*Article* **Gd/Mn Co-Doped CaBi4Ti4O<sup>15</sup> Aurivillius-Phase Ceramics: Structures, Electrical Conduction and Dielectric Relaxation Behaviors**

**Daowen Wu <sup>1</sup> , Huajiang Zhou <sup>1</sup> , Lingfeng Li <sup>1</sup> and Yu Chen 1,2,\***


**\*** Correspondence: chenyu01@cdu.edu.cn; Tel.: +86-28-8461-6169

**Abstract:** In this work, Gd/Mn co-doped CaBi4Ti4O<sup>15</sup> Aurivillius-type ceramics with the formula of Ca1-*x*Gd*x*Bi4Ti4O<sup>15</sup> + *x*Gd/0.2wt%MnCO<sup>3</sup> (abbreviated as CBT-*x*Gd/0.2Mn) were prepared by the conventional solid-state reaction route. Firstly, the prepared ceramics were identified as the single CaBi4Ti4O<sup>15</sup> phase with orthorhombic symmetry and the change in lattice parameters detected from the Rietveld XRD refinement demonstrated that Gd3+ was successfully substituted for Ca2+ at the A-site. SEM observations further revealed that all samples were composed of the randomly orientated plate-like grains, and the corresponding average grain size gradually decreased with Gd content (*x*) increasing. For all compositions studied, the frequency independence of conductivity observed above 400 ◦C showed a nature of ionic conduction behavior, which was predominated by the long-range migration of oxygen vacancies. Based on the correlated barrier hopping (CBH) model, the maximum barrier height *WM*, the dc conduction activation energy *E*dc, as well as the hopping conduction activation energy *E*p were calculated for the CBT-*x*Gd/0.2Mn ceramics. The composition with *x* = 0.06 was found to have the highest *E*dc value of 1.87 eV, as well as the lowest conductivity (1.8 <sup>×</sup> <sup>10</sup>−<sup>5</sup> S/m at 600 ◦C) among these compositions. The electrical modules analysis for this composition further illustrated the degree of interaction between charge carrier *β* increases, with an increase in temperature from 500 ◦C to 600 ◦C, and then a turn to decrease when the temperature exceeded 600 ◦C. The value of *β* reached a maximum of 0.967 at 600 ◦C, indicating that the dielectric relaxation behavior at this temperature was closer to the ideal Debye type.

**Keywords:** CaBi4Ti4O15; ion doping; electrical conduction; dielectric relaxation; oxygen vacancies
