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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.

Topochemical reduction of the n = 1 Ruddlesden-Popper phases LaSrCo 0.5 Rh 0.5 O 4 and LaSrNi 0.5 Rh 0.5 O 4 with Zr yields LaSrCo 0.5 Rh 0.5 O 3.25 and LaSrNi 0.5 Rh 0.5 O 3.25 , respectively. Magnetization and XPS data reveal that while the rhodium centers in LaSrCo 0.5 Rh 0.5 O 3.25 and LaSrNi 0.5 Rh 0.5 O 3.25 have an average oxidation state of Rh 2+ , these are actually mixed valence Rh(I,III) compounds, with the disproportionation of Rh 2+ driven by the favorability of locating d 8 Rh 1+ and d 6 Rh 3+ cations within square-planar and square-based pyramidal coordination sites, respectively.