It is reported that 90◦ domain can

It is reported that 90◦ domain can be formed to minimize the transformation stress from cubic to tetragonal phase transformation [13]. It is also reported that 90◦ domain was expected to be vanished at grain size less than 400 nm [1]. Therefore, with increased grain size,gradually increased tetragonal content suggest the increased area of the domain wall per volume, which can enhance the dielectric properties. As for fine grain size, especially less than 400 nm, however, this is not applicable. Disappearance of domains makes transformation from cubic to tetragonal type more difficult because a localized shear strain presented at the grain boundaries, which hindered the transformation cannot be relieved by domains [14].So metastable orthorhombic structure starts to present in BaTiO3 mixed with tetragonal structure with reduced grain size since the unit cell of the orthorhombic is characterized by a shear deformation of the cubic perovskite cell, which is relatively easy to happen compared with tetragonal structure [15]. For nano-grained BaTiO3, pure orthorhombic structure can be expected. The gradually decreased tetragonal content with reduced grain size can deteriorate the dielectric properties resulting in the lower dielectric constant.