Formation of the incommensurate ordered phase in TaC$_y$ carbide

Structural neutron diffraction studies indicate that only one ordered phase arises after the disorder–order transition in nonstoichiometric cubic tantalum carbide TaC$_y$. This phase arises in the composition range $y=0.79{-}0.89$ due to long-term annealing with a decrease in temperature from 1600 to 300 K. It is incommensurate in the $[1{-}1\,1]_{B1}$ direction, but it is close to commensurate M$_6$C$_5$ structures ($C2/m$ and $P3_1$ space groups) in mutual arrangement of atoms and vacancies in nonmetallic $(1{-}1\,1)_{B1}$ planes. The disorder–order transition channel that is associated with the formation of the incommensurate superstructure in TaC$_y$ carbide includes two arms $\bf{k}_5^{(6)}\approx 0.473\bf b_2$ and $\bf {k}_5^{(5)}=-\bf{ k}_5^{(6)}$ of the $\{\bf{k}_5\}$ star and arms of the $\{\bf{k}_4\}$ and $\{\bf{k}_3\}$ stars. The translation period of the incommensurate phase in the $[1{-}1\,1]_{B1}$ direction is 8.9–9.1 nm, which is larger than that in the commensurate phase M$_6$C$_5$ by a factor of about 18.