Abstract
Topochemical reduction of the cation-disordered perovskite oxides LaCo0.5Rh0.5O3 and LaNi0.5Rh0.5O3 with Zr yields the partially anion-vacancy ordered phases LaCo0.5Rh0.5O2.25 and LaNi0.5Rh0.5O2.25, respectively. Neutron diffraction and Hard X-ray photoelectron spectroscopy (HAXPES) measurements reveal that the anion-deficient phases contain Co1+/Ni1+ and a 1:1 mixture of Rh1+ and Rh3+ cations within a disordered array of apex-linked MO4 square-planar and MO5 square-based pyramidal coordination sites. Neutron diffraction data indicate that LaCo0.5Rh0.5O2.25 adopts a complex antiferromagnetic ground state, which is the sum of a C-type ordering (mM5+) of the xy-components of the Co spins and a G-type ordering (mΓ1+) of the z-components of the Co spins. On warming above 75 K, the magnitude of the mΓ1+ component declines, attaining a zero value by 125 K, with the magnitude of the mM5+ component remaining unchanged up to 175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co1+ cations in MO4 square-planar and MO5 square-based pyramidal coordination sites. LaNi0.5Rh0.5O2.25 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 Ni1+ centers.
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