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The B-site ordered double perovskite oxides LaPbMSbO6 (M=Co, Ni) have been synthesized via the modified Sol–Gel precursor two-step route. Rietveld refinements reveal strong abnormal structural distortion and BO6 octahedral deformation appearing along the ab plane. Owing to the cooperative Jahn–Teller effect of Co2+ and Pb2+ ions, the Co-related compound exhibits almost complete Co2+–Sb5+ order. For magnetic properties, spin-canted antiferromagnetic state with high extent of magnetic frustration is confirmed. The Ni-related compound presents heavier magnetic frustration for introducing tiny disorder on site occupation accompanied with valence state and further enhancing the complexity of magnetic competition. Dielectric measurements present a considerable temperature dependent dielectric relaxation with great dc-like loss feature in the LaPbCoSbO6. For LaPbNiSbO6, however, the permittivity with low dielectric loss is shown to be insensitive to either temperature or frequency. The corresponding electronic active energy manifests that the weakly bounded 3d-electron is prone to hop in a more distorted Co–Sb sublattice.

A-site ordered double perovskites hold promise for potential device applications, where how their magnetic objects actually interact is known to influence functionality. Here, we investigate, by first-principles calculations, electronic and magnetic properties of YMn3Al4O12, and demonstrate that it is an antiferromagnetic insulator. Our calculations reveal the extended (Mn-O)-(O-Mn) superexchange as a primary magnetic coupling mechanism in YMn3Al4O12, and further identify the O-O bonds as a key factor to dominate coupling strength. We suggest that this superexchange mode is the origin of antiferromagnetic behavior, opening an additional avenue to understand magnetic coupling in A-site ordered double perovskites. (C) 2012 Elsevier Masson SAS. All rights reserved.

We have systematically investigated the electronic and magnetic properties of hypothesized A-site-ordered perovskite YMn₃Sc₄O₁₂ using first-principle calculation based on the density functional theory. Our calculated results predict that YMn₃Sc₄O₁₂ is both thermodynamically and mechanically stable and its ground state is antiferromagnetic insulator. The Mn(3+) is in the high-spin state. More importantly, by comparison to YMn₃Sc₄O₁₂, we point out that the empty Sc 3d orbital provides the Mn--O--Sc--O--Mn superexchange interaction, which is similar to its isostructural perovskite CaCu₃Ti₄O₁₂, and enhances the antiferromagnetic interaction between Mn ions. From these calculations, we can clearly see that the empty 3d orbital plays an important role to realize superexchange interaction.

A-site ordered double perovskites hold promise for potential device applications, where how their magnetic objects actually interact is known to influence functionality. Here, we investigate, by first-principles calculations, electronic and magnetic properties of YMn3Al4O12, and demonstrate that it is an antiferromagnetic insulator. Our calculations reveal the extended (Mn-O)-(O-Mn) superexchange as a primary magnetic coupling mechanism in YMn3Al4O12, and further identify the O-O bonds as a key factor to dominate coupling strength. We suggest that this superexchange mode is the origin of antiferromagnetic behavior, opening an additional avenue to understand magnetic coupling in A-site ordered double perovskites. (C) 2012 Elsevier Masson SAS. All rights reserved.

The crystal structures, magnetic and dielectric properties for the ordered double perovskites LnPbNiSbO6 (Ln = La, Pr) have been investigated. Rietveld refinements of x-ray diffraction data have been indexed for the monoclinic symmetry in space group P21/n (No. 14) and a highly rock-salt ordered arrangement of NiO6 and SbO6 octahedra. The B-site lattices are distorted strongly due to the substitution of rare Earth ions at the A-site. The magnetization measurements show an antiferromagnetic ordering. The effective magnetic moments μ eff are larger than the spin-only values, suggesting that the orbital component for Ni2+ is significant. The maximum values of isothermal magnetization increase due to the lattice distortion of BO6 octahedra, which may weaken the antiferromagnetic interaction via Ni2+–O–Sb5+–O–Ni2+ paths. The dielectric constants for LaPbNiSbO6 present frequency dependence and the tan δ curves exhibit relaxor-like dielectric response. The e' decreases with the reduction of the magnetic moments of B-site transition metal ions, which reveals a relationship between the dielectric and magnetic properties.

The crystal structures, magnetic, and dielectric properties for the ordered double perovskites LnPbCoSbO6 (Ln = La, Pr, Nd) have been investigated. The crystal structure has been solved by Rietveld refinements of X-ray diffraction data in the monoclinic space group P21/n (No. 14). The Co2+ and Sb5+ ions are almost fully ordered over the B-site, and the octahedral framework displays significant tilting distortion according to the Glazer's tilt system a–a–c+. As the result of lanthanide contraction from La3+ to Nd3+, the B-site sublattice distortions become stronger accompanying with the reduction of the tolerance factor and coordination number. The magnetization measurements show an antiferromagnetic ordering with large effective magnetic moments (μeff) suggesting that the orbital component is significant. The maximum values of isothermal magnetization increase with the decrease in radii of rare earth ions, which is attributed to the weakening of antiferromagnetic interaction via Co2+–O–Sb5+–O–Co2+ paths. ...