Lone-pair self-containment in pyritohedron-shaped closed cavities: optimized hydrothermal synthesis, structure, magnetism and lattice thermal conductivity of Co 15 F 2 (TeO 3 ) 14

International audienceThe new oxofluoride Co15F2(TeO3)14 has been prepared by optimized hydrothermal synthesis involving complex mineralization process. The crystal structure consists of a three-dimensional network of CoO5(O,F) octahedra, distorted CoO5 square pyramids, TeO3 trigonal pyramids and grossly distorted TeO3+3 octahedra, which are linked by sharing corners and edges. The Te(IV) lone pairs are accommodated within novel pyritohedron-shaped [(TeO3)14] 28units. This special framework provides a much larger free space that allows Te atoms to vibrate with large amplitude, which leads to extremely low lattice thermal conductivity. Magnetic susceptibility data for Co15F2(TeO3)14 show antiferromagnetic ordering below 9.6 K with a substantial orbital component to the effective magnetic moment. An S=3/2 honeycomblike spin network was carefully analyzed by experimental techniques and first principles calculation