Weak‐Beam Dark‐Field Microscopy of Complex Stress States in X7R‐Type BaTiO3 Dielectric Core–Shell Structures

Abstract

X7R‐type BaTiO3 materials were analyzed using X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Powder XRD indicated that the materials had pseudocubic lattices, but core–shell grain structures predominated in bright‐field (BF) TEM images. Electron diffraction patterns across the core–shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant–temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak‐beam dark‐field (WBDF) microscopy, and strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution.