Dear Bajarangi Prasad Sahoo

You're talking about replacing Mn³⁺ (67 pm) with Co³⁺ (65 pm) in a perovskite structure like LaMnO₃ or similar. Since both are in the +3 oxidation state, this is an isovalent substitution, but the ionic radius difference will have several important consequences.

• Structural Effects: The smaller ionic radius of Co³⁺ causes slight lattice contraction and increased octahedral tilting, leading to structural distortion from ideal symmetry (e.g., cubic to orthorhombic).
• Magnetic Properties: Co³⁺ can exist in multiple spin states (low, intermediate, or high spin). If it adopts a low-spin state (S = 0), magnetic ordering weakens significantly compared to the high-spin Mn³⁺ (S = 2). This alters superexchange interactions and may reduce or disrupt long-range magnetic ordering.
• Electronic & Transport Behavior: Differences in electronic configuration and bonding can affect charge transport, potentially changing conductivity or inducing metal–insulator transitions depending on the degree of substitution.
• Overall Impact: This substitution tunes the perovskite’s structure, magnetism, and electronic properties, offering pathways to engineer materials for specific functions like magnetoresistance, spintronics, or catalysis.

Thanks for your time.

Dear Bajarangi Prasad Sahoo

Yes, in case of a perovskite material and you replacing the the magnetic ion with same oxidation number but lesser ionic radius as seen in your question text ( replacing Co^3+ ion 65pm by Mn^3+ ion 67pm) . So you have to expect the magnetic properties become improved well , since magnetism properties are usually related to unfilled 3d electron shells of transition metal ions incorporated on the bulk ion site (electronic configuration of Co^3+ ion is [Ar] 3d6 and for a Mn3+ ion is [Ar]3d4) . On other hand, the difference in ionic radius is small and it's has limmited effect on the degree of crystallographic distortions of most perovskitessize and only falls in the context of a rough estimate error of structure tolerance factor .

Best regards ....

https://repositorio.ulisboa.pt/bitstream/10451/1643/10/19741_ulsd_re487_10_Chapter2.pdf