Structural and magnetic phase transitions in Pr2NiO4

Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr₂NiO₄₊(0), which at room temperature (RT) is orthorhombic (Bmab). Ni²⁺ becomes three dimensionally (3D) antiferromagnetically ordered atT_N325K, with a propagation vectork=[100], and spins oriented along thea axis (parallel to the propagation vector). The magnetic structure belongs to the_7g (––+)-representation (g_x mode, Shubnikov groupB_pmab') ofBmab, and a magnetic moment of 1.5_B is measured at 155K. This compound undergoes two different structural phase transitions. First, going down in temperature, Pr₂NiO₄ transforms by a first order phase transition from orthorhombic (Bmab) to tetragonal (P4₂/ncm) atT_c1115K. At high temperature, we can predict a transformation from orthorhombic (Bmab) to tetragonal (I4/mmm) in a continuous way. The extrapolated temperature for this second structural transition isT_c21500±100K. The low temperature structural transformation allows a change in the magnetic structure which forT<T_c1 is better described in the orthorhombic symmetry. Just belowT_c1 the magnetic structure is described by the:_3g (–+)-representation ofPccn(g_xc_yf_z mode, Shubnikov groupPccn), this magnetic phase begins to disappear going down in temperature and at the same time a new magnetic phase grows. This new magnetic structure is described by the_1g ofPccn (c_xg_ya_z mode, Shubnikov groupPccn). Both magnetic structures coexist in a certain temperature range. At 1.5 K thec_xg_ya_z mode represents the total of the magnetic ordering. To reproduce the observed magnetic intensities we are forced to consider that Pr³⁺ is polarized below 40 K, with a magnetic structure which is coupled to the Ni sublattices (i.e._1g and_3g). The magnetic moment at low temperature for Pr is about 1.28_B.