Influence of the annealing temperature on the physical properties of the vacancy manganites Ln1-x□xMnO3

Vacancy manganites Ln(1-x)square(x)MnO(3) powders, where Ln is a rare earth (Ln = Nd, Pr, Sm or La) and 0 is a vacancy, were prepared using the classic ceramic methods with different thermal treatments. The study of physical properties of these oxides can be helpful to obtain accurate information on the role of the oxidation degree of the Mn cation, the sintering temperature and the cooling rate. Annealed at high temperature (1773 K) the samples have perovskite-like structure with an orthorhombic deformation. The lattice volume decreases when annealed at 973 K and the structure is rhombohedral according to the type of the used rare earth. The structural transition is accompanied by different magnetic transitions. The Nd compound sintered at high temperature is ferromagnetic, when further annealed at 973 K different magnetic phases were observed. These structural and magnetic transitions are related to the oxidation of Mn3+ by oxygen uptake leading to the increase of (Mn3+-Mn4+). A sharp drop in resistivity and a magnetoresistance value (rho(0) - rho(5T))/rho(0) of 85% are observed close to T-c = 268 K, for an annealed La(0.8)square(0.2)MnO(3) sample. (C) 2004 Elsevier B. V. All rights reserved.