Studies on the crystal structure, magnetic and conductivity properties of titanium oxycarbide solid solution (TiO_1-xC_x)

摘要

Titanium oxides and carbides are often considered as electrode materials in energy conversion and storage devices due to their high potential conductivity and good stability. Titanium monoxide and titanium carbide have structures that can both be described as rocksalt with the same cubic close packed titanium sublattice with oxygen and carbon, respectively, occupying the octahedral interstices; however, the oxide is characterised by extensive defects on both sublattices whilst the carbide is stoichiometric and might be considered as an interstitial metal. Despite the anticipated very different natures of the oxide and carbide sublattices, these two phases actually form a complete solid solution. In the present investigation, we carefully characterise this titanium oxycarbide solid solution, reporting on the crystal structure, magnetic and electronic conduction properties. Titanium oxycarbide powders (TiOC with x = 0 ≤ x ≤ 1) have been prepared by solid state reactions of TiO and TiC powder under controlled environments at elevated temperatures. X-ray diffraction and pycnometric density measurements illustrate the gradual transition of the crystal structure of titanium oxycarbides from a vacancy containing rock-salt structure of TiO to fully occupied TiC with increase in carbon content in the oxycarbide lattice. The variation of the lattice parameter of the oxycarbide crystal as a function of the carbon content has been found to be non-linear which can be attributed to variations in the level of vacancies present in metal as well as non-metal sublattices. The existence of a short-range ordering of anion vacancies in oxycarbide with a nominal composition of TiOC where half of oxygen of TiO is replaced by carbon has been confirmed by selected-area electron diffraction studies. Low temperature magnetic and conductivity measurements confirm that all oxycarbide compositions are Pauli paramagnetic and good metallic conductors.