Reaction of titanium with carbon in a laser heated diamond anvil cell and reevaluation of a proposed pressure-induced structural phase transition of TiC

The formation of cubic TiC_xfrom the elements was studied in a laser-heated diamond anvil cell at pressures up to 26 GPa. Annealed samples at these pressures show no splitting of the cubic (1 1 1) or (2 2 2) reflection. This is in contrast to an earlier study, in which a splitting of the (1 1 1) reflection was observed above 18 GPa. The recovered sample had a lattice parameter of 4.3238(6) Å, which implies that the synthesis gave fully stoichiometric TiC. Density functional theory-based model calculations were used to study the dependence of the total energy of a rhombohedral distortion. In these model calculations the total energy was minimal for the undistorted (cubic) lattice. Therefore, the results obtained here imply that at least for titanium carbides with a high carbon content no pressure-induced structural phase transition up to at least 26 GPa occurs. The appearance of a trigonal TiC_x polymorph during the synthesis is discussed in terms of its relative stability with respect to the cubic phase.