Table S1. The calculated relative energy of Fe2P2O7 in different phases. FIG. S1 (a) Temperature dependent SXRD patterns of Fe2P2O7 from 100 to 700 K, (b) The low temperature unit cell structure with the C1 ̅ space group. Fig. S2 XPS spectra of Fe2P2O7 (a) and Fe 2p (b), P 2p (c) and O 1s (d). FIG. S3 (a) The ∠O-Fe-O bond angles and (b) the O-O distance change with temperature; (c) Unit cell structure in which the movements of O atoms with increasing temperature are marked by arrows; (d) The relationship of the mean octahedral quadratic elongation against the σ^2, which deviate seriously from the linear relationship above 325 K. FIG. S4 The balance of the distortion index D and octahedral quadratic elongation with temperature, which can qualitatively reproduce the trends of the thermal expansion of Fe2P2O7 below 475 K; (b) The Fe-O-P bond angle changes with temperature.