**1. Introduction**

Bismuth layer—structured ferroelectric (BLSF) compounds (also called Aurivillius phase compounds), with the general formula [Bi2O2][A*m*−1B*m*O3*m*+1], where A with a dodecahedral coordination is a low valence element (less than or equal to trivalent), B with a octahedral coordination is a transition metal element (e.g., Cr3+, Ce4+, Ti4+, Nb5+ , Ta5+, W6+), and 1 <sup>≤</sup> *<sup>m</sup>* <sup>≤</sup> 6, constructed by *<sup>m</sup>*[ABO3] <sup>2</sup><sup>−</sup> layers that alternate with [Bi2O2] 2+ layers [1–3]. Due to their relatively high Curie point (*T*C) and excellent fatigue resistance, they have attracted extensive attention for their potential application in high-temperature piezoelectric systems [4,5] and ferroelectric random-access memory (FeRAM) [6]. However, the piezoelectricity of these compounds is limited because of the 2D orientation restriction of their spontaneous polarization (*P*s) rotation and their high coercive fields (*E*c), and high conductivity also restricts their applications in high temperature environment [7]. The conductivity mechanisms of most ferroelectric materials are approximately divided into three categories: electronic conduction, oxygen vacancies ionic conduction, and mixed

**Citation:** Zhou, H.; Wang, S.; Wu, D.; Chen, Q.; Chen, Y. Microstructures and Electrical Conduction Behaviors of Gd/Cr Codoped Bi3TiNbO<sup>9</sup> Aurivillius Phase Ceramic. *Materials* **2021**, *14*, 5598. https://doi.org/ 10.3390/ma14195598

Academic Editor: Andres Sotelo

Received: 31 August 2021 Accepted: 23 September 2021 Published: 26 September 2021

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conduction of ions and holes. BLSF compounds' conduction mechanisms are still indistinct. Various mechanisms have been put forward to explain the conductivity property. These mechanisms may be very correct according to the corresponding experiments, but none of them is widely accepted.

Bi3TiNbO<sup>9</sup> (BTN), which is made up of (Bi2O2) 2+ layers between which two (BiTiNbO7) 2− layers (*m* = 2) are inserted, has a very high *T*<sup>C</sup> of ~914 ◦C. As a sensing material, BTN is promising for fabricating piezoelectric accelerometers with operating temperature above 500 ◦C [8], which can be used for the high-temperature vibration monitoring of some large power equipment such as aircraft engines, gas turbines, power generators, etc. However, the piezoactivity of pure BTN ceramics is very low (*d*<sup>33</sup> ≤ 7 pC/N) [9]. The resistivity of BTN is only about 10<sup>7</sup> <sup>Ω</sup>·cm at 400 ◦C, for example [10]. Up to now, most reported studies about BTN have concentrated on its crystal structure [11–14], the electrical properties of pure BTN ceramics [15–18], and improvement in its piezoelectric ability [19–21]. For example, S.V. Zubkov doped Gd elements in BTN, which can increase the Curie temperature to 950 ◦C [22]. Gd element was also found to provide high insulation and low loss [23]. Chen et al. codoped W/Cr into Bi4Ti3O<sup>12</sup> of BLSFs, which increased the resistivity (*σdc* (600 ◦C)) and piezoelectric properties (*d*<sup>33</sup> (RT)) to 2.94 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>Ω</sup>·cm and 28 pC/N, respectively [24,25]. In addition, Chen et al. codoped Mo/Cr into CaBi2Nb2O9, which increased the multifaceted performances of CaBi2Nb2O<sup>9</sup> (*d*<sup>33</sup> = 15 pC/N, *T*<sup>C</sup> = 939 ◦C, *σdc* (600 ◦C) = 3.33 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>Ω</sup>·cm) [26]. However, there has been no significant improvement of the piezoelectric properties of BTN-based ceramics. There is also no clear understanding of their conduction behavior.

In this work, we studied the effects of Gd/Cr codoped on the microstructure, AC conduction mechanisms, and electrical impedance spectrum of BGTN−0.2Cr ceramic, focusing on the effect of grain size on conduction mechanism and impedance spectroscopy. A type of piezoelectric ceramic that can be used as a sensing material for piezoelectric sensors with operating temperatures above 600 ◦C was developed in this work.
