STRUCTURAL AND MAGNETIC PHASE-TRANSITIONS IN PR2NIO4

Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr2NiO4 + delta-(delta-almost-equal-to 0), which at room temperature (RT) is orthorhombic (Bmab). Ni2+ becomes three dimensionally (3D) antiferromagnetically ordered at T(N) almost-equal-to 325 K, with a propagation vector K = [100], and spins oriented along the a axis (parallel to the propagation vector). The magnetic structure belongs to the GAMMA-7g(- - +)-representation (g(x) mode, Shubnikov group B(p)mab') of Bmab, and a magnetic moment of 1.5 mu-B is measured at 155 K. 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 (P4(2)/ncm) at T(c1) almost-equal-to 115 K. 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 is T(c2) almost-equal-to 1500 +/- 100 K. The low temperature structural transformation allows a change in the magnetic structure which for T < T(c1) is better described in the orthorhombic symmetry. Just below T(c1) the magnetic structure is described by the: GAMMA-3g (- +)-representation of P c c n (g(x)c(y)-closed-integral-z mode, Shubnikov group P c' c' n), 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 GAMMA-1g(+ +)-representation of P c c n (c(x)g(y)a(z) mode, Shubnikov group P c c n). Both magnetic structures coexist in a certain temperature range. At 1.5 K the c(x)g(y)a(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. GAMMA-1g and GAMMA-3g-representations). The magnetic moment at low temperature for Pr is about 1.28 mu-B.