Discovery of carbon-vacancy ordering in Nb(4)AlC(3–x) under the guidance of first-principles calculations

The conventional wisdom to tailor the properties of binary transition metal carbides by order-disorder phase transformation has been inapplicable for the machinable ternary carbides (MTCs) due to the absence of ordered phase in bulk sample. Here, the presence of an ordered phase with structural carbon vacancies in Nb(4)AlC(3–x) (x ≈ 0.3) ternary carbide is predicted by first-principles calculations, and experimentally identified for the first time by transmission electron microscopy and micro-Raman spectroscopy. Consistent with the first-principles prediction, the ordered phase, o-Nb(4)AlC(3), crystalizes in P6(3)/mcm with a = 5.423 Å, c = 24.146 Å. Coexistence of ordered (o-Nb(4)AlC(3)) and disordered (Nb(4)AlC(3–x)) phase brings about abundant domains with irregular shape in the bulk sample. Both heating and electron irradiation can induce the transformation from o-Nb(4)AlC(3) to Nb(4)AlC(3–x). Our findings may offer substantial insights into the roles of carbon vacancies in the structure stability and order-disorder phase transformation in MTCs.