Abstract
High resolution neutron powder diffraction has been used to study the charge states and spin order in the A-site-ordered perovskite LaCu3Fe4O12. This undergoes a first-order phase transition between cubic Im-3 structures with charge distributions LaCu3+3Fe3+4O12 and LaCu2+3Fe3.75+4O12 at TCT ≈ 400 K. Bond valence sums confirm that these charge states are adopted in the two phases, and there are no substantial valence fluctuations near the charge transfer transition. G-type antiferromagnetic order of B-site Fe3+ spins is observed in the low temperature phase LaCu3+3Fe3+4O12 and the ordered moment at 50 K is 4.0 μB. Magnetic moment is absent at the A′-site Cu3+ cation. The thermal evolution of the ordered moment enables an intrinsic TN′ ≈ 600 K to be estimated, although the actual upper limit for the spin order is TCT. No long range magnetic ordering was found in the high temperature phase, LaCu2+3Fe3.75+4O12, showing that any ordering transition for this regime has TM < TCT.
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