Coupling of lattice dynamics and configurational disorder in metal deficient Al1−δB2 from first-principles

• Published in: Journal of applied physics Vol. 130; no. 1
• Main Authors: Johansson, Erik, Eriksson, Fredrik, Ektarawong, Annop, Rosen, Johanna, Alling, Björn
• Format: Journal Article
• Language: English
• Published: 07.07.2021
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/5.0047275

We investigate the role metal vacancies play in the phase stability and properties of Al 1 − δB 2 using first-principles calculations, alloy theory simulations, phonon calculations, and experimental in situ x-ray diffraction measurements of thermal expansion. The relevant concentrations and configurations of metal vacancies are analyzed using cluster expansion and special quasirandom structure methods combined with vibrational free energy calculations within the quasiharmonic approximation for ordered and disordered phases. We find that electronic structure effects stabilize Al 1 − δB 2 in a narrow composition range of 0.073 ≤ δ ≤ 0.096 depending only weakly on temperature while they destabilize ideal stoichiometric AlB 2. This composition corresponds to the narrow range 0.311 ≤ x ≤ 0.317 in the formulation of Al xB 1 − x, which can explain the appearance of this phase as a line compound with the ideal AlB 2 stoichiometry in most phase diagrams. The ordered structures of vacancies found at low temperature are destabilized and disordered already at a low temperature of ∼ 200 K. Our experiments observe linear thermal expansion coefficients α a = 4.8 × 10 − 6 K − 1 and α c = 10.85 × 10 − 6 K − 1 at room temperature. Only thermal expansion calculations of disordered phases compare well with these measurements, and, in particular, stoichiometric vacancy-free AlB 2 shows dramatic overestimations of the experimental thermal expansion. These results highlight the importance of disordered Al vacancies on both stability and vibrational properties of Al 1 − δB 2.