Combined Experimental and Mechanoelastic Modeling Studies on the Low-Spin Stabilized Mixed Crystals of 3D...

Structural studies involving single-crystal and powder X-ray diffraction analysis have been performed on dehydrated coordination networks of the [NiCo_1-(bpy)₃][LiCr(ox)₃] series, 0 ≤ x ≤ 1, (bpy = 2,2'-bipyridine). The high-symmetry cubic 3D structure of these materials is formed by oxalate anions bridging alternating Cr³⁺ and Li⁺ ions into an anionic framework, which contains large cavities that incorporate the [NiCo_1-(bpy)₃]²⁺ cations. Irrespective of the Co/Ni ratio, all of the mixed samples are phase-pure and retain the high-symmetry cubic structure, with the lattice parameters gradually decreasing upon increasing Ni(II) concentration. The influence of the Ni(II) dilution on the magnetic behavior of these materials is substantial. For pure [Co(bpy)₃][LiCr(ox)₃], a gradual but incomplete thermal spin-crossover is evident due to the effect of the chemical pressure applied by the [LiCr(ox)₃]^2- framework, which stabilizes the low-spin (LS) ²E state relative to the high-spin (HS) ⁴T₁ state of the Co(II) ion. Upon increasing the Ni(II) content, the spin-crossover becomes even more gradual and incomplete and eventually is not observed for pure [Ni(bpy)₃][LiCr(ox)₃]. The average spin-crossover temperature increases with the increasing Ni(II) content, suggesting a higher degree of chemical pressure applied by the oxalate framework manifested by changing the ΔE⁰_HL toward positive values. The magnetic behavior of all these framework materials has been explained by the mechanoelastic model, considering different radii for Co and Ni molecules and different interactions between Co-Co sites and Co-Ni sites. The model reproduced the incomplete transition, with the HS residual fraction at 300 K decreasing with increasing Ni concentration, and provided microscopic snapshots of the systems, showing how the existence of impurities prevented the spreading of Co atoms in the HS state.