Probing the incoherent admixture of low and high-spin states of Co in (LaPr)CoO3perovskite: focus on structural phase transitions.

ABSTRACT

We report the mixed valence and intermediate spin-state (IS) transitions in Pr substituted LaCoO3perovskites in the form of bulk and nanostructures. Various compositions (x) of La1-xPrxCoO3(0 ⩽x⩽ 0.9) were synthesized using the sol-gel process under moderate heat treatment conditions (600 °C). The structural analysis of these compounds reveals a phase crossover from the monoclinic phase (space group, s.g.I2/a) to an orthorhombic one (s.g.Pbnm), and a rhombohedral phase (s.g.R-3c) to an orthorhombic one (s.g.Pnma) in the bulk and nanostructures, respectively, for the composition range 0 ⩽x⩽ 0.6. Such a structural transformation remarkably reduces the Jahn-Teller distortion factor ΔJT 0.374 → 0.0016 signifying the dominant role of the IS state (SAvg= 1) of trivalent Co ions in the investigated system. Magnetization measurements reveal the ferromagnetic (FM) nature of bulk LaCoO3along with a weak antiferromagnetic (AFM) component coexisting with an FM component. This coexistence results 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 (JEX/kB∼ 11.25 K) between the tetravalent and trivalent Co ions. Significant decrease in the ordering temperatures was noticed in the nanostructures (TC∼ 50 K) as compared to the bulk counterpart (∼90 K) due to the finite size/surface effects in the pristine compound. However, the incorporation of Pr leads to the development of a strong AFM component (JEX/kB∼ 18.2 K) and enhances the ordering temperatures (∼145 K forx= 0.9) with negligible FM correlations in both bulk and nanostructures of LaPrCoO3due to the dominant super-exchange interaction Co3+/4+‒O‒Co3+/4+. Further evidence of the incoherent mixture of low-spin (LS) and high-spin (HS) states comes from theM-Hmeasurements which yields a saturation magnetization ofMS∼ 275 emu mol-1(under the limit of 1/H→ 0) consistent with the theoretical value of 279 emu mol-1corresponding to the spin admixture 65% LS + 10% IS of trivalent Co along with 25% of LS Co4+in the bulk pristine compound. A similar analysis yields Co3+[30% LS + 20% IS] + Co4+[50% of LS] for the nanostructures of LaCoO3,yet the Pr substitution decreases the spin admixture configuration. The Kubelka-Munk analysis of the optical absorbance results in a significant decrease in the optical energy band gap (Eg1.86 → 1.80 eV) with the incorporation of Pr in LaCoO3which corroborates the above results.