Thermal expansion and crystal structure of cementite, Fe₃C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

Abstract

The cementite phase of Fe 3 C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic ( Pnma ) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with T c ≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above T c was found to be 4.1 (1) × 10 −5 K −1 ; below T c it is considerably lower (< 1.8 × 10 −5 K −1 ) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third-order Grüneisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-field theory.