Insulating to metallic behaviour in the cation ordered perovskites Ba2Nd1−xFexMoO6

Abstract

The series of cation ordered double perovskites Ba₂Nd_1−xFe_xMoO₆ 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 Ba₂Nd_0.9Fe_0.1MoO₆ shows freezing of magnetic moments at 5 K. Neutron scattering indicates an absence of long range magnetic ordering suggesting the formation of a spin glass phase below this temperature. Ba₂Nd_0.75Fe_0.25MoO₆ 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 Ba₂Nd_0.10Fe_0.90MoO₆ and Ba₂Nd_0.05Fe_0.95MoO₆ 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 Ba₂Nd_0.10Fe_0.90MoO₆, indicate ferrimagnetic ordering of Fe³⁺ and delocalised Mo⁵⁺.