Probing the incoherent admixture of low and high-spin states of Co in (LaPr)CoO 3 Perovskite: focus on structural phase transitions.

We report the mixed valence and intermediate spin-state (I.S.) transitions in Pr substituted LaCoO 3 perovskites in the form of bulk and nanostructures. Structural analysis of these compounds reveal phase crossover from monoclinic ( I2/a ) to Orthorhombic ( Pbnm ), and Rhombohedral ( R-3c ) to Orthorhombic (Pnma) in bulk and nanostructures, respectively for the composition range 0 ≤ x ≤ 0.6. Such a structural transformation markedly reduces the Jahn-Teller distortion factor Δ JT : 0.374→0.0016 signifying the dominant role of intermediate spin state (S Avg =1) of trivalent Co ions in the investigated system. Magnetization measurements reveal the ferromagnetic (FM) nature of bulk LaCoO 3 along with a weak antiferromagnetic (AFM) component coexisting with an FM component resulting in a weak loop-asymmetry (zero-field exchange-bias effect ~134 Oe) at low temperatures. Here the FM ordering occurs due to the double exchange interaction (J EX ~11.25 K) between the tetravalent and trivalent Co ions. Significant decrease in the ordering temperatureswas noticed in the nanostructures (T C ~ 50 K) as compared to the bulk counterpart (~ 90 K) due to the finite size/surface effects in the pristine compound. However, Pr incorporation leads to the development of a strong AFM component (J EX ~ -18.2 K) and enhances the ordering temperatures (~ 145 K for x = 0.9) with negligible FM correlations in both bulk and nanostructures of LaPrCoO 3 due to the dominant super-exchange interaction: Co 3+/4+ ‒ O ‒ Co 3+/4+ . Further evidence to the incoherent mixture of low-spin (L.S.) and high-spin (H.S.) states come from the M-H measurements which yields saturation magnetization MS ~ 275 emu/mol (under the limit of 1/H →0) consistent with the theoretical value of 279 emu/mol corresponding to the spin admixture: 65% L.S +10% I.S. of trivalent Co along with 25% of L.S. Co 4+ in bulk pristine compound. Similar analysis yields: Co 3+ [30% L.S +20% I.S.]+Co 4+ [50% of L.S.] for the nanostructures of LaCoO 3 , yet the Pr substitution decreases the spin admixture configuration. The Kubelka-Munk analysis of the optical absorbance results a significant decrease in the optical energy band gap (E G :1.81→ 1.70 eV) with incorporation of Pr in LaCoO 3 which corroborates the above results.&#xD.