High pressure synthesis and properties of ternary titanium (III) fluorides in the system KF-TiF3 containing regular pentagonal bipyramids [TiF7]

Titanium trifluoride TiF₃ has the distorted ReO₃ structure composed of corner sharing TiF₆ octahedra linked with Ti-F-Ti bridges. Potassium fluoride KF was inserted into the bridges using high-pressure and high-temperature conditions (5 GPa, 1000-1200 °C). When the molar ratio KF/TiF₃≥1, a few low dimensional compounds were obtained forming non-bridged F ions. At the composition KF/TiF₃=1/2, a new compound KTi₂F₇ was formed, which crystallizes with the space group Cmmm and the lattice parameters of a=6.371(3), b=10.448(6), c=3.958(2) Å, consisting of edge-sharing pentagonal bipyramids TiF₇] forming ribbons running along the a axis. The ribbons are linked by corners to construct a three-dimensional framework without forming non-bridged F ions. The compound is antiferromagnetic with the Néel temperature T_N=75 K, and the optical band gap was 6.4 eV. A new fluoride K₂TiF₅ (KF/TiF₃=2) with the space group Pbcn and the lattice parameters of a=7.4626(2), b=12.9544(4) and c=20.6906(7) Å was also obtained by the high pressure and high temperature treatment (5 GPa at 1000 °C) of a molar mixture of 2 KF+TiF₃. The compound contains one-dimensional chains of corner-sharing TiF₆ octahedra.