NEUTRON POWDER DIFFRACTION STUDIES OF TRANSITION-METAL HEMICARBIDES M2C1-X .2. INSITU HIGH-TEMPERATURE STUDY ON W2C1-X AND MO2C1-X .69. OVERVIEW

In the first part of this paper the authors 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, the authors report the results: in Mo2C1-x, a first order transformation between an orthorhombic ζ-Fe2N-type superstructure and a hexagonal ΕFe2N-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 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.