual-doped BiFeO 3 is observed which favors reduced eddy current loss thereby increasing its application in high-frequency devices. Keywords Doped BiFeO 3 . Band gap . Dielectric constant . Impedance . Nyquist plot 1 Introduction Perovskite bismuth ferrite, BiFeO 3 (BFO), is a promising multiferroic material, intensively studied owing to its potential applications in information storage, spintronics, smart sensors, lead-free ferroelectric photovoltaic, photocatalysis, etc. [1–4]. For the past decade, tremendous research has been focused on the improvement of multiferroic properties of BFO by simul- taneous substitution of A and B sites by transition/rare earth metal ions [5–20]. Earlier reports have confirmed that co- doping in BFO has resulted in enhancement of dielectric prop- erties and lowering of dielectric loss [15–22]. Pr and Ti co- doping in BFO has resulted in improved dielectric properties which has been attributed to the reduction in oxygen vacan- cies by the substituted ions [23]. Moreover, weaker frequency dependence of ε o and tan δ has been witnessed. Coondoo et al. [20] have realized significant increase in dielectric constant by La and Nd substitution at the Bi and Fe sites, respectively. They have accredited the lanthanide substitution for suppress- ing the formation of oxygen vacancies in BFO. Ahmad et al. [21] have studied the effect of Yb and Co co-doping of BFO and have reported enhanced dielectric properties. Similar re- sults have been observed in Er/Ti co-doped BFO [22] and Ba and Ho co-doped BFO [24], and decrease in oxygen vacancies has been established as the reason for that. Moreover, co- doping of transition metals instead of rare earth metals have also yielded the same result. Godara et al. [25] have observed high values of dielectric constant for Ba- and Co-modified bismuth ferrite. They have reported improved stability of di- electric constant with temperature at all frequencies. Similarly, co-doping of Zn, Ti [26] and Ba, Ta [27] has also resulted in enhanced dielectric constant (ε) and reduced dielectric loss. Bismuth ferrite is a narrow bandgap metal oxide semicon- ductor that has attracted considerable interest as a potential candidate for solar cells and optoelectronic devices [28]. The narrow bandgap of this material is appropriate for the absorp- tion of visible light and hence is useful for visible light active photocatalysis. BiFeO 3 is a charge transfer type material where the valence band is formed due to the 2p oxygen states hybridized with the Fe 3d. The conduction band is due to the formation of Fe 4s states and Bi 6p states [29, 30]. Reported values for the optical bandgap of BiFeO 3 at room temperature * D. Geetha geetha@mitindia.edu 1 Department of Physics, Madras Institute of Technology, Anna University, Chennai 600044, India Brazilian Journal of Physics https://doi.org/10.1007/s13538-020-00822-2