Order-disorder transition and structure of the ordered vacancy compound Nb6C5: powder neutron diffraction studies

High-resolution neutron diffraction studies of NbC 0.83 powders are described. In a first set of experiments the order-disorder transition was studied by measuring the integrated Bragg intensities. A first-order transition is observed around 1025+or-5 degrees C on heating, which shows a hysteresis. In a second set of experiments, a profile refinement method was used to get information on the crystal structure of a well ordered Nb 6 C 5 sample. The V 6 C 5 -type trigonal superstructure previously proposed was confirmed (rather than the V 6 C 5 monoclinic form). A shift of the niobium atoms ( approximately=0.05 AA) away from their first-nearest-neighbour vacant site, towards a first-nearest-neighbour occupied site (trans to the former) is found.