Effect of temperature and stoichiometry on the long-range 1:2 cation order in BaZn1/3Ta2/3O3

We report on the compositional stability range, the degree of atomic order and Raman and optical spectra of the off-stoichiometric BaZn1/3Ta2/3O3 (BZT) within the BaO-ZnO-Ta2O5 ternary diagram. Almost all off-stoichiometric BZT compositions equilibrated at 1200 °C show significant degree of the long-range 1:2 cation order ranging from 60% to 80%. Ceramics equilibrated at 1550 °C and annealed at 1450 °C show strong effect of composition on the 1:2 order. The regions where an 1:2 atomic order is robust to the deviation from stoichiometry include the off-stoichiometric compositions along the BZT-Ba4Ta2O9, BZT-Ba3Ta2O8, BZT-BaTa2O6, BZT-Ta2O5, BZT-ZnTa2O6 and BZT-Zn4Ta2O9 (pseudo) tie lines. At the same time ceramics formulated along the BZT-BaO, BZT-ZnO, BZT-BaZnO2, BZT-Ba2ZnO3 tie lines and BZT-'Ba3ZnO4' pseudo tie line show complete disorder. There is a very close correlation between the degree of the 1:2 order on one hand and the unit cell volume and lattice distortion on the other hand. The ordered BZT show contraction of the unit cell whereas disordered ceramics show expansion of the unit cell in the off-stoichiometric region. The pronounced signatures of the order-disorder phase transition in the Raman and optical spectra are discussed.