Formation of new structural states in compressed BaTiO$_{3}$ nanopowders

The structural changes in BaTiO$_{3}$ nanocrystal powder and tablets have been probed via X-ray diffraction, scanning electron and transmission microscopy, and differential calorimetry after successive hightemperature annealing in air. It is shown that, beginning with the annealing temperature of 1200$^\circ$C, significant amount of the Ba$_{2}$TiO$_{4}$ phase forms in the tablets together with the BaTiO$_{3}$ phase. This phase is equilibrium one; it practically vanishes when the annealing temperature decreases to 700–600$^\circ$C; and this phase practically restored to the initial state when the annealing temperature is again increased to 1200$^\circ$C. Annealing the powders causes no formation of new phases, but an increase in their crystallite sizes. A probable reason of the emergence of Ba$_{2}$TiO$_{4}$ phase in tablets and its absence in free powder is discussed, as well.