Emergent Negative Thermal Expansion in Amorphous Fe-Y-Zr-B Alloys

Negative thermal expansion (NTE) is an unusual yet highly useful phenomenon that has been extensively studied in numerous crystals, including ceramics, alloys, and metal-organic frameworks. This Letter reports an unprecedented NTE in an amorphous Fe87.5Y₃Zr1.5B₈ alloy that lacks a periodic atomic arrangement. Such an NTE is significant for metallic materials and extends over a wide temperature range (α₁=-6.9×10⁻⁶ K⁻¹, 200-375 K). We demonstrate that this NTE is intrinsic to the amorphous nature of the alloy and is correlated with Fe moment. Extended x-ray absorption fine structure reveals a strong NTE for nearest neighboring Fe-Fe pairs. Further analysis using a x-ray pair distribution function indicates that the amorphous Fe87.5Y₃Zr1.5B₈ alloy, serving as a transition state, exhibits a tendency toward local ordered atomic arrangement. A complex interplay among local structure, magnetic interaction, and thermal relaxation results in volume contraction upon heating below TC. This Letter introduces amorphous alloys as a new family of materials with NTE functionality, offering interesting prospects for both scientific research and practical applications.