Crystal structure and high temperature X-ray diffraction study of thermoelectric chimney-ladder FeGe_γ (γ ≈ 1.52)

The crystal structure of Nowotny chimney-ladder phase FeGe_γ with γ ≈ 1.52 was studied by X-ray diffractometry from 300 K to 850 K. The diffraction patterns were fitted by Rietveld refinement considering an incommensurate composite crystal structure. The refined crystal structure of FeGe_γ is described in details and compared to MnSi_γ, a thermoelectric material with γ ≈ 1.74. The lattice parameters a, c_Fe and c_Ge were found to increase with the temperature following a polynomial law, while the modulation vector component γ remained constant up to the peritectoid decomposition into FeGe and FeGe₂. Within the temperature range considered, the linear and volumetric thermal expansion parameters increased from about 3 × 10⁻⁶ K⁻¹ to 10 × 10⁻⁶ K⁻¹ and from 9.6 × 10⁻⁶ K⁻¹ to 31.5 × 10⁻⁶ K⁻¹, respectively.