A new ordered triple Hollandite: Ba1.33Sb2.66Al5.33O16

Ba_1.33Sb_2.66Al_5.33O₁₆ is a triple Hollandite, which crystallizes in a tetragonal cell (I4/m no. 87) with a=b=9.86090(5) Å and c=8.77612(6) Å. Its crystal structure was characterized using electron diffraction and X-ray powder diffraction; it is isotypic to K_1.33Mg_3.11Sb_4.89O₁₆, K_1.76Mg_3.25Sb_4.75O₁₆ and to K_1.8Li_2.45Sb_5.55O₁₆. In the rutile chains of Ba_1.33Sb_2.66Al_5.33O₁₆, the ordering of Al and Sb atoms into unmixed sites induces the tripling of the c parameter compared to a ‘single’ Hollandite structure. The Ba²⁺ cations are dispersed along c, in the largest tunnels on non-split and fully occupied sites. They lie into Ba-Ba pairs separated by vacancies. Their regular arrangement has been confirmed by high resolution electron microscopy. Electrochemical experiments have also been performed in Li-ion cell but no Li insertion was detected.