Structure, Properties, and Disorder in the New Distorted-Hollandite PbIr4Se8

The synthesis and physical properties of the new distorted-Hollandite PbIr4Se8 are reported. Powder X-ray diffraction and transmission electron microscopy show that the structure consists of edge- and corner-sharing IrSe6 octahedra, with one-dimensional channels occupied by Pb. The structure contains Se-Se anion-anion bonding, leading to an electron count of Pb2+(Ir3+)4(Se2)2-(Se2-)6, confirmed by bond-valence sums and diamagnetic behavior. Structural and heat capacity measurements demonstrate disorder on the Pb site, due to the combination of lone-pair effects and the large size of the one-dimensional channels. Comparisons are made to known Hollandite and pseudo-Hollandite structures, which demonstrates that the anion-anion bonding in PbIr4Se8 distorts its structure, to accommodate the Ir3+ state. An electronic structure calculation indicates semiconductor character with a band gap of 0.76(11) eV.