Compounds of general composition A3MO7 where A3+ = La, Pr, Nd and M5+ = Nb, Ta, Sb, Ru, Mo, crystallizes in an orthorhombic fluorite-related superstructure: the weberite structure. There are three different crystallographic sites for the cation, distorted cubic A3+ site, distorted pentagonal bipyramidal A3+ and the octahedral M5+ site. The M5+ cation is octahedrally coordinated to six oxygen ions and the MO6 octahedra share corners, forming a zig-zag chain. In this structure slabs are formed in the bc plane, in which one dimensional MO6 chains run parallel to the c-axis alternating with rows of edge shared AO8 pseudo cubes consisting of one third of A3+ ions. These slabs are separated by the remaining two thirds of A ions which are seven-coordinated by oxygen ions.
These compounds display a wide variety of physical properties depending on the electronic configuration of the metals ions and the interaction between the different metal sites. Fox example, when M is a d0 or d10 cation the oxides are electronically inert. But when M5+ has a dn configuration, the materials are electronically and magnetically active. In the case that A is also a magnetic ion, its magnetic moment can interact whit the magnetic moments of the M5+ species, leading to a more complicated magnetic behavior.
In this work we present the synthesis, structure and magnetic characterization of Gd3MO7 with M = Nb, Ta and Sb. These phases were synthesized as polycrystalline powders by standard ceramic method. All are orthorhombic, space group C2221, determined through Rielveld refinement from X-ray powder diffraction data.
Magnetic measurements were made in a commercial SQUID in a 5 ? 400 K Temperature range. M vs T measurements indicated paramagnetic behaviour at room temperature for all the samples. The temperature dependence of the magnetic susceptibility obeys the Curie-Weiss law with a magnetic moment close to the expected value of 7.94 calculated assuming the free ion moment for 4f7 Gd3+. In the case of Gd3SbO7 it is observed at round 125 K a deviation in the susceptibility, not observed en the case of Nb and Ta.
