Formation of the incommensurate ordered phase in TaCy carbide

Abstract

Structural neutron diffraction studies indicate that only one ordered phase arises after the disorder-order transition in nonstoichiometric cubic tantalum carbide TaCy. 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 [1–11]B1 direction, but it is close to commensurate M6C5 structures (C2/m and P31 space groups) in mutual arrangement of atoms and vacancies in nonmetallic (1–11)B1 planes. The disorder-order transition channel that is associated with the formation of the incommensurate superstructure in TaCy carbide includes two arms k5(6) ≈ 0.473b2 and k5(5) = −k5(6) of the {k5} star and arms of the {k4} and {k3} stars. The translation period of the incommensurate phase in the [1–11]B1 direction is 8.9–9.1 nm, which is larger than that in the commensurate phase M6C5 by a factor of about 18.