Effect of density functionals on the vibrational and thermodynamic properties of Fe2VAl and Fe2TiSn compounds

Phonon calculations along with first-principles Kohn-Sham density functional theory (DFT) is an essential tool to study the lattice dynamics, thermodynamical properties and phase-transitions of materials. The two full-Heusler compounds Fe2VAl and Fe2TiSn are studied for lattice vibration dependent properties using finite displacement method and supercell approach. For the investigation, four exchange-correlation functionals viz., LDA, PBE, PBEsol and meta-GGA SCAN are employed. Using these functionals, phonon dispersion, phonon density of states (DOS), partial density of states (PDOS), thermal properties and zero-point energy are calculated at equilibrium lattice parameters under harmonic approximation. For the two compounds the Debye temperatures corresponding to average maximum frequency phonons are calculated from the obtained phonon DOS which are ∼649 K and ∼510 K, respectively. The obtained results from different functionals are compared among each other. The overall phonon energy in the dispersion is found to be ∼15 meV higher in Fe2VAl than the Fe2TiSn compound. For the two compounds PBE is yielding the lowest phonon frequencies while LDA or SCAN functional is giving the highest. The same behavior is observed in phonon DOS plots of two compounds. The zero-point energy calculated is the highest from SCAN (21.04 and 16.95 kJ/mol) and the lowest from PBE functionals (20.09 and 16.02 kJ/mol) obeying the same trend as frequency for both compounds. A general prediction of nature of lattice thermal conductivity is made based on the characteristics of acoustic phonon dispersion curves which is in agreement with the qualitative behavior of reported experimental thermal conductivity of two compounds. Phonon spectra obtained from PBE and SCAN have similar general features while those from LDA and PBEsol have resembling features for Fe2VAl, while this trend is not observed for the case of the compound Fe2TiSn.