A novel titanium oxycarbide phase with metal-vacancy (Ti_1-yC_xO_1-x): Structural and thermodynamic basis

A unique phenomenon of TiC–TiO₂ reaction at 900 °C is established, forming a quasi-pure phase of abnormal titanium oxycarbide, since it is commonly synthesized at temperatures above 1500 °C. Impurity of Magnéli phase (Ti_nO_2n-1, 4≤n≤9) is also detected in the product. Thermodynamics analysis indicates that the critical C/O concentration of titanium oxycarbide (x in TiC_xO_1-x) equilibrated with Magnéli phase at 900 °C is around 0.66, typically 0.651 for Ti₄O₇. The obtained titanium oxycarbide phase contains metal-vacancy in Ti site, as Ti_1-yC_xO_1-x which is reported the first time, differing from stoichiometric vacancy of both Ti site and C/O site in conventional TiC_xO_1-x. The dominating part of TiC–TiO₂ reaction at 900 °C is reaction via solid solution as directly introducing the Ti and O atoms of TiO₂ into the FCC structure of TiC, which leads to the formation of metal-vacancy without altering the original lattice size, instead maintaining at the same level of TiC regardless of the variation of corresponding C/O concentration. The obtained maximum concentration of metal-vacancy in Ti_1-yC_xO_1-x is 18%, close to the theoretical capacity of 18.9% which exists when the starting molar ratio of TiC and TiO₂ is aiming to the formation of TiC_0.651O_0.349, as the critical composition equilibrated with Ti₄O₇. The stability of metal-vacancy in titanium oxycarbide is verified by first-principles calculation in the perspective of energy.