High-Temperature Zero Thermal Expansion in HfFe_2+δ from Added Ferromagnetic Paths

Deformation caused due to the thermal expansion of a material at high temperatures impairs the functioning of the device. Hence, high-temperature zero thermal expansion (ZTE) compounds are widely used in many high-precision devices. However, the domination of magnetic behavior over the thermal expansion of magnetic compounds makes it difficult to display ZTE at high temperatures. Herein, we report a high-temperature ZTE in a Fe-rich HfFe_2+δ compound, whose ZTE operating temperature could reach 583 K, the highest temperature reached by ZTE metal-based compounds. Synchrotron X-ray diffractometry (SXRD), neutron powder diffractometry, Mössbauer spectroscopy, first-principle calculations, and macroscopic magnetic measurements revealed that the additional Fe atoms occupy the Hf sites and introduced extra ferromagnetic exchange interaction paths with the neighboring Fe atoms, thereby enhancing the magnetic transition temperature and the ZTE temperature region. Moreover, it was experimentally shown that the generation of ZTE by HfFe_2.5 was due to the mutual cancellation of lattice shrinkage caused by the transformation of magnetic moments of Fe from ordered to disordered state and lattice expansion caused by lattice vibration. This study not only reports a high-temperature ZTE material but also provides an unusual method to modulate the magnetic systems to obtain high-temperature ZTE compounds.