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

Abstract

Tungsten subcarbide (W2C) 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 W2C including its disordered and different ordered phases have been often reported in the literature, and atomic-scale understanding of W2C 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 W2C up to a unit cell containing ten formula units, and the phase transition among five stable ordered W2C 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 W2C. The high-temperature disordered β-W2C with L′3-type lattice is more likely to be stabilized by configuration entropy. The stable ordered W2C 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 W2C structures with formation energies lower than those of β″-W2C and ε-W2C 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 W2C modifications. Our results bring a deep insight into the understanding of the stable W2C polymorphic structures and would be very helpful to identify the ordered structures of highly nonstoichiometric tungsten carbide and other transition metal carbides in experiments.

Memory Effect on the Synthesis of Perovskite-Type Li-Ion Conductor LixLa2/3–x/3TiO3 (LLTO)

“…Furthermore, CaCO 3 polymorphs can be selected and using different amorphous CaCO 3 precursors results in different crystalline polymorphs being formed . In addition, vacancy distribution in WC precursors results in different W 2 C polymorphs, indicating that the disorder in the precursors may affect the structure of the product.…”

Grain Boundary-Rich Tungsten Carbide Nanoparticle Films Exhibit High Intrinsic Activity Toward Hydrogen Evolution

“…For the intermediate carbon dose (WC2–WC4), the GIXRD (1.5°) patterns clearly demonstrated the formation of β-W 2 C, with peaks located at 34.6, 38, and 39.5° and indexed to the (100), (002), and (101) planes, respectively (Figures S12–S15). ,− Remarkably, under the fast carburization conditions of carbon ion implantation, β-W 2 C was synthesized in WC1–WC4, which previously has proven difficult to achieve. , However, a transition from W 2 C toward WC occurred at the highest carbon fluence (2.5 × 10 17 at. cm –2 , WC5), with new peaks at 36.5, 42.4, and 32.5° assigned to γ-WC(111), γ-WC(200), and δ-WC(001), respectively. ,,,, Clearly, at such high carbon concentrations, the formation energy for WC was significantly lowered, and a transition toward the thermodynamically favored WC was observed.…”

“… ,− Remarkably, under the fast carburization conditions of carbon ion implantation, β-W 2 C was synthesized in WC1–WC4, which previously has proven difficult to achieve. , However, a transition from W 2 C toward WC occurred at the highest carbon fluence (2.5 × 10 17 at. cm –2 , WC5), with new peaks at 36.5, 42.4, and 32.5° assigned to γ-WC(111), γ-WC(200), and δ-WC(001), respectively. ,,,, Clearly, at such high carbon concentrations, the formation energy for WC was significantly lowered, and a transition toward the thermodynamically favored WC was observed. Previous DFT calculations have shown that the FCC γ-WC phase predominantly forms smaller nanoparticles, compared to its hexagonal counterpart (δ-WC). , This suggested that the WC5 film contained WC nanoparticles with a range of sizes.…”

First-Principles Insights into Tungsten Semicarbide-Based Single-Atom Catalysts: Single-Atom Migration and Mechanisms in Oxygen Reduction

“…The resulting SACs, characterized by their nonsaturated surface, single-atom configuration, and varying W and/or C compositions, demonstrate distinctive responses under different reaction environments. − This makes them suitable models for investigating the structural evolution of catalysts and their reaction mechanisms. Previous studies have underscored the enhanced activity and stability of tungsten carbide-supported catalysts in oxygen reduction reaction (ORR) applications. , Notably, W 2 C, with its differing interstitial carbon atom count compared to that of WC, alters the symmetry of surface sites and atomic coordination, offering broader possibilities for structural modification. , Therefore, W 2 C-supported SACs are of significant interest, especially in catalyzing the ORR, for fundamental research into catalyst structural evolution and understanding the structure–activity relationship through systematic exploration. Our research is thus directed toward an in-depth investigation of the formation of the W 2 C substrate, the mechanisms of single-atom anchorage, the adaptability of the active phase under ORR conditions, and, ultimately, deciphering the fundamental principles governing the catalytic mechanism.…”