Giant negative thermal expansion covering room temperature in nanocrystalline GaN_xMn₃

Nanocrystalline antiperovskite GaN_xMn₃ powders were prepared by mechanically milling. The micrograin GaN_xMn₃ exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50 K as the average grain size (⟨D⟩) is reduced to ∼30 nm. The corresponding coefficient of linear thermal expansion (α) reaches ∼ −70 ppm/K, which are comparable to those of giant NTE materials. Further reducing ⟨D⟩ to ∼10 nm, ΔT exceeds 100 K and α remains as large as −30 ppm/K (−21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling ⟨D⟩, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions.