Room-temperature and low-temperature structure of Nd1−x Ca x MnO3 (0.3 ≤ x ≤ 0.5)

The structure of the insulating manganites Nd(1-x)Ca(x)MnO(3) (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 GdFeO(3)-type structure (a approximately 2(1/2)a(p), b approximately 2a(p), c approximately 2(1/2)a(p)). Below the charge-ordering temperature of approximately 250 K, a doubling of the lattice is observed along the a axis which could be interpreted as an ordering of the d(z(2)) Mn(3+) orbitals. The low-temperature polymorph has ortho-rhombic 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 GdFeO(3)-type structure and indicates the existence of defective areas with a lower symmetry.