A novel titanium oxycarbide phase with metal-vacancy (Ti1-yCxO1-x): Structural and thermodynamic basis

A unique phenomenon of TiC–TiO 2 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 n O 2 n -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 x O 1- x ) equilibrated with Magnéli phase at 900 °C is around 0.66, typically 0.651 for Ti 4 O 7 . The obtained titanium oxycarbide phase contains metal-vacancy in Ti site, as Ti 1- y C x O 1- x which is reported the first time, differing from stoichiometric vacancy of both Ti site and C/O site in conventional TiC x O 1- x . The dominating part of TiC–TiO 2 reaction at 900 °C is reaction via solid solution as directly introducing the Ti and O atoms of TiO 2 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- y C x O 1- x is 18%, close to the theoretical capacity of 18.9% which exists when the starting molar ratio of TiC and TiO 2 is aiming to the formation of TiC 0.651 O 0.349 , as the critical composition equilibrated with Ti 4 O 7 . The stability of metal-vacancy in titanium oxycarbide is verified by first-principles calculation in the perspective of energy.