E.p.r. and endor of Tb4+ in thoria

E.p.r. and endor spectra have been measured in ThO2 containing Tb4+. The crystalline electric field is cubic, and the splittings are very large compared with other S state ions. The values of the parameters in the standard cubic spin-Hamiltonian are: g = 2·0146 ±0·0004, 60B4 = —2527·53 ±0·10 Mc/s, 1260B6 = —24·84 ± 0·04 Mc/s, A = —73·891 ±0·023 Mc/s, B = + 6·194 ± 0·038 Mc/s, μN(159Tb) = + 1·994 ± 0·004 nuclear magnetons. There are also additional small high-order terms. There are very marked differences between these parameters and those for the other S state ions Gd3+ and Eu2+. In addition to the much larger 60B4, the g value is in excess of the free spin value; at the nucleus, the electrons produce a smaller magnetic field (proportional to A/g1) and a larger electric field gradient (proportional to B/Q) than they do in Gd3+ and Eu2+. These differences are probably due to covalent bonding. The value of the nuclear moment of 159Tb has been used to obtain a value of = 8·23 a.u. for Tb3+ from the known hyperfine structure in Tb3+.