Extended Data Fig. 4: High-temperature X-ray powder diffraction of BiTeI.

(a) Lattice constants and the cell volume as a function of temperature upon heating up to 550°C. Dashed black curves are first derivatives of corresponding lattice parameters, which show clear anomalies at 460°C. (b) Selected X-ray diffraction spectra of BiTeI near the transition temperature upon heating, demonstrating continuous and smooth shifts of X-ray peaks. (c) (100) X-ray diffraction peak of BiTeI at selected temperatures upon heating with no signs of phase coexistence near the transition temperature. The continuous evolution of lattice parameters without abrupt jumps and signs of phase coexistence near the transition unambiguously confirms the second-order nature of this phase transition.