Phonon and thermodynamic properties of Al-Mn compounds: A first-principles study

Based on first-principles calculations within the projector augmented wave method and the generalized gradient approximation, the structural, thermodynamic as well as phonon properties of Al-Mn compounds have been investigated. The compounds studied include Al₁₂Mn, Al₆Mn, Al ₈Mn₅, Al₁₀Mn₃, Al ₁₁Mn₄ (low temperature phase), γ-AlMn, ε-AlMn, and τ-AlMn. Besides phonon calculations, Debye model is also used to evaluate the thermodynamic properties at elevated temperatures, which are compared with available data from experiments and thermodynamic modeling. A good agreement is found between first-principles calculations and experimental values or CALPHAD-type calculations. For Al-Mn compounds with lower Mn content, it is observed that (i) the equilibrium volume decreases roughly linearly while the bulk modulus increases roughly linearly with increasing Mn content, (ii) the bonding strength follows the trend of Al-Al > Al-Mn (and Mn-Mn) at a fixed bond length, and (iii) roughly the higher the Mn content of Al-Mn compounds, the smaller the vibrational contribution to entropy, and in turn, to Gibbs energy. The demonstrated methodology herein, as well as the predicted thermodynamic properties, provides helpful insights into the stability of phases and thermodynamic modeling, especially for system where the experimental information is lacking or less reliable.