Magnetic Ordering and Structural Transition in the Ordered Double-Perovskite Pb2NiMoO6

The B-site-ordered double-perovskite Pb2NiMoO6 was prepared at high pressure and high temperature. The structural analysis of synchrotron powder X-ray diffraction data shows that Pb2NiMoO6 crystallizes into monoclinic symmetry with the space group Pc (no. 7), where the Ni and Mo ions are ordered in a rock-salt-type manner. The magnetic and specific heat characterizations reveal unusual two-step antiferromagnetic (AFM) transitions at 18 and 26 K for Pb2NiMoO6. The X-ray absorption spectra at the Ni-L-2,L-3 edge and the Mo-L-3 edge and the high-resolution partial fluorescence yield at the Pb-L-3 edge indicate Pb(2+)2Ni(2+)Mo(6+)O(6) valence states. Although in A(2)NiMoO(6) (A = Sr2+, Pb2+, and Ba2+), the size of the A cation increases gradually from Sr2+ (1.44 angstrom) to Pb2+ (1.49 angstrom) to Ba2+ (1.61 A), Pb2NiMoO6 exhibits much lower symmetry structure and AFM transition temperature, T-N, compared with Sr2NiMoO6 (I4/m, TN = 81 K) and Ba2NiMoO6 (Fm (3) over barm, T-N = 64 K), which is attributed to the large distortion of NiO6 and MoO6 octahedra induced by the lone pair electron effect of Pb2+ with a 6s(2) electronic configuration. Moreover, symmetry-breaking phase transition from a high-temperature centrosymmetric, cubic Fm (3) over barm phase to a low-temperature non-centrosymmetric, monoclinic Pc phase was observed at 393-413 K in Pb2NiMoO6.