Abstract
The new compounds Mn4(TeO3)(SiO4)X2 (X=Br, Cl) were synthesized by solid state reactions in sealed evacuated silica tubes. The compounds crystallize in the monoclinic space group P21/m with the unit cell parameters a=5.5463(3)Å (5.49434(7)Å), b=6.4893(4)Å (6.44184(9)Å), c=12.8709(7)Å (12.60451(18)Å), β=93.559(5)° (94.1590(12)°) and Z=2 for the respective Br and Cl analogues. Manganese adopts various distorted coordination polyhedra; [MnO6] octahedra, [MnO5] tetragonal pyramids and [MnO2X2] tetrahedra. Other building blocks are [SiO4] tetrahedra and [TeO3] trigonal pyramids. The structure is made up from layers having no net charge that are connected via weak Van der Waal interactions. The layers that are parallel to (110) consist of two manganese oxide sheets which are separated by [SiO4] tetrahedra. On the outer sides of the sheets are the [MnO2X2] tetrahedra and the [TeO3] trigonal pyramids connected so that the halide ions and the stereochemically active lone pairs on the tellurium atoms protrude from the layers. Magnetic susceptibility measurements reveal a Curie law with a Weiss temperature of θ=−153(3)K for temperatures ≥100K and indicate antiferromagnetic ordering at TN ~4K. Possible structural origins of the large frustration parameter of f=38 are discussed.
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