Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets

The design of zero thermal expansion (ZTE) materials is urgently required as dimension-stable components in widespread modern high-precision technologies. Local chemical order has been of great importance in engineering advanced inorganic materials, but its role in optimizing the ZTE is often overlooked. Herein, we propose local composition heterogeneity for developing superior ZTE via a nonstoichiometric strategy. A remarkably low coefficient of thermal expansion of α = +1.07 × 10^-6 K^-1 is achieved from 3 to 440 K in a quaternary Zr-Nb-Fe-Co pyrochlore magnet, which is the widest temperature range among known cubic ZTE metals. High-resolution synchrotron X-ray diffraction and magnetization measurements reveal that all the Bragg peaks split as resulting from two cubic phases with different magnetic orders. Scanning transmission electron microscopy, Mössbauer spectroscopy and theoretical calculations indicate that such phase separation intimately derives from excess Co dopant preferentially clustering on the Fe pyrochlore-lattice (16d) and simultaneously yielding an antisite Fe on Zr/Nb sublattice (8a). The Co content in pyrochlore-lattice has weaker exchange interactions than that of Fe, but the antisite Fe introduces extra positive exchange interactions between 8a and 16d sites. Local composition fluctuation of Co and Fe thus affects the interplanar ferromagnetic order of pyrochlore-lattice and balances the normal phonon effect successively on heating. Superior corrosion resistance to both acid and alkaline conditions merits potential applications of the present ZTE metal.