NEUTRON-DIFFRACTION STUDY ON STRUCTURAL AND MAGNETIC-PROPERTIES OF LA2NIO4

An overall survey of the structural and magnetic features of the La2NiO4-sigma system is presented as a result of neutron diffraction experiments. The stoichiometric compound (delta = 0) presents two structural phase transitions. At T0 congruent-to 770 K. La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 congruent-to 80 K, from orthorhombic to a new tetragonal (P4(2)/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 (T(N) = 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.