Room-temperature and low-temperature structure of Nd1-xCaxMnO3 (0.3 <= x <= 0.5)

The structure of the insulating manganites Nd1-xCaxMnO3 (0.3 less than or e qual to x less than or equal to 0.5) has been studied against temperature u sing 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 approximate to 2(1/2)a(p), b approximate to 2a(p), c appr oximate to 2(1/2)a(p)). Below the charge-ordering temperature of similar to 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-temperatu re 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 wa s refined using neutron diffraction data. The HREM study at room temperatur e confirms the average GdFeO3-type structure and indicates the existence of defective areas with a lower symmetry.