Lanthanide contraction effect on crystal structures, magnetic, and dielectric properties in ordered double perovskites LnPbCoSbO6 (Ln = La, Pr, Nd)

摘要

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 (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. The dielectric constants present frequency dependence and monotonically decrease with the ionic radii reduction from La3+ to Nd3+ due to the suppression of electron transfer. These results indicate that the magnetic and dielectric properties can be tuned by controlling the degree of lattice distortion, which is realized by introducing different Ln3+ ions at the A-site.