Synthesis and Characterization of Ti⁴⁺ Containing Carbonates Ti[CO₄] and Ti₂O₃[CO₃]

Two titanium carbonates, Ti[CO₄] and Ti₂O₃[CO₃], were synthesized by the reaction of TiO₂ with CO₂ at high pressures and temperatures in a laser heated diamond anvil cell. Their structures were solved by in situ single crystal X-ray diffraction at high pressures. At pressures above ≈33 GPa, Ti[CO₄]-$I\overline{4}2d$ is stable. It transforms to Ti[CO₄]-I4₁/amd on pressure release. The distinguishing feature of the Ti[CO₄]-structures is the presence of isolated [CO₄]^4--groups, while Ti₂O₃[CO₃] has isolated [CO₃]^2--groups. Both structures contain 8-fold coordinated tetravalent Ti⁴⁺-cations. Full geometry optimizations based on DFT calculations reproduced the crystal structures. The DFT models were used to complement the experimental compression data and for the assignment of Raman modes. While carbonates are typically compressible compounds, Ti[CO₄] is rather incompressible with a bulk modulus of K₀ ≈ 200 GPa. As we observed the pressure-induced transformation of TiO₂, we provide the Raman spectra of TiO₂-OI, and TiO₂-OII.