Influence of Order-Disorder Transformations on the Electrical Resistivity of Vanadium Carbide

The electrical resistivity of single crystals of vanadium carbide has been studied as a function of composition and temperature. Particular attention has been devoted to the resistivity of the two ordered phases of vanadium carbide, V₆{C}₅$ and ${V}₈C₇. Both solids have been shown to undergo first-order transformations to the disordered state at elevated temperature. The results of this investigation have been used to assess the role of carbon vacancies in determining the resistivity of the disordered solids. Vacancy scattering accounts for over 90% of the residual resistivity (resistivity at liquid-He temperatures) of the disordered solids but reduces in importance as temperature increases. The change in the resistivity of V₆{C}₅$ at its critical point was found to be 5.4 ±{} 0.3 {μ}{Ω} cm or (3.6 ±{} 0.2)% of the total resistivity of the disordered solid. The corresponding change found for V₈{C}₇$ was 20.1 ±{} 1.4 {μ}{Ω} cm or (14.7 ±{} 1.0)%. The critical temperatures for disordering in the two ordered phases were determined to be (1275 ±{} 8) ^∘{}C for V₆{C}₅and (1124 ± 15) ^∘C for{V}₈C₇. The vanadium-carbon system was also shown to have a strong preference for one or the other of the ordered phases for nearly all compositions in the range 0.83≤x≤0.90, where x is the carbon-to-metal ratio.