Comparative studies on the structural, magnetic, and optical properties of perovskite Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) manganite nanoparticles synthesized by sol–gel method

Comparative studies of the structural, magnetic, and optical properties of the sol–gel synthesized Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) nanoparticles were carried out focusing on the effect of the A-site average cation size ⟨rA⟩. Rietveld refinements of x-ray diffraction data demonstrate all nanoparticles crystallize in an orthorhombic crystal structure (Pnma space group). Their unit cell volumes and the Mn–O–Mn bond angle decreased with reducing ⟨rA⟩, whereas the Mn–O bond length increased. The morphologies of nanoparticles evolved from spherical to irregular shapes, and their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching mode of the Mn–O bond near 600 cm−1, and the softening of this phonon mode as reducing ⟨rA⟩ is ascribed to the elongation of the Mn–O bond length. X-ray photoelectron spectroscopy reveals the mixed Mn3+ and Mn4+ cations with a content ratio of Mn3+/Mn4+ = 2:1, divalent Ca cations, and trivalent rare earth Ln cations in all nanoparticles and oxygen element existing as lattice oxygen and chemically absorbed oxygen. The La0.67Ca0.33MnO3 nanoparticles exhibited ferromagnetic behavior, whereas Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles displayed antiferromagnetic behavior and strong exchange bias effect. Temperature dependence of dc magnetizations suggests the spin-glass behavior established in the La0.67Ca0.33MnO3 nanoparticles, while magnetic cluster-glass behavior formed in the Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles, in which the charge-ordered and antiferromagnetic phases were completely suppressed. Electronic bandgaps of the nanoparticles were about 1.55 eV–1.66 eV, which was ascribed to the electronic charge transfer between two eg bands of the Mn cation with up-spins and down-spins separated by Hund’s coupling energy.