First principles study of phase stability in Ba-based tantalate complex double perovskites

Despite the practical importance of Ba-based perovskite dielectrics in microwave applications, the literature does not consistently agree which structural phases form. To provide insight, we use density functional methods to calculate Gibbs energies of structural polymorphs of 1:1 ordered Ba-based perovskites. Ba(In_1/2Ta_1/2)O₃ has a ground state Fm$3¯$m cubic phase at 0 K, while Ba(Y_1/2Ta_1/2)O₃, Ba(Gd_1/2Ta_1/2)O₃, and Ba(Nd_1/2Ta_1/2)O₃ form lower symmetry ground state phases. With increasing temperature, the structural phases change from monoclinic I2/m to trigonal R$3¯$ and finally to cubic Fm$3¯$m. For the zero and higher temperature conditions, the energy differences between the ground state and next higher energetic polymorphs are below 50 meV/atom and arise predominantly from changes in the octahedral tilts. The resulting coexistence of two or more polymorphs at room temperature and similar x-ray diffraction patterns may explain why so many papers report different phases for the same compound.