Abstract
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 Nb4AlC3–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 (Nb4AlC3–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 Nb4AlC3–x. Our findings may offer substantial insights into the roles of carbon vacancies in the structure stability and order-disorder phase transformation in MTCs.
Highlights
VC8, blue, red and purple balls denote the Nb, Al and C atoms
First-principles calculation is a powerful tool to investigate the point defects, crystal structure and properties of the MTCs7,15–19. It is frustrating for the phase stability of stoichiometric Nb4AlC3
Since Nb4AlC3 bears striking resemblance to V4AlC3, this puzzling and unsolved inconsistence necessitates the revisiting of the crystal structure of Nb4AlC3 with considerations of carbon vacancies
Summary
The grey squares denote the carbon vacancies. First-principles calculation is a powerful tool to investigate the point defects, crystal structure and properties of the MTCs7,15–19. It is frustrating for the phase stability of stoichiometric Nb4AlC3. Since Nb4AlC3 bears striking resemblance to V4AlC3, this puzzling and unsolved inconsistence necessitates the revisiting of the crystal structure of Nb4AlC3 with considerations of carbon vacancies. Under the guidance of the first-principles prediction, an ordered phase bearing structural carbon vacancies in Nb4AlC3–x (x ≈ 0.3), o-Nb4AlC3 (Nb12Al3C8), is unambiguously identified in experiment, demonstrating the validity to investigate the carbon vacancies in the MTCs with the combination of first-principles calculations, electron diffractometry and Raman spectroscopy
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