**1. Introduction**

It is well-known that bismuth layer structure ferroelectrics (BLSFs) are one of the important ferroelectric oxides, which have a general formula (Bi2O2) 2+ (Am−1BmO3m+1) <sup>2</sup>−, and their crystal structure composed of pseudo-perovskite blocks (Am−1BmO3m+1) <sup>2</sup><sup>−</sup> interleaved with bismuth oxide layers (Bi2O2) 2+ along the *c*-axis [1–3]. Generally, A represents a tetravalent, pentavalent, and hexavalent ion (such as k<sup>+</sup> , Li1+, Zn2+, Ca2+, Sr2+, Cr3+ , or La3+) [4], or the mixture of them. About B, it represents a tetravalent, pentavalent, or hexavalent ion (such as Ti4+, Ta5+, Nd5+). m is the number of BO<sup>6</sup> octahedra in the pseudoperovskite block (m = 1, 2, 3, 4, or 5) [5]. The CaBi4Ti4O15(CBT) shows the structure of A21am space group at room temperature, composing four perovskite-like TiO<sup>2</sup> octahedron units stacked in between (Bi2O2) 2+ layers.

For Aurivillius oxides, CBT ceramics attracted much attention from years ago, with simple preparation, transferring speed, a high fatigue strength, and low leakage current density, which are widely used in large equipment [6]. With the advancement of the aerospace industries, the research of high temperature piezoelectric acceleration sensor is urgent and necessary. Due to the high cure temperature (*T*<sup>c</sup> = 790 ◦C) [7] and excellent

**Citation:** Wu, D.; Zhou, H.; Li, L.; Chen, Y. Gd/Mn Co-Doped CaBi4Ti4O<sup>15</sup> Aurivillius-Phase Ceramics: Structures, Electrical Conduction and Dielectric Relaxation Behaviors. *Materials* **2022**, *15*, 5810. https://doi.org/10.3390/ma15175810

Academic Editor: Andres Sotelo

Received: 26 July 2022 Accepted: 16 August 2022 Published: 23 August 2022

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fatigue resistance [8,9], Bismuth layered piezoelectric ceramics are widely used in piezoelectric acceleration sensors. However, the low piezoelectric property limits the application of Pure CBT, because its own layer structures limit the material transportation when sintering progress and spontaneous polarization (along *a-b* plane) [10–12]. Moreover, a low spontaneous polarization (*P*s) and higher coercive field (*E*c) requires higher polarization voltage, and high electrical conductivity leads to high leakage current [13]. Therefore, it is of certain significance to study the high-temperature conductivity of CBT for operating in high-temperature environment. For Aurivillius piezoceramics, it is necessary to study electrical resistivity and conduction behavior at high temperature. Until now, many studies about CBT have been reported that concentrated on the structures and how to improve the *T*<sup>c</sup> or piezoelectricity [14–16]. For example, Gd3+ was found to reduce the leakage current and low loss [17]. Generally, the *p*-type conduction is mainly a conducting type for Aurivillius piezoceramics. As such, the dc conductivity can be reduced by donor doping [18]. There are few studies about the conduction behavior of CBT. For example, Xie et al. doped W<sup>+</sup> into CaBi4Ti4O<sup>15</sup> piezoceramics, the relaxation activation energy of the doped system was 1.45 eV, and its hopping conduction energy was 1.50 eV, while dc conduction energy was 1.39 eV [19], but the *d*<sup>33</sup> of this system was only 17.8 pC/N. Many studies revealed that V5+, Nb5+, and W6+ can decrease the high-temperature conductivity and increase the piezoelectric properties of BLSF ceramics, since these donor-type substituted ions could release the distortion of the oxygen octahedral, as well as reduce the concentration of oxygen vacancies in the lattice [20–22]. This means that CBT ceramics may have two different conductive types at different temperatures. However, there are many studies on the conduction mechanism of bismuth layered oxide ceramics and various mechanisms are still not widely adopted. Therefore, it would be necessary to study the conductance mechanism of the CaBi4Ti4O<sup>15</sup> ceramics, which is conducive to understanding of the microscopic motion energy of charge carriers [23].

In this work, a kind of Gd/Mn co-doped CaBi4Ti4O<sup>15</sup> ceramics were prepared using the solid-state reaction method and the structures of samples were characterized by using XRD and SEM. The effects of Gd/Mn co-doping on the electrical conduction and dielectric relaxation behaviors of CaBi4Ti4O<sup>15</sup> were studied in terms of the temperature dependent conductivity spectrum and electrical modulus analysis, with emphasis on the thermally activated motion of ionic defects, which predominates the dielectric behaviors at high temperature.
