Magnetic and structural features of the NdNi1-xMnxO3 perovskite series investigated by neutron diffraction

Selected members of the perovskite series NdNi1-xMnxO3 (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 21/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni3+ + Mn3+ to give Ni2+ + Mn4+ cations. The Ni2+O6 octahedra are considerably larger than the Mn4+O6 octahedra. This compound can be considered as a double perovskite of composition Nd2NiMnO6. Unlike NdNiO3 and NdMnO3, which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum TC of 200 K is observed for x = 0.5, whereas TC = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO3 is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.