research papers
The structure of the insulating manganites Nd1−xCaxMnO3 (0.3 ≤ x ≤ 0.5) has been studied against temperature using electron diffraction and neutron diffraction. At room temperature, the three compounds with x = 0.3, 0.4 and 0.5 exhibit the orthorhombic GdFeO3-type structure (a ≈ 21/2ap, b ≈ 2ap, c ≈ 21/2ap). Below the charge-ordering temperature of ∼250 K, a doubling of the lattice is observed along the a axis which could be interpreted as an ordering of the dz2 Mn3+ orbitals. The low-temperature polymorph has orthorhombic symmetry for x = 0.5 and monoclinic symmetry for x = 0.4 and 0.3. The low-temperature structure of the x = 0.4 sample was refined using neutron diffraction data. The HREM study at room temperature confirms the average GdFeO3-type structure and indicates the existence of defective areas with a lower symmetry.