Discovery of carbon-vacancy ordering in Nb4AlC3-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 Nb₄AlC_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-Nb₄AlC₃, crystalizes in P6₃/mcm with a = 5.423 Å, c = 24.146 Å. Coexistence of ordered (o-Nb₄AlC₃) and disordered (Nb₄AlC_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-Nb₄AlC₃ to Nb₄AlC_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. [ABSTRACT FROM AUTHOR]