Structure–Thermoelectric Property Relations of the Nowotny Chimney–Ladder Phase Fe2Ge3

Abstract

The Nowotny chimney–ladder (NCL) phase in the Fe–Ge system near Fe2Ge3 composition is a promising thermoelectric material owing to its characteristic coexistence of degenerate electronic transport and strongly suppressed lattice thermal conductivity, yet the structural and compositional origins of this behavior remain incompletely understood. Synchrotron diffraction shows that samples prepared with nominal Fe2Ge3 composition crystallize as an incommensurately modulated NCL phase close to FeGe1.52. From the superspace description, we derive and refine for the first time a three-dimensional commensurate approximant, Fe27Ge41 (tP272). Chemical-bonding analysis based on the model of Fe2Ge3 reveals a three-dimensional framework of heteroatomic multiatomic Fe–Ge interactions without homoatomic Fe–Fe or Ge–Ge bonds. DFT calculations for the Fe27Ge41 approximant show that slight Ge excess in comparison with Fe2Ge3 shifts the Fermi level into the conduction band, yielding a nominal carrier concentration of ∼1021 cm–3, consistent with Hall measurements, and explaining the degenerate n-type behavior without extrinsic donors. The electronic structure is characterized by a high density-of-states effective mass (∼11 me), arising from convergence of flat conduction bands near the Fermi level. Despite high elastic moduli and a moderate Grüneisen parameter, the lattice thermal conductivity is low (∼1.8 W m–1 K–1 at 300 K), corresponding to an estimated phonon mean free path of approximately 8 Å. The failure of the Debye–Callaway model to reproduce this value indicates additional phonon scattering, which may be related to the incommensurate structure and bonding inhomogeneity. Overall, the incommensurate Fe–Ge NCL phase combines degenerate electronic transport with phonon-glass-like thermal behavior.