Ultrawide Temperature Range Super-Invar Behavior of R2(Fe,Co)17 Materials (

Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of ${R}_{2}{(\mathrm{Fe},\mathrm{Co})}_{17}$ materials ($R=\text{rare}$ Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3--461 K, almost twice the range of currently known SIV. In situ neutron diffraction, M\"ossbauer spectra and first-principles calculations reveal the $3d$ bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve ``ultrawide'' SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.