Structural Phase Transitions and Three-Dimensional Magnetic Ordering in the Nd₂NiO₄ Oxide

Neutron diffraction on polycrystalline samples of stoichiometric Nd₂NiO₄ shows a complex structural and magnetic behaviour as a function of temperature. The room temperature (RT) phase is orthorhombic (Bmab) and Ni²⁺ 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_Pmab'). The magnetic moment for Ni²⁺ is 1.57μ_B at 160 K. The system undergoes a structural phase transition from orthorhombic to tetragonal (P4₂/ncm) at T₁ ≈ 130 K. The tetragonal phase allows the existence of a ferromagnetic component along c-axis in the Ni²⁺ spin structure, the magnetic structure can be described either as a g_xc_yf_z mode (Shubnikov group (Pc' c' n) or as a g_x + c_yf_z mode (Shubnikov group P4₂/nc' m'). At a temperature as high as 70 K the polarization of the Nd³⁺ ions becomes noticeable. At low temperature (T_N ≈ 8 K) Nd³⁺ 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 P4₂/nc' m'). Fully oxidized samples of Nd₂NiO_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.