Tunable Negative Thermal Expansion in Fe/Cr‐Substituted Nd ₂ Co ₁₇ Compounds via Magnetoelastic Coupling

ABSTRACT Precisely tunable thermal expansion materials are essential for high‐precision applications, yet composition‐dependent bidirectional switching between positive and negative thermal expansion (NTE) poses a significant challenge. Here, we present a magnetoelastic approach to tailor anisotropic thermal expansion in Nd ₂ (Co _1‐x Fe _x ) _17‐y Cr _y compounds. Synchrotron X‐ray diffraction studies reveal that increasing Fe content induces a reversible lattice response: while Co‐rich compositions (e.g., Nd ₂ (Co _0.5 Fe _0.5 ) _13.7 Cr _3.3 ) display conventional positive expansion, Fe‐rich variants (e.g., Nd ₂ (Co _0.2 Fe _0.8 ) _14.7 Cr _2.3 ) exhibit pronounced uniaxial NTE along the c ‐axis ( α _c = −5.23 × 10 ⁻⁶ K ⁻¹ ) below Curie temperature ( T _C ). This transition originates from strong magnetoelastic coupling at specific crystallographic sites, where enhanced occupancy at the 6c Wyckoff position strengthens negative exchange interactions, constraining the c ‐axis via magnetostriction and inducing an anomalous thermal response with negative linear expansion. Compositional tuning not only suppresses the volume expansion coefficient (reducing α _V by 20% at x = 0.7), but also induces a systematic, non‐monotonic modulation of T _C (442–625 K). Critical exponent analyses near T _C align with mean‐field theory, confirming the dominant role of long‐range magnetic interactions. Our findings establish a new strategy for achieving zero or near‐zero thermal expansion in rare‐earth intermetallic, making these materials promising candidates for precision thermal management.