In the high magnification image (Fi

In the high magnification image (Figure 3A) and corresponding EDS mapping for Sr (Figure 3B), the presence of Sr-rich area near the grain boundaries and Sr-poor grain boundaries is clearly observed. The clear difference in the Sr distribution suggests the precipitation areas might be influenced by the pressure direction (illustrated by arrows in Figure 3A,B), which is consistent with what we would expect from the PSC mechanism.18 To be more specific, the low Sr concentration in some grain boundaries, as shown in Figure 3B, suggests that the dissolved ions preferentially diffuse from the high stressed grain-to-grain contacts (high P in Figure 3C) to less stressed pore surfaces (low P in Figure 3C). Subsequently, dissolved ions react to form Ba1-xSrxTiO3 at the low stressed areas, leading to pore elimination and densification (inset in Figure 3D). Thermodynamically speaking, the preferential precipitation of BaTiO3, instead of Ba1-xSrxTiO3, is very limited under Sr excess conditions.24,31 Therefore, it implies that Sr-poor grain boundaries are formed as a result of the dissolution and directional diffusion. Such directional diffusion and precipitation in adapting to the surroundings could result in the flat grain boundaries (Figure 3D). In comparison to the Sr-rich area near the grain boundary (Area 1 in Figure 3E), no Sr was detected in the center area of the grain (Area 2 in Figure 3E), indicating the phenomena mainly occur at the solid/flux interface. It is assumed that Sr-rich areas such as Area 1 were less stressed pore, which worked as the precipitation area during CSP. The pressure-dependent phenomena are further considered in Figure 3F, whereby the measured grain boundary angles using a low magnification image with respect to the applied pressure direction are determined. The 0° and 90° angles are defined as directions respectively parallel and perpendicular to the direction of the applied uniaxial pressure (Figure 3F). For example, Figure 3B represents the case where the angle is 90°. Interestingly, the angles of grain boundaries in the sample exhibited randomly (black in Figure 3F), yet with the tendency of higher angle for the Sr-poor grain boundaries (blue in Figure 3F). This result also reveals that the applied pressure is strongly related to the formation of the Sr-poor grain boundaries, namely it is not likely formed along the applied pressure direction.