Complex Magnetic Order in Topochemically Reduced Rh(I)/Rh(III) LaM(0.5)Rh(0.5)O(2.25) (M = Co, Ni) Phases

[Image: see text] 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.