The crystal structure of Nowotny chimney-ladder phase FeGeγ 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 FeGeγ is described in details and compared to MnSiγ, a thermoelectric material with γ ≈ 1.74. The lattice parameters a, cFe and cGe 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.
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