Figure 2(a) shows X-ray diffraction

Figure 2(a) shows X-ray diffraction of the initial powder (BT-powder) and sintered specimens, respectively. It appears all the specimens possess a perovskite phase as a major phase. The small amount of the impurity phase (BaCO3) is only seen in the initial powder and disappeared after CSP. The molten hydroxide may have provided the thermodynamically stable conditions that preferred the perovskite formation over the BaCO3, as already observed under hydrothermal conditions[26,27]. No obvious peaks from hydroxide phases are found in X-ray diffraction pattern, probably due to their small proportion and/or their low crystallinity.The peak shape is nearly same in the form of the ceramic pellet and grinded ceramic (Figure S5). It should be addressed that the XRD is not able to sufficiently resolve the spontaneous distorted structure in the size of 100 nm scale.Therefore, Raman Spectroscopy was performed, as it has already been successfully used to reveal the local structure of BaTiO3[23,28].