Order-Disorder Phase Transition Channel in Niobium Carbide

Using a neutron diffraction structure analysis method it is found that an order-disorder phase transition of the first kind occurs in a nonstoichiometric niobium carbide NbCy at a temperature of ≈ 1300 K in the composition range NbC0,81-NbC0,88. The phase transition channel which incorporates five nonequivalent superstructural vectors belonging to three stars is determined. A carbon atom distribution function is obtained which depends on three one-component order parameters.