KNIGHT SHIFTS OF C13 IN CARBIDES OF URANIUM AN THORIUM

The C13 nuclear magnetic resonance has been observed in the unenriched carbides of thorium and uranium ThC, ThC2, UC, U2C3, and UC2. 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 P31 shift in UP and PrP. The C13 Knight shift in all of the carbides except U2C3 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 C13 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 UC2 and three in U2C3. Only the lowest crystal-field level of the H43 configuration (the nonmagnetic level Γ1 in the case of UC) is appreciably populated in UC and UC2. 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. © 1968 The American Physical Society.