Lone-pair self-containment in pyritohedron-shaped closed cavities: optimized hydrothermal synthesis, structure, magnetism and lattice thermal conductivity of Co15F2(TeO3)14

Abstract

A new oxofluoride Co₁₅F₂(TeO₃)₁₄ has been prepared by optimized hydrothermal synthesis involving a 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]²⁸⁻ units. This special framework provides a much larger free space that allows Te atoms to vibrate with a 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 honeycomb-like spin network was carefully analyzed by experimental techniques and first principles calculations.