Neutron diffraction study on structural and magnetic properties of La₂NiO₄

An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T0 approximately=770 K, La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 approximately=80 K, from orthorhombic to a new tetragonal (P42/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La2NiO4 is three-dimensional (3D) antiferromagnetically ordered at room temperature (TN=330 K). A weak ferromagnetic component appears below T1. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.