Scanning TEM mode reveals that some

Scanning TEM mode reveals that some internal defect inclusions exist in grains of BaTiO3 in Figure 1(c,g). These are often observed in the sintered BaTiO3 ceramics when low temperature hydrothermally- or solvochemically- synthesized BaTiO3 powders are used, due to the presence of hydroxyl group.[22–24] As FTIR and TGA showed the presence of hydroxyl group in the initial nanopowder(Figure S2) and the final microstructure is very similar to the reported literature in the conventionally sintered BaTiO3 ceramics with hydrothermally-derived starting powder,[22] it should be reasonable to conclude that the internal defects in Figure 1(c,g) are not a specific consequence of CSP. The observed dense shell region in Figure 1(c) may grow during CSP. Some minor pores at core region may affect some properties under high voltage.[25] The CSP for BaTiO3 using a powder derived in the solid-state route is under investigation. Besides these minor defects and closed pores, specimens show equiaxed grains with equilibrated triple points which are a feature of the final stage of the sintering. Ferroelectric ferroelastic non-180º domain structures are not observed in the grains. Figure 1(d) and (h) revealed the well-crystallized grain boundaries after the cold sintering. The clean grain boundary formation without glassy phase was previously not observed until a second heat treatment was applied after cold sintering to temperatures 800-900 oC.[5] An elemental analysis with energy dispersion spectroscopy in the TEM indicates there are no detectable alkaline ions in the grains or grain boundaries (Figure S3). It should be noted that the presence of the remaining hydroxides islands can be observed on a fracture surface by Scanning Electron Microscopy, (Supplementary Figure S4). The majority of the hydroxide flux is believed to exist as segregated precipitates with relatively large scale (> 1 µm), not in grains. The absence of NaOH-KOH in the TEM-EDS is probably because the hydroxides were preferentially removed during the sample preparation for TEM.