Insulating to metallic behaviour in the cation ordered perovskites Ba₂Nd₁−_xFe_xMoO₆

The series of cation ordered double perovskites Ba2Nd1−xFexMoO6 undergo a compositionally-driven transition from localised to delocalised electronic behaviour, as exhibited in the end members Ba2NdMoO6 and Ba2FeMoO6 respectively. Rietveld structural analyses against neutron diffraction data indicate that all compounds are stoichiometric in oxygen and show replacement of Nd3+ with Fe3+ on the larger of the two octahedral sites in the cation-ordered structure. A tetragonal distortion persists up to x=0.25 and Ba2Nd0.9Fe0.1MoO6 shows freezing of magnetic moments below a spin glass transition temperature of 5 K. Ba2Nd0.75Fe0.25MoO6 shows high electrical resistivity with a temperature dependence indicative of fully localised electronic behaviour. Despite the Fe3+ occupation (0.25) being above the percolation limit (0.195) for the face centred cubic lattice, this compound shows no magnetic ordering at 2 K. Compositions in the range 0.30 ≤ x ≤0.85 give a mixture of two perovskite phases with lattice parameters of ca. 8.4 and 8.1 Å. The single phase compositions Ba2Nd0.10Fe0.90MoO6 and Ba2Nd0.05Fe0.95MoO6 form face centred cubic structures with long range magnetic ordering of the Fe3+ moments below ferrimagnetic ordering transitions of 270 and 285 K respectively. Neutron diffraction shows almost complete parallel alignment of the Fe3+ moments and, combined with conductivity measurements showing delocalised electronic behaviour in Ba2Nd0.10Fe0.90MoO6, indicate ferrimagnetic ordering of Fe3+ and delocalised Mo5+.