Self-diffusion of C in single crystals of NBCx

The self-diffusion coefficients of ¹⁴C in NbC_x single crystals have been measured as a function of composition in the temperature range 1900–2315 K, and can be represented by the expressions

$\text{D}{}^1{}_{\mathrm{C}}\text{(}\text{NbC}{}_{0.868}\text{) = (2.59}{}_{\mathrm{?1.07}}{}^{\mathrm{+1.82}}\text{)}\text{exp}\text{(?}\text{100.42 ± 2.2}\text{kcal}\text{mol}\text{RT}\text{)}\text{cm}{}^2\text{s}$

$\text{D}{}^1{}_{\mathrm{C}}\text{(}\text{NbC}{}_{0.834}\text{) = (7.44}{}_{\mathrm{?4.14}}{}^{\mathrm{+9.36}}\text{)}\text{exp}\text{(?}\text{105.0 ± 3.3}\text{kcal}\text{mol}\text{RT}\text{)}\text{cm}{}^2\text{s}$

$\text{D}{}^1{}_{\mathrm{C}}\text{(}\text{NbC}{}_{0.766}\text{) = (2.22}{}_{\mathrm{?1.04}}{}^{\mathrm{+1.98}}\text{) × 10}{}^{\mathrm{?2}}\text{exp}\text{(?}\text{76.02 ± 2.7}\text{kcal}\text{mol}\text{RT}\text{)}\text{cm}{}^2\text{s}$

The lower values of the activation energy and the pre-exponential term in NbC_0.766 are attributed to a change in the path of C mass transport from that of an octahedral-tetrahedral-octahedral mechanism in NbC_0.868 and NbC_0.834 involving a C-metal divacancy mechanism. The effect of lattice geometry and the electronic charge distribution on the diffusion mechanism is also discussed.