Neutron diffraction investigation of magnetic phase transitions to long-range antiferromagnetic ordering in the “free-electron” praseodymium halides Pr 2X5 ( X = Br, I)

Abstract

Powder neutron and single-crystal X-ray diffraction experiments of the halides Pr 2X 5 ≜ ( Pr 3+) 2(X −) 5( e −) were performed in order to determine temperature dependences of the structure and to establish the nature of the magnetic phase transitions. Bulk magnetic measurements in the temperature range of 1.6 to 300 K showed remarkably high ordering temperatures. Pr 2Br 5 orders antiferromagnetically at T N = 50(1) K, corresponding to the Shubnikov space group P2 1 m′ , i.e., k = 0. Below T ≈ 25 K the ordered magnetic moments of the two inequivalent Pr 3+ ions increase abruptly from μ Pr1 = 1.2 μ B and μ Pr2 = 2.2 μ B at 30 K to the saturation values μ Pr1 = 1.9 μ B and μ Pr2 = 2.6 μ B. Below T N = 37(1) K, Pr 2I 5 shows antiferromagnetic ordering corresponding to k = [ 1 2 ,0,0] and the Shubnikov space group P a 2 1 with ordered magnetic moments at saturation: μ Pr1 = 2.1 μ B and μ Pr2 = 1.8 μ B. Measurements of average electrical conductivities of polycrystalline Pr 2 X 5 indicate semiconducting behavior.