The {111}‐Modulated Domains in Tetragonal BaTiO 3

Barium titanate (BaTiO 3 ) base‐metal electrode multilayer ceramic capacitors of an X7R‐formulation, sintered at 1200°C under low oxygen partial pressures (of p O 2 ≈10 −9 and 10 −11 atm, respectively), followed by annealing at 1000°C in an atmosphere containing a higher oxygen partial pressure (of p O 2 ≈10 −5 –10 −6 atm), have been analyzed for crystalline phases using X‐ray diffractometry, for microstructure using transmission electron microscopy, and for microchemistry using energy‐dispersive X‐ray spectroscopy and electron energy loss spectroscopy. The classical core–shell structure characterized by a core consisting of tetragonal ferroelectric {011) domains and featureless shell (designated type I) was observed only in sample A sintered in p O 2 ≈10 −9 atm. For sample B sintered in p O 2 ≈10 −11 atm, the core–shell structure is predominantly type II, consisting of a featureless shell similar to type I, but a core of modulated domains. The core of type II contained incommensurately modulated {111} superlattice domains along 〈111〉. The superlattice can be described by a displacive modulation with incommensurate wave vectors k 1 =0.58a * , k 2 =0.58b * , and k 3 =0.58c * . It is due to the ordering of defect associates , generated extrinsically from sintering in low p O 2 . Shell thickness was determined by the lattice diffusion of Ca 2+ solute cations into BaTiO 3 grains during sintering. The core–shell interface became less distinguishable in type II because defect associates , unlike those in type I, were not completely eliminated by re‐oxidizing in p O 2 ≈10 −5 –10 −6 atm, but became ordered along 〈111〉 and gave rise to structural modulation.