Neutron powder diffraction of carbon-coated FeCo alloy nanoparticles

Neutron powder diffraction is used to study the order–disorder transformation in carbon-coated FexCo1−x nanoparticles produced using a radio frequency plasma torch. The nanoparticles, nominally Fe50Co50, are produced from alloy powder and acetylene precursors by gas-phase nucleation from the plasma. The resulting nanoparticles undergo an order–disorder transformation near 730 °C, passing from an ordered B2 (CsCl) structure to a disordered A1 (body-centered-cubic) structure upon heating, similar to the transformation seen in bulk equiatomic FeCo. Although it is very difficult to quench the disordered state in bulk samples, the extreme cooling rates present in the plasma reactor produce metastable disordered nanoparticles. Neutron powder diffractograms acquired during a heating–cooling cycle at 27, 500, 710, 800, 710, and 400 °C indicate the particles relax to their equilibrium ordered state upon heating first, disorder as they pass through the transformation temperature, and reorder upon cooling.