Crystal structures of (Ba_(1-x)La_x)[Mg_((1+x)/3)Nb_((2-x)/3)]O_3 with 0.9 ≤ x ≤ 1.0

摘要

Based on a structure model of mixture of phases, the crystal structure analysis of (Ba_(1-x)La_x)[Mg_((1+x)/3)Nb_((2-x)/3)]O_3 (BLMN) (x = 0.9 and 1.0) by Rietveld refinements was carried out. The pure La(Mg_(2/3)Nb_(1/3)O_3 (LMN) and BLMN (x = 0.9) actually consist of a two-phase mixture rather a single phase. One is Fm-3m, which is cubic and 1:1 ordered and the other is P2_1, 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-3m phase is 17.5% at x = 0.9 and further decreases to 7.5% in pure LMN. Meanwhile, that of the P2_1 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-3m to P2_1 phase in the matrix.