Origin of the charge density wave state inBaFe2⁢Al9

Recently, a first-order phase transition associated with charge density wave (CDW) has been observed at low temperatures in intermetallic compound BaFe2⁢Al9. However, this transition is absent in its isostructural sister compound BaCo2⁢Al9. Consequently, an intriguing question arises as to the underlying factors that differentiate BaFe2⁢Al9 from BaCo2⁢Al9 and drive the CDW transition in BaFe2⁢Al9. Here, we set out to address this question by conducting a comparative ab initio study of the electronic structures, lattice dynamics, and electron-phonon interactions of their high-temperature phases. We find that both compounds are dynamically stable with similar phonon dispersions. The electronic structure calculations reveal that both compounds are nonmagnetic metals; however, they exhibit distinct band structures around the Fermi level. In particular, BaFe2⁢Al9 exhibits a higher density of states at the Fermi level with dominant partially filled Fe-3⁢𝑑 states and a more intricate Fermi surface. This leads to an electronic instability of BaFe2⁢Al9 toward the CDW transition, which is manifested by the diverged electronic susceptibility at the CDW wave vector 𝐪CDW=(0.5,0,0.3), observable in both the real and imaginary parts. Conversely, BaCo2⁢Al9 does not display such behavior, aligning well with experimental observations. Although the electron-phonon interactions in BaFe2⁢Al9 surpass those in BaCo2⁢Al9 by two orders of magnitude, the strength is relatively weak at the CDW wave vector, suggesting that the CDW in BaFe2⁢Al9 is primarily driven by electronic factors.