Crystal structure of (Ba_1-xLa_x) [Mg_(1+x)/3Nb_(2-x)/3]O₃ ceramics

The crystal structure of (Ba_1-xLa_x)[Mg_(1+x)/3Nb_(2-x)/3]O ₃ (BLMN) ceramics with 0 ≤ × ≤ 1 was investigated using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). When the La content, x, was above 0.1, the 1:2 ordered hexagonal structure found in Ba(Mg_1/3Nb_2/3)O₃ (BMN) was transformed into the 1:1 ordered cubic structure. The 1:1 ordered cubic structure was maintained up to x = 0.7. When x exceeded 0.7, however, BLMN exhibited a 1:1 ordered monoclinic structure, rather than a 1:1 ordered cubic structure. La(Mg_2/3Nb_1/3)O₃ (LMN) has a 1:1 ordered monoclinic P2₁/n structure with a = 5.6004 Å, b = 5.6414 Å, c = 7.9346 Å, and β = 89.9819°. The monoclinic LMN has the in-phase and the anti-phase tilt of oxygen octahedra. The anti-parallel shift of A-site cations was also found in LMN.