Structural and magnetic phase transitions in Pr2NiO4

Abstract

Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr2NiO4+δ(δ≈0), which at room temperature (RT) is orthorhombic (Bmab). Ni2+ becomes three dimensionally (3D) antiferromagnetically ordered atT N ≈325K, with a propagation vectork=[100], and spins oriented along thea axis (parallel to the propagation vector). The magnetic structure belongs to theΓ7g (−−+)-representation (g x mode, Shubnikov groupBpmab') ofBmab, and a magnetic moment of 1.5μ 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) atTc1≈115K. 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 isTc2≈1500±100K. The low temperature structural transformation allows a change in the magnetic structure which forT<Tc1 is better described in the orthorhombic symmetry. Just belowTc1 the magnetic structure is described by the:Γ3g (−+)-representation ofPccn(gxcyfz mode, Shubnikov groupPc′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 theΓ1g ofPccn (cxgyaz mode, Shubnikov groupPccn). Both magnetic structures coexist in a certain temperature range. At 1.5 K thecxgyaz 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.Γ1g andΓ3g). The magnetic moment at low temperature for Pr is about 1.28μ B .