Journal of Applied Crystallography

Volume 37, Part 1 (February 2004)

J. Appl. Cryst. (2004). 37, 82-90    [doi:10.1107/S0021889803024695]

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

I. G. Wood, L. Vocadlo, K. S. Knight, D. P. Dobson, W. G. Marshall, G. D. Price and J. Brodholt

Abstract: The cementite phase of Fe₃C 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.

Online 17 January 2004

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