B-site ordered perovskite LaSrMnNbO6: Synthesis, structure and antiferromagnetism

LaSrMnNbO6 has been synthesized by high temperature solid state reaction under 1% H-2/Ar dynamic flow. The structure is determined by Rietveld refinement of the powder X-ray diffraction data. It crystallizes in the monoclinic space group P2(1)/n with the unit cell parameters: a=5.69187(12), b=5.74732(10), c=8.07018(15) angstrom and beta=90.0504(29)degrees, which were also confirmed by electron diffraction. The Mn2+ and Nb5+ ions, whose valence states are confirmed by X-ray absorption near-edge spectroscopy, are almost completely ordered over the B-site (<1% inversion) of the perovskite structure due to the large differences of both cationic size (0.19 angstrom) and charge. The octahedral framework displays significant tilting distortion according to Glazer's tilt system a(-)b(-)C(+). Upon heating, LaSrMnNbO6 decomposes at 690 degrees C under O-2 flow or at 775 degrees C in air. The magnetic susceptibility data indicate the presence of long-range antiferromagnetic ordering at T-N = 8 K; the experimentally observed effective paramagnetic moment, mu(eff)=5.76 mu(B) for high spin Mn2+ (3d(5), S=5/2) is in good agreement with the calculated value (mu(calcd)=5.92 mu(B)). (C) 2010 Published by Elsevier Inc.