High-pressure synthesis and characterization of BiCu3(Mn4−xFex)O12 (x=0, 1.0, 2.0) complex perovskites

Abstract We have studied the series of nominal composition BiCu 3 (Mn 4− x Fe x )O 12 ( x =0, 1.0, 2.0) where Mn is replaced by Fe cations in the ferrimagnetic perovskite BiCu 3 Mn 4 O 12 . These compounds have been prepared from citrate precursors under moderate pressure conditions (3.5 GPa) and 1000 °C in the presence of KClO 4 as oxidizing agent. All the samples have been studied by x-ray and neutron powder diffraction (NPD) at room temperature and 4 K. The crystal structure has been defined in a cubic Im 3 ¯ (No. 204) space group with a 2 a 0 ×2 a 0 ×2 a 0 unit-cell. The doubling of the unit-cell occurs due to the ordering of Bi 3+ and Cu 2+ cations over A sites of the AA′ 3 B 4 O 12 structure. The A -site accommodates 12-fold coordinated Bi 3+ ions and, at the A ′-site, Jahn–Teller Cu 2+ ions form pseudo-square planar units aligned perpendicular to each other. Mn 4+ /Fe 3+ cations randomly occupy the centre of slightly distorted octahedra. These materials have also been characterized by magnetic and magnetotransport measurements. We found that all the samples are ferrimagnetic and show a progressive decrease of T C as the Fe content increases, since Fe ions disturb the ferromagnetic interactions within the B magnetic sublattice. In fact, the Curie temperature diminishes from T C =360 K ( x =0) to T C =219 K ( x =2). The magnetic structures, studied by low-temperature NPD data, correspond to an antiferromagnetic arrangement of spins at 8 c and 6 b sites; the ordered moments are in excellent agreement with those obtained from the saturation magnetization at 4 K. A significant magnetoresistant effect is determined for the x =1.0 oxide, with low-field values as high as 5% at 300 K and 1 T.