Negative and near-zero thermal expansion driven by cooperative Jahn–Teller effect in Fe2P2O7

Fe2P2O7 is a multifunctional material and has potential applications in a variety of fields but there exist controversies in the postulated space groups and studies on its phase transition and thermal expansion properties are still lacking. High-resolution synchrotron radiation x-ray diffraction, first-principles calculations, and specific heat capacity analyses are applied to solve these problems. The results reveal that Fe2P2O7 crystallizes in a triclinic structure with the C1¯ space group at low temperatures (α phase) and undergoes successive phase transitions to an intermediate phase (α′) at about 346 K and a monoclinic structure with the B21/c space group (β phase) at about 456 K. Fe2P2O7 exhibits a low, giant negative, and near-zero thermal expansion in the regions of 100–325, 325–375, and 375–475 K, respectively. The Jahn–Teller effect of Fe2+ ions and their transition from the static to dynamic one by thermal activation are proposed to account for the unique thermal expansion/contraction properties.