Tunable Negative Thermal Expansion in Fe/Cr-Substituted Nd₂Co₁₇ Compounds via Magnetoelastic Coupling

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 Nd₂(Co_1-xFe_x)_17-yCr_y compounds. Synchrotron X-ray diffraction studies reveal that increasing Fe content induces a reversible lattice response: while Co-rich compositions (e.g., Nd₂(Co_0.5Fe_0.5)_13.7Cr_3.3) display conventional positive expansion, Fe-rich variants (e.g., Nd₂(Co_0.2Fe_0.8)_14.7Cr_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.