Structural phase transition between γ-Ti₃O₅ and δ-Ti₃O₅ by breaking of a one-dimensionally conducting pathway

The phase transition between gamma-trititanium-pentoxide (γ-Ti₃O₅) and delta-trititanium-pentoxide (δ-Ti₃O₅) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic I2/c crystal structure of γ-Ti₃O₅ was found to switch to a monoclinic P2/a crystal structure of δ-Ti₃O₅ due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti₃O₅ behaves like a metallic conductor with a σ value of 4.7 S cm^-1 at 320 K, while δ-Ti₃O₅ shows a semiconductive property with a σ value of 2.5 × 10^-5 S cm^-1 at 70 K. Optical measurement also supports that γ-Ti₃O₅ is a metallic conductor, while δ-Ti₃O₅ is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti₃O₅ is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with one-dimensional linkage along the crystallographic c-axis. On the contrary, δ-Ti₃O₅ has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti₃O₅ and δ-Ti₃O₅ is caused by breaking of a one-dimensionally conducting pathway due to a Mott-Hubbard metal-insulator phase transition.