Crystal Structure and Magnetic Properties of Li_2Mn_2(SO_4)_3

摘要

The crystal structure of dilithium dimanganese trisulfate Li_2Mn_2(SO_4)_3 has been established by single-crystal X-ray diffraction. This compound crystallizes in the orthorhombic system (space group Pbca, z = 8) with cell parameters a = 8.686(2) A, b = 8.792(2) A, and c = 24.146(4) A. The structure was refined to R = 0.035 and Rw = 0.037 using 2622 independent reflections. Lithium and sulfur atoms are in most regular tetrahedral oxygen coordination, manganese atoms are in slightly distorted octahedral oxygen coordination. Formally, the 3-D framework can be described, starting from the Li_2Mg_2(SO_4)_3 structure, by condensation of Sc_2(WO_4)_3-type slabs. As a result, half of the LiO_4 and MnO_6 polyhedra share an edge, forming Li_2O_6 and Mn_2O_(10) dimeric entities. Below T_N = 10 K, the magnetic susceptibility evolution is characteristic of a long-range antiferromagnetic order. In the high-temperature region, the susceptibility follows a Curie-Weiss law with C = 8.51 cm~3 K mol~(-1) and #theta# = -19 K. In a Mn_2O_(10) bioctahedron the Mn~(2+) cations are indeed antiferromagnetically coupled with an interaction parameter J = -6.5 cm~(-1).