Structural Phase Transitions and Three-Dimensional Magnetic Ordering in the Nd 2 NiO 4 Oxide

Neutron diffraction on polycrystalline samples of stoichiometric Nd 2 NiO 4 shows a complex structural and magnetic behaviour as a function of temperature. The room temperature (RT) phase is orthorhombic ( Bmab ) and Ni 2+ ions are 3D antiferromagnetically ordered ( T N ≈ 320 K), with a propagation vector k = [100] and spins oriented parallel to the propagation vector, i.e. along the a -axis. The magnetic structure can be described as a g x mode (Shubnikov group B P mab') . The magnetic moment for Ni 2+ is 1.57μ B at 160 K. The system undergoes a structural phase transition from orthorhombic to tetragonal ( P 4 2 / ncm ) at T 1 ≈ 130 K. The tetragonal phase allows the existence of a ferromagnetic component along c -axis in the Ni 2+ spin structure, the magnetic structure can be described either as a g x c y f z mode (Shubnikov group ( Pc' c' n ) or as a g x + c y f z mode (Shubnikov group P 4 2 / nc' m' ). At a temperature as high as 70 K the polarization of the Nd 3+ ions becomes noticeable. At low temperature ( T N ≈ 8 K) Nd 3+ ions cooperative order takes place with a magnetic moment of 3.2μ B at 1.5 K and a magnetic structure belonging to the same Shubnikov group (either Pc' c' n or P 4 2 / nc' m' ). Fully oxidized samples of Nd 2 NiO 4+δ as obtained in air atmosphere, seems to be also orthorhombic at RT and do not show static magnetic order or structural phase transitions between 1.5 K and 300 K. Partially oxidized samples (δ = 0.04) are also orthorhombic at RT, and show a similar magnetic behaviour as stoichiometric ones, but no structural phase transition at low temperatures is observed.