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Order–Disorder Transition Involving the A‑Site Cations in Ln3+Mn3V4O12 Perovskites
journal contribution
posted on 2014-01-06, 00:00 authored by Yuichi Shimakawa, Shoubao Zhang, Takashi Saito, Michael W. Lufaso, Patrick M. WoodwardA crossover
from the A-site-ordered double-perovskite structure with Im3̅ cubic symmetry to the simple-perovskite structure with Pnma orthorhombic symmetry is found in LnMn3V4O12 (Ln = La, Nd, Gd, Y, Lu) synthesized under high-pressure
conditions. Relatively large Ln3+ ions (La3+, Nd3+, and Gd3+) induce the a+a+a+ in-phase cooperative tilting
of the VO6 octahedra, resulting in the A-site-ordered double-perovskite
structure with chemical composition Ln3+Mn2+3V3.75+4O12. Compounds with small Ln3+ ions like
Y3+ and Lu3+, on the other hand, crystallize
with the Pnma simple-perovskite structure with chemical
composition (Ln3+1/4Mn2+3/4)V3.75+O3, where the Ln3+ and Mn2+ ions are disordered
at the A site. The random distribution of the small A-site cation
induces the a–b+a– tilting distortion
of the VO6 octahedra. The observed phase crossover is well
explained by the structural stability calculation based on the bond-valence-sum
model, and the most stable crystal structure gives the smallest unit-cell
volume. This A-site-cation size-dependent phase transition between
the A-site-ordered double-perovskite and A-site-disordered simple-perovskite
structures in LnMn3V4O12 is thus a result of the structural stability due to the
cooperative tilting of the VO6 octahedra. The Mn2+ ions at the A′(A) site contribute local magnetic moments,
whereas the V3.75+ ions at the B site play a role in metallic
conduction. The observed magnetic behaviors are consistent with the
order–disorder distribution of the Mn2+ ions at
the A site, antiferromagnetism in the A-site-ordered double perovskites,
and magnetic spin glass in the A-site-disordered simple perovskites.