Toward a Unified Understanding of W₂C Polymorphic Structures by First-Principles Calculations

Abstract

Tungsten subcarbide (W 2 C) is widely applied to industrial catalysts, military industries, and aerospace facilities because it possesses excellent high-temperature performance and superior mechanical properties. However, contradictory data on the crystal structure of W 2 C including its disordered and different ordered phases have been often reported in the literature, and atomic-scale understanding of W 2 C polymorphic structures has not yet reached a consensus. Based on the L ′3-type lattice, we have performed first-principles calculations to study the stability of L ′3-WC, the interaction of dilute carbon vacancies in L ′3-WC, the stable ordered structures of W 2 C up to a unit cell containing ten formula units, and the phase transition among five stable ordered W 2 C structures. Our results indicate that carbon vacancies in L ′3-WC can exhibit an attractive interaction, which provides an essential driving force to stabilize the ordered structures of W 2 C. The high-temperature disordered β-W 2 C with L ′3-type lattice is more likely to be stabilized by configuration entropy. The stable ordered W 2 C structures are all extended in the ab plane of the L ′3-type lattice rather than along the c axis and possess some specific distribution patterns of aggregate carbon vacancies. New ordered W 2 C structures with formation energies lower than those of β″-W 2 C and ε-W 2 C are found to be mechanically and dynamically stable. Carbon atom migration between the interlayers of the L ′3-type lattice via a sequential mechanism is an energetically favorable pathway for the phase transition among different W 2 C modifications. Our results bring a deep insight into the understanding of the stable W 2 C polymorphic structures and would be very helpful to identify the ordered structures of highly nonstoichiometric tungsten carbide and other transition metal carbides in experiments.

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