Structure and magnetism in Pr3ReO7 and Nd3ReO7—materials with an ordered, defect fluorite structure

Nd3ReO7 and Pr3ReO7 were prepared by solid-state reaction involving Ln2O3, ReO3 and ReO2 in sealed quartz tubes and a carefully controlled thermal sequence. Attempts to prepare the corresponding compound with Ln=La was unsuccessful with the same procedure. The crystal structures were refined from powder neutron diffraction data in space group Cmcm. The unit cell constants (Å) are a=10.941(1), b=7.4565(8) and c=7.5993(9) for Ln=Pr and a=10.867(1), b=7.4594(7) and c=7.5740(8) for Ln=Nd. The structure type is a well-known variant of an ordered, defect fluorite. There are two Ln sites with pseudo-cubic 8-fold and pentagonal bipyramidal 7-fold coordination and Re5+ in an octahedral site. The Re–O octahedra share corners to form a zig-zag chain parallel to the c-axis. In both materials, Re5+ displays a local magnetic moment consistent with S=1 and both show evidence for cooperative magnetic effects at low temperature. Pr3ReO7 appears to show spin glass order below about 10K in the form of a zero-field-cooled–field-cooled divergence in the susceptibility, a hysteresis in the isothermal magnetization curves and the absence of magnetic Bragg peaks in the neutron powder pattern. For Nd3ReO7, by contrast, the bulk magnetic behavior is very complex but magnetic Bragg peaks appear in the neutron diffraction pattern below Tc=9K, thus confirming long-range order. The magnetic Bragg peaks can be indexed with a wave vector, k=(000).