High-pressure synthesis and crystal structure of the mixed-valent titanium borate Ti5B12O26

The new titanium borate was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1350 °C. Ti5B12O26 is built up exclusively from corner-sharing BO4-tetrahedra and shows a structural relation to the Zintl phase NaTl. Consisting of B12O26-clusters as fundamental building blocks, the structure of Ti5B12O26 can be described via two interpenetrating diamond structures as in NaTl, where each atom corresponds to one B12O26-cluster. The tetragonal titanium borate crystallizes with eight formula units in the space group I41/acd and exhibits lattice parameters of a=1121.1(2) pm and c=2211.5(4) pm. Ti5B12O26 is a mixed-valent compound with TiIII and TiIV cations. The environment of the titanium cations, as well as charge distribution calculations, leads us to the assumption that TiIII and TiIV are located on different crystallographic sites.