Al27 NMR insight into the phase transition in BaFe2Al9

We have applied $²⁷\mathrm{Al}$ nuclear magnetic resonance (NMR) spectroscopy to investigate the iron-based aluminide of $\mathrm{Ba}{\mathrm{Fe}}₂{\mathrm{Al}}₉$. This material has been a subject of current interest due to indications of charge density wave behavior below the transition temperature ${T}_C\ensuremath{\simeq}100$ K. Two sets of the $²⁷\mathrm{Al}$ NMR resonance lines that are associated with two nonequivalent crystallographic aluminum sites have been well resolved. We have discussed the obtained electric field gradient and anisotropic Knight shift for each individual aluminum site, revealing the strong $ab$ plane bonding configuration for the structural properties of $\mathrm{Ba}{\mathrm{Fe}}₂{\mathrm{Al}}₉$. Pronounced features in the isotropic Knight shift and nuclear spin-lattice relaxation rate have been observed in the vicinity of ${T}_C$. Furthermore, the detailed $²⁷\mathrm{Al}$ NMR analyses have provided evidence for the decrease of the electronic density of states upon lowering temperature below ${T}_C$.