M 6 X 5 Superstructures in Ordered Niobium Carbide NbC 0.83]]>

摘要

A new model of defect structure for ordered niobium carbide is proposed. The model is a superimposure of two partially disordered M ~(6) X ~(5)superstructures. The first of these is an energetically advantageous low-temperature ordered phase (spatial group C 2), while the other is an auxiliary superstructure (space group C 2/ m ) that serves to increase the contribution from configurational entropy to the free energy as the temperature rises. The superimposure of superstructures is due to an incomplete order–order phase transition and explains the reasons for the discrepancy between the experimental and calculated intensities of superstructure reflexes in the neutron diffraction patterns of the ordered phase.