*3.2. Microstructures*

Figure 2 shows the SEM images of the CBT-*x*Gd/0.2Mn ceramics of the original surface. It can be seen from Figure 2 that the CBT-*x*Gd/0.2Mn ceramics presented a dense structure composed of many plate-like grains with random orientation. Such a special morphology was formed due to the structurally highly anisotropic grain growth, which had a much higher grain growth rate in the direction perpendicular to the *c*-axis of the BLSFs crystal [26]. Horn et al [27] reported that the (0 0 *l*)-type planes of the BLSFs crystal possessed a lower surface energy, which developed predominantly during sintering. Although the platelike grains with their *c*-axis were normally oriented to the major surface preferred to grow up in the BLSFs ceramics, the grain orientation was random in the CBT-*x*Gd/0.2Mn piezoceramics, as the ceramics were fabricated by pressure-less sintering.

**Figure 2.** SEM images focused on the original surfaces of the CBT−*x*Gd/0.2Mn ceramics.

In order to explore the grain characteristics of the CBT-*x*Gd/0.2Mn piezoceramics quantitatively, the linear intercept method (performed by the Nano Measurer software) was used to obtain the grain size distribution from SEM images, the results were shown in Figure 3. When *x* increased from 0 to 0.11, the average grain size (*D*λ) gradually decreased from 2.65 µm to 2.30 µm, and the corresponding size distribution became more inhomogeneous. Among the CBT-*x*Gd/0.2Mn piezoceramics, the composition with *x* = 0.11 had the smallest grain size (*D*<sup>λ</sup> = 2.30 µm) and the widest size distribution; such refined grains and compact structure could reduce the oxygen vacancy concentration and improve the activation energy of grain boundary, so as to provide a higher poling electric field to the ceramic. Alternatively, an obvious grain refinement, which was accompanied by more random grain orientation, occurred in those samples with *x* = 0.08, indicating that enough Gd3+ entering into the A-site of perovskite unit would influence the growth behavior of ceramic grains. This phenomenon could be attributed to the reduced boundary energy for grain boundary migration or the increased activating energy for ion migration [28].

**Figure 3.** Grain size distribution of the CBT−*x*Gd/0.2Mn ceramics derived from SEM images.
