Abstract Dielectric and ultrasound measurements of berthierite FeSb 2 S 4 reported in the literature revealed a phase transition at T c =348 K. In the present study the phase transition was confirmed by Mossbauer spectra exhibiting anomalous behavior close to 350 K. Precise lattice parameters measured in the temperature region 305–380 K show a change of slope at about 350 K. The crystal structure of FeSb 2 S 4 has been determined using single-crystal X-ray diffraction data at both room temperature and 370 K above the phase transition. The crystal structure is orthorhombic, Z =4. At 295 K a =11.412(2) A, b =3.763(1) A, c =14.161(3) A, and R =0.0280, and at 370 K a =11.421(2) A, b =3.771(1) A, c =14.173(3) A, and R =0.0274. No symmetry change has been observed. At both temperatures the space group of FeSb 2 S 4 is Pnma with small changes in atomic parameters and interatomic distances. The transition observed at 348 K is, therefore, to be classified as a weak phase transition going on without essential changes in the crystal structure.
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