Abstract
In the first part of this paper [Acta metall.36, 1891 (1988)], we have presented the experimental background for a high temperature neutron powder diffraction study of transition metal hemicarbides M2C1−x having various compositions. In this second part, we report the results: in Mo2C1−x, a first order transformation between an orthorhombic ζ-Fe2N-type superstructure and a hexagonal ϵ-Fe2N-type one has been identified in a middle temperature range (1100–1400°C); in the case of W2C1−x, the same ϵ-Fe2N-type phase appears to be the major constituent in our powders. In both carbides, the disordering of this ϵ-phase occurs through a second order transition at elevated temperatures. The ζ-Fe2N-type phase is also observed as a minor constituent of a W2C0.91 powder; a consistent discussion of these results points out a general scheme for the structural evolution of these M2C1−x compounds as a function of the temperature; this conclusion is also correlated to the refinement of the ζ and ϵ-type structures which has been performed at room temperature.
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