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Topochemical reduction of the cation-disordered perovskite oxides LaCo 0.5 Rh 0.5 O 3 and LaNi 0.5 Rh 0.5 O 3 with Zr yields the partially anion-vacancy ordered phases LaCo 0.5 Rh 0.5 O 2.25 and LaNi 0.5 Rh 0.5 O 2.25 , respectively. Neutron diffraction and Hard X-ray photoelectron spectroscopy (HAXPES) measurements reveal that the anion-deficient phases contain Co 1+ /Ni 1+ and a 1:1 mixture of Rh 1+ and Rh 3+ cations within a disordered array of apex-linked MO 4 square-planar and MO 5 square-based pyramidal coordination sites. Neutron diffraction data indicate that LaCo 0.5 Rh 0.5 O 2.25 adopts a complex antiferromagnetic ground state, which is the sum of a C-type ordering (mM 5 + ) of the xy -components of the Co spins and a G-type ordering (mΓ 1 + ) of the z -components of the Co spins. On warming above 75 K, the magnitude of the mΓ 1 + component declines, attaining a zero value by 125 K, with the magnitude of the mM 5 + component remaining unchanged up to 175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co 1+ cations in MO 4 square-planar and MO 5 square-based pyramidal coordination sites. LaNi 0.5 Rh 0.5 O 2.25 shows no sign of long-range magnetic order at 2 K - behavior that can also be explained on the basis of the d-orbital occupation of the Ni 1+ centers.

The exchange mechanism and magnetic structure of the organic-inorganic layered molecule-based magnet [Co2(bta)]n (1) (H4bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800 G (metamagnetic behavior), and a slow magnetic relaxation from this ferromagnetic phase to the antiferromagnetic one is observed. The magnetic structure of 1 has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be of antiferromagnetic nature with the local cobalt(II) spins (magnetic moments) being aligned ferromagnetically in the ac plane and antiferromagnetically coupled along the crystallographic b axis. No evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment.