Origin of Suppression of Charge Ordering Transition in Nanocrystalline Ln0.5Ca0.5MnO3 (Ln = La, Nd, Pr) Ceramics

The bulk and nanocrystalline samples of half doped, rate earth, perovskite manganites, Ln(0.5)Ca(0.5)MnO(3). (Ln = Nd, Pr) were investigated by X-ray diffraction and magnetic measurements at various temperatures to understand the origin of suppression of charge ordering transitions in nanocrystalline samples of these manganites. The controversial reports regarding the effect of crystallite size reduction on the unit cell volume has been resolved by studying these three manganites prepared by combustion synthesis method. As reported by earlier authors, reduction-of the particle size to nanocrystalline range leads to suppression of charge ordering transition and stabilization of ferromagnetic phase at low temperatures. The unit cell volume is found to systematically increase for all the three manganites with decreasing particle size, which results in increased bandwidth and is responsible for suppression of charge ordering transition. Even through the crystal structure of both bulk and nanocrystalline samples is orthorhombic with space group Pnma, crystallite size reduction into nanocrystalline form affects the orthorhombic strain, lattice parameter, atomic coordinates, and unit cell volume. A. comparative study for the Ln(0.5)Ca(0.5)MnO(3) samples prepared by sol gel route is also presented to show that the reduction of the unit cell volume with decreasing crystallite is linked with nonstoichiometry of the samples, which can also lead to the suppression of the charge ordering transition and stabilization of ferromagnetic state in nanocrystalline form reported by some earlier authors. The role of inherent anisotropic strain in nanocrystalline samples On the magnetic state and phase transitions is also investigated.