An overall survey ofthe structural and magnetic features ofthe La2Ni04,d system is presented as a result of neutron diffraction experiments. The stoichiometric compound (a = 0) presents two structural phase transitions. At Tu = 770 K, La,NiOd transforms from tetragonal (141mmm) to orthorhombic (Bmab); at 7, = 80 K, from orthorhombic to a new tetragonal (P4dncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. LaiNiOl is three-dimensional ( 3 ~) antiferromagneticallyordered at room temperature (T, = 330 K). A weak ferromagnetic component appears below T,. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to B non-stoichiometricmmpoundwith interstitial oxygens. A tentative phase diagram is proposed

The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific heat, and neutron diffraction measurements. By cooling, HoMnO3 exhibits three singularities at T N = 41 K, T ≈ 26 K, and T ≈ 6.5 K, suggesting a rich magnetic phase diagram. The neutron diffraction data show that below T N = 41 K, the Mn3+ magnetic moments become ordered in an antiferromagnetic arrangement, adopting a modulated sinusoidal magnetic structure characterized by the wave vector k = (k x ,0,0) (k x = 0.40 at 41 K) and defined by the magnetic mode (C x ,0,0). When the temperature is decreased, the propagation vector varies and at T ≈ 29 K a transition to a commensurate magnetic structure defined by k = (1/2,0,0) takes place. Below T ≈ 22 K, a small ordered magnetic moment appears on the Ho3+ cations, strongly increasing below 9 K and reaching 7.3(1) μB at T = 1.8 K. The magnetic structure of the Ho3+ moments is defined by a (A x ,0,C z ) mode. The (H,T) phase diagram has been mapped out and the different magnetic structures interpreted on the basis of competing superexchange interactions.

Hexagonal, nonperovskite HoMnO3 oxide, containing a triangular arrangement of Mn3+ cations, has been prepared in polycrystalline form by the thermal decomposition of metal citrates. The crystal structure has been refined from neutron powder diffraction data. Magnetic and specific-heat measurements anticipate a complex phase diagram:  HoMnO3 becomes magnetically ordered at T N ≈ 72 K, and another two magnetic transitions take place at lower temperatures. Neutron powder diffraction measurements demonstrate that, below the ordering temperature, the moments of the Mn3+ cations adopt a triangular spin arrangement, the magnetic moments lying in the basal plane and parallel to the [100] axis. At T = 44.6 K, the moments suddenly reorientate within the basal plane and become aligned perpendicularly to the initial direction. Below T = 25.4 K, an ordered magnetic moment is observed on the Ho atoms at the 4b sites of the crystal structure, whereas those of the 4a site remain in a paramagnetic state. The Ho atoms adopt an antiferromagnetic structure with the moments parallel to the c axis. At 1.7 K, the ordered moment on the Mn3+ cations is 3.05(2) μB, and that on the Ho3+ cations is 2.97(3) MB.