Structural chemistry of magnéli phases TinO2n−1 (4 ≤ n ≤ 9): IV. Superstructure in Ti4O7 at 140 K

The structure of Ti₄O₇ was reexamined at 298, 140, and 115 K. The room-temperature and the 115 K structures are essentially as previously reported. Two new observations were made: a considerable cell-parameter change at the lower transition with a 0.3% increase in cell volume on cooling and the existence of a fivefold superstructure at 140 K only. The 140 K phase with cell parameters a = 6.918(1), b = 11.142(2), c = 15.127(3) α = 90.64(1), β = 92.79(1), γ = 91.45(1) in space group $\text{P}\text{1}$ corresponds to long-range order of the Ti valences. It shows alternate Ti³⁺ and Ti⁴⁺ slabs parallel to (1 0 $\text{2}$) and a few valences with intermediate values at the contact between the slabs with the overall appearance of a modulated structure. Pairing of Ti³⁺ ions is observed. Similar to Ti₆O₁₁, expansion and contraction of the TiTi distances in the rutile-like chains is correlated with the electrostatic repulsion of the Ti ions so that the 3⁺ slabs are on the whole contracted along c_rutile while the 4⁺ slabs are expanded. The previous interpretation of the 140 K phase as a “liquid of bipolarons” should therefore be revised.