Layered polymorph of titanium triiodide

A previously unreported layered triangular lattice polymorph of TiI3 is described, synthesized under 6 GPa of applied pressure at 900°C, but stable at atmospheric pressure. This air-sensitive material has a CdI2-type layered structure [P-3m1 (no. 164), a=4.012Å and c=6.641Å at 120 K, Z=1 of Ti0.667I2] with an in-plane triangular lattice, related to that of TiI4 (Ti0.5I2). Although the TiI3 formula is consistent with expectations for a layered honeycomb lattice of spin 1/2 Ti(III), there is disorder in the crystal structure, and the complex magnetic behavior observed is inconsistent with a simple local moment picture of spin 1/2 per Ti(III). Magnetic susceptibility and heat capacity measurements suggest that the material undergoes several low-temperature phase transitions.