Crystal structures of (Ba_1-xLa_x)[Mg_(1+x)/3Nb_(2-x)/3]O ₃ with 0.9 ≤ x ≤ 1.0

Based on a structure model of mixture of phases, the crystal structure analysis of (Ba_1-xLa_x)[Mg_(1+x)/3Nb_(2-x)/3]O ₃ (BLMN) (x = 0.9 and 1.0) by Rietveld refinements was carried out. The pure La(Mg_2/3Nb_1/3)O₃ (LMN) and BLMN (x = 0.9) actually consist of a two-phase mixture rather a single phase. One is Fm-3̄m, which is cubic and 1:1 ordered and the other is P2₁/n, which is monoclinic and has both 1:1 ordering and the antiphase and inphase tilting of oxygen octahedra with a^-a^-c⁺. The weight fraction of the Fm-3̄m phase is 17.5% at x = 0.9 and further decreases to 7.5% in pure LMN. Meanwhile, that of the P2₁/n phase is 82.5% at x = 0.9 and further increases to 92.5% in pure LMN. The I4/m phase, which is present as a minor phase from x = 0.3 to 0.7, disappeared when x ≥ 0.9. The abrupt change of the x-ray diffraction pattern at x = 0.9 is caused by a change of major phase from Fm-3̄m to P2₁/n phase in the matrix.