Coupling of lattice dynamics and configurational disorder in metal deficient Al1-delta B2 from first-principles

• We investigate the role metal vacancies play in the phase stability and properties of Al1-delta B2 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 Al1-delta B2 in a narrow composition range of 0.073 < delta < 0.096 depending only weakly on temperature while they destabilize ideal stoichiometric AlB2. This composition corresponds to the narrow range 0:311 <= x <= 0.317 in the formulation of AlxB1-x, which can explain the appearance of this phase as a line compound with the ideal AlB2 stoichiometry in most phase diagrams. The ordered structures of vacancies found at low temperature are destabilized and disordered already at a low temperature of similar to 200 K. Our experiments observe linear thermal expansion coefficients alpha(a) = 4.8 x 10(-6) K-1 and alpha(c) = 10.85 x 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 AlB2 shows dramatic overestimations of the experimental thermal expansion. These results highlight the importance of disordered Al vacancies on both stability and vibrational properties of Al1-delta B2. (c) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Ämnesord

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

Publikations- och innehållstyp

ref (ämneskategori)

art (ämneskategori)