E.p.r. and endor spectra have been measured in ThO
2
containing Tb
4+
. 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, 60
B
4
= —2527·53 ±0·10 Mc/s, 1260
B
6
= —24·84 ± 0·04 Mc/s,
A
= —73·891 ±0·023 Mc/s,
B
= + 6·194 ± 0·038 Mc/s,
μN
(
159
Tb) = + 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 Gd
3+
and Eu
2+
. In addition to the much larger 60
B
4
, the
g
value is in excess of the free spin value; at the nucleus, the electrons produce a smaller magnetic field (proportional to
A
/
g
1
) and a larger electric field gradient (proportional to
B
/
Q
) than they do in Gd
3+
and Eu
2+
. These differences are probably due to covalent bonding. The value of the nuclear moment of
159
Tb has been used to obtain a value of <r
-3
> = 8·23 a.u. for Tb
3+
from the known hyperfine structure in Tb
3+
.
