Ion channeling inNbC1−c: Determination of local atomic displacements around carbon vacancies

The results of channeling experiments on $\mathrm{Nb}{\mathrm{C}}_{1-c}$ single crystals as a function of the vacancy concentration ($0.02\le c\le 0.18$) are presented. Angular yield curves using Rutherford backscattering and nuclear reactions for the Nb and the C yields, respectively, were measured at temperatures of $T=5, 78, \mathrm{and} 295$ K. Statical lattice calculations were used to construct a defect model with displacements of the first and second Nb neighbors of a C vacancy. Monte Carlo calculations have been performed with this defect model to analyze the experimental channeling data. Displacements of the first Nb neighbors of $u_1=0.12\mathrm{}\pm 0.025$ Å and of the second Nb neighbors of $u_2\le 0.03\pm 0.01$ Å around isolated C vacancies have been determined. At high vacancy concentrations ($c=0.18\mathrm{}$) the displacements of the first neighbors decreased to values of $u_1=0.09\mathrm{}\pm 0.007$ Å and of the second neighbors to values of $u_2\le 0.02\pm 0.007$ Å. The static displacements determined with the channeling method were in good agreement with the results of x-ray diffraction experiments.