First-principles Investigation of B-site Ordering in Ba(Mg_xTa_1-x)O₃ Microwave Dielectrics with the Complex Perovskite Structure

The B-site cation ordering of Ba(Mg 1/3 Ta 2/3 )O 3 microwave dielectrics with the complex perovskite structure has been studied using a combination of first-principles calculations, a cluster expansion technique, and Monte Carlo simulations. Our calculations confirm the experimentally observed hexagonal superstructure with space group P3m1 (D 3 3d ) as the ground state. The order-disorder transition between the low-temperature 1:2 ordered hexagonal phase (P3m1) and high-temperature simple perovskite phase (Pm3m) is predicted to occur at ∼3770 K. This indicates that Ba(Mg 1/3 Ta 2/3 )O 3 in equilibrium should be fully ordered at all practical temperatures. Sintering at high temperature for a long time or prolonging the anneal should therefore be effective in enhancing the degree of cation order in Ba(Mg 1/3 Ta 2/3 )O 3 . The charge density distribution and one electron density of states (DOS) for the 1:2 ordered structure indicate that Ta and O atoms possess some degree of covalency with some overlap between the O-2 p orbitals and the Ta-5 d orbitals.