Room-temperature and low-temperature structure of Nd_1−xCa_xMnO₃ (0.3 ≤ x ≤ 0.5)

The structure of the insulating manganites Nd_1−xCa_xMnO₃ (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 ≈ 2^1/2a_p, b ≈ 2a_p, c ≈ 2^1/2a_p). 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 d_z² 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.