Synthesis, structure and magnetic properties of the pillared perovskites La5Re3MO16(M = Mg, Fe, Co, Ni)

Four new compounds La5Re3MgO16 La5Re3FeO16 La5Re3CoO16 La5Re3NiO16 have been prepared by solid-state reaction and characterized by X-ray and neutron powder diffraction and SQUID magnetometry. Rietveld refinement revealed that the four compounds are isostructural with La5Re3MnO16 and crystallize in space group C (1) over bar with cell parameters a = 7.9370(3), 7.9553(5), 7.9694(7), and 7.9383(4)Angstrom; b = 7.9998(3), 7,9960(6), 8.0071(8), and 7.9983(5)Angstrom; c = 10.1729(4), 10.1895(7), 10.182(l), and 10.1732(6)Angstrom; alpha = 90.190(3)degrees, 90.270(3)degrees, 90.248(4)degrees, 90.287(3)degrees; beta = 94.886(2)degrees, 95.082(3)degrees, 94.980(4)degrees, 94.864(3)degrees; 7 = 89.971(4)degrees, 90.001(5)degrees, 89.983(6)degrees, 89.968(4)degrees for Mg, Fe, Co, and Ni, respectively. The structures are related to a layered perovskite. The layers of corner-sharing octahedra Re5+ M2+ O-6 (M2+ = Mg, Fe, Co, Ni) are pillared by diamagnetic edge-sharing octahedra dimers, Re2O10, involving a Re = Re double bond. Three crystallographically independent lanthanum atoms occupy the three-dimensional interstices. All compounds obey the Curie-Weiss law at sufficiently high temperatures with Curie constants or effective magnetic moments near the expected values for the combination of Re5+ (S = 1) and M2+ (S = 0, 2, 3/2, 1 for Mg, Fe, Co, and Ni, respectively). Weiss constants, theta(C), are negative (-575, -84, -71, and -217 K for Mg, Fe, Co, and Ni, respectively) indicating the predominance of antiferromagnetic exchange coupling. The phases for M = Fe, Co and Ni show long-range order at 155, 33, 36 and 14 K, respectively. Neutron diffraction discloses a magnetic structure for the Fe series member consisting of ferrimagnetic perovskite layers coupled antiparallel along the stacking c-axis, direction which is consistent with the magnetic structure found recently for La5Re3MnO16. (C) 2002 Elsevier Science (USA). All rights reserved.