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The series of title compounds is prepared from citrate precursors at 3.5 GPa and 1000 °C (1 h) in the presence of KClO4.

We have studied the series of nominal composition BiCu3(Mn 4-xFex)O12 (x=0, 1.0, 2.0) where Mn is replaced by Fe cations in the ferrimagnetic perovskite BiCu3Mn 4O12. These compounds have been prepared from citrate precursors under moderate pressure conditions (3.5 GPa) and 1000 °C in the presence of KClO4 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 Im3̄ (No. 204) space group with a 2a0 × 2a0 × 2a0 unit-cell. The doubling of the unit-cell occurs due to the ordering of Bi 3+ and Cu2+ cations over A sites of the AA′3B4O12 structure. The A-site accommodates 12-fold coordinated Bi3+ ions and, at the A'-site, Jahn-Teller Cu2+ ions form pseudo-square planar units aligned perpendicular to each other. Mn4+/Fe3+ 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 TC as the Fe content increases, since Fe ions disturb the ferromagnetic interactions within the B magnetic sublattice. In fact, the Curie temperature diminishes from TC=360 K (x=0) to TC=219 K (x=2). The magnetic structures, studied by low-temperature NPD data, correspond to an antiferromagnetic arrangement of spins at 8c and 6b 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. © 2013 Elsevier Inc., We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2010-16404 and of the Comunidad de Madrid to the project S2009PPQ-1551.

New members of the family of complex-perovskite oxides with the formula RCu(3)(Mn(3)Fe)O(12) (R = Ce, Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples have been prepared from citrate precursors treated under moderate pressure conditions (2-3.5 GPa) and 1000 °C in the presence of KClO(4) as an oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at 300 K and 2 K. These oxides crystallize in the cubic space group Im3 (no. 204). Mn(4+)/Mn(3+) and Fe(3+) occupy at random the octahedral B positions of the perovskite structure. These materials have also been characterized by magnetic and magnetotransport measurements. The observed enhancement of T(C) along the RCu(3)(Mn(3)Fe)O(12) series is understood as an effect of the chemical pressure on the (Mn,Fe)-O bonds as R(3+) size decreases. The semiconducting behaviour observed in all of the samples is related with the introduction of Fe at B position. Despite the drastic change of the transport properties, significant negative magnetoresistance values are observed in the Fe-containing compounds both at 10 K and 300 K.

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T(C) = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 μ(B)/Cu atom.

High-pressure synthesis is a powerful technique to stabilize metastable oxides, either containing transition metals in unusual oxidation states, or favouring the formation of dense perovskite-related phases. Happily, many solids synthesized at high pressure-high temperature conditions (where they are fhermodynamically stable) can be "quenched" to ambient conditions, where they are termodynamically metaestable, yet they remain indefinitely kinetically stable. In this paper we illustrate the example of a new family of oxides derived from the CaCu3Mn4O12 perovskite. We have studied the series of nominal composition NdCu3(Mn3M)O12 (M = Fe, Cr) where Mn is replaced by Fe(Cr) cations in the ferrimagnetic perovskite NdCu3Mn4O12. These materials have been synthesized in poly crystalline form under moderate pressure conditions of 2 GPa, in the presence of KClO4 as oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) below and above the ferromagnetic Curie temperatures. These oxides crystallize in the cubic space group Im (No. 204). Mn4+/Mn3+ and Fe3+(Cr3+) occupy at random the octahedral B positions of the perovskite structure. The materials have also been characterized by magnetic and magnetotransport measurements. All the samples are ferrimagnetic and show a decrease of TC upon Fe(Cr) introduction since these ions disturb the ferromagnetic interactions within this magnetic sublattice. The introduction of Fe changes the resistivity response from metallic to a semiconductor behavior. However, the magnetoresistance is still considerable at 300 K upon Fe doping, and it is enhanced at 100 K probably due to the decrease in the number of charge carriers from the pure oxide to the Fe-doped compound.