Complex Magnetic Order in Topochemically Reduced Rh(I)/Rh(III) LaM_0.5Rh_0.5O_2.25 (M = Co, Ni) Phases

Topochemical reduction of the cation-disordered perovskite oxides LaCo_0.5Rh_0.5O₃ and LaNi_0.5Rh_0.5O₃ with Zr yields the partially anion-vacancy ordered phases LaCo_0.5Rh_0.5O_2.25 and LaNi_0.5Rh_0.5O_2.25, 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 LaCo_0.5Rh_0.5O_2.25 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. LaNi_0.5Rh_0.5O_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 Ni¹⁺ centers.