Knight Shifts ofC13in the Carbides of Uranium and Thorium

The ${\mathrm{C}}^{13}$ nuclear magnetic resonance has been observed in the unenriched carbides of thorium and uranium ThC, Th${\mathrm{C}}_2$, UC, ${\mathrm{U}}_2$ ${\mathrm{C}}_3$, and U${\mathrm{C}}_2$. In contrast to nearly all nonmagnetic metals and semimetallic compounds, the Knight shift in the semimetallic thorium carbides is negative. On the other hand, the Knight shifts in all of the uranium carbides are positive, which is consistent with the ${\mathrm{P}}^{31}$ shift in UP and PrP. The ${\mathrm{C}}^{13}$ Knight shift in all of the carbides except ${\mathrm{U}}_2$ ${\mathrm{C}}_3$ was temperature-independent. On the basis of magnetic susceptibility evidence, it is found that the uranium carbides are similar to other semimetallic uranium compounds in that their magnetic properties are best explained at higher temperatures with a localized $f$-electron model large crystal-field splitting. The ${\mathrm{C}}^{13}$ Knight shift in the uranium carbides is interpreted in terms of an effective hyperfine field resulting from the presence of two $5f$ electrons in UC and U${\mathrm{C}}_2$ and three in ${\mathrm{U}}_2$ ${\mathrm{C}}_3$. Only the lowest crystal-field level of the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ configuration (the nonmagnetic level ${\Gamma }_1$ in the case of UC) is appreciably populated in UC and U${\mathrm{C}}_2$. A comparison of hyperfine fields in a variety of lanthanide and actinide compounds suggests that the fields in semimetallic compounds and insulators have similar origins and that the differences are associated largely with structure, covalency, and overlap, and not with the presence or absence of conduction electrons.