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Electron paramagnetic resonance of 249Bk4+ in ThO2 single crystals
Volume 42 A, number 1
PHYSICS LETTERS
ELECTRON
6 November 1972
PARAMAGNETIC RESONANCE ThO, SINGLE CRYSTALS*
OF 249Bk4+ IN
L.A. BOATNER, R.W. REYNOLDS Advanced
Technology
Center, Inc., Dallas, Texas 75222,
USA
C.B. FINCH and M.M. ABRAHAM Oak Ridge National Luboratory,
Oak Ridge, Tennessee 37830,
USA
Received 28 August 1972
The EPR spectrum of Bk4’ has been obtained for the fist time. For a ThOa host, a l?e doublet lies lowest. The nuclear spin of 249Bk is confirmed as 7/2 and the estimated nuclear moment is 2.2 fiN.
This work reports
the first observation
by EPR of
tetravalent berkelium which was present as a dilute impurity in a ThO, host crystal. The thorium dioxide
crystals used in this study were grown by a thermal gradient technique [l] and were doped by addition of the oxide of 24gBk to the molten solvent. At a temperature of 1.5 K, the isotropic, eight-line EPR spectrum shown in fig. 1 was observed. At ~9.5 GHz, this spectrum is described by the. conventional spin HamiltonianH=g~BH.StAf.SwithS= l/2,1= 7/2,g = 4.488 + 0.004 and A = 175.2 + 0.5 X 1O-4 cm -l. The observed nuclear spin of 7/2 is consistent with the previous determination [2] made for 249Bk by means of optical spectroscopy. Tetravalent berkelium has the electronic configuration [Rn] 5fl and is the actinide analog of the rareearth ion Tb4+. Previous studies of the electronic properties of actinide ions have shown that the large spin-orbit interaction present in these ions produces considerable departures from pure Russell-Saunders coupling (i.e., intermediate coupling effects are important). In particular, EPR studies of the 5f 7 configuration ions Am2+ and Cm3+ [3-51 have indicated that appreciable admixing of higher-lying states into the pure Russell-Saunders 8s 7,2 ground state occurs. The resulting increase in the ground-state orbital angular momentum of Am2+ and Cm3+ produces crystahine-
* Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation.
ti
!I03
4300
4500
1700
1900
gOUS
Fig. 1. EPR spectrum of ThOa: 249Bk4+ at ‘L 1.5 K. The nuclear spin is equal to 7/2 and the spectrum is isotropic for all crystal orientations at this frequency.
electric-field splittings which are much larger than those of their rare-earth analogs, Eu2+ and Gd3+. Correspondingly, the crystal-field splitting of Bk4+ is expected to be considerably larger than that of Tb4+. For large values of the fourth-order cubic crystal field parameter relative to the electronic Zeeman interaction?, an isotropic EPR transition is observed whose value is either (7/3)gJ or 3gJ depending on whether the ground level is a F6 or F7 doublet. A F6 doublet ground state has been observed previously for Am2+ and Cm3+ in all of the eight-coor+ The sixth-order crystal fieldparameter is also assumed to be somewhat smaller than the fourth-order term.
93
Volume 42 A, number 1
PHYSICS LETTERS
dinated cubic hosts. The g-value of 4.488 determined for berkelium in ThO, results in a value ofg, = 1.923 for a r6 ground state. This value ofgt is in good agreement with all of the values obtained for Am2+ and Cm3+ in g-fold cubic hosts and indicates that the Bk4+ ground state is also a r6 doublet. The sign of the fourth-order crystal field parameter is therefore determined to be negative, as in the case for Tb4+ in ThO, [6]. At sufficiently high microwave frequencies, an admixture of the first excited I’S level by the applied magnetic field will produce an anisotropy in the Bk4+ spectrum. This anisotropy can then be used to determine the r8 - r, separation. An attempt* to make such a determination at 35 GHz was unsuccessful since, within the experimental error, the spectrum remained isotropic. This result does show, however, that for Bk4+ in ThO,, the rs - r6 separation is greater than 50 cm-l. Since the nuclear magnetic moment PI is proportional to the product AZ, where 1 is the nuclear spin, it is possible to determine an approximate value of constant obPI for 249Bk by using a proportional$y of A and I_(~ tained from the known values [3,7]
for the isoelectronic ion 241Am2+. This procedure yields a value of IpI (24gBk) I = 2.2 + 0.4 nuclear magnetons. When this result is modified to account for differences in the values of (r-3) between Am2+
and Bk4+ using the results of Lewis et al. [S] a. value of 1.9 + 0.4 nuclear magnetons is obtained. The value of 5.1 pN obtained from optical data [9] was calculated using assumptions which are no longer believed to be valid [lo]. A preliminary measurement of t5.79 barns for the nuclear quadrupole moment has recently been made [ 1 l] using nuclear coulomb excitation techniques. References [l] C.B. Finch and G.W. Clark, J. Appl. Phys. 36 (1965) 2143.
[ 21 E.F. Warden, E.K. Hulet and R. Lougheed, J. Opt. [ 31 [4]
[ 51 [6]
[ 71 * The experiment at 35 GHz was performed at the Lawrence Radiation Laboratory, Berkeley, with the co ** operation of W. Kolay,andzN; Edelstein. rrI = 1.59 given for Am was used in the estimate of the 249Bk4+ nuclear moment.
94
6November 1972
[8] [9]
[lo] [ 1 l]
SOC.
Am. 57 (1967) 550. N. Edelstein and W. Easley, J. Chem. Phys. 48 (1968) 2110 M.M. Abraham, L.A. Boatner,C.B. Finch, R.W. Reynolds and H. Zeldes, Phys. Rev. B-l (1970) 3555. W. Kolbe, N. Edelstein, C.B. Finch and M.M. Abraham, J. Chem. Phys 56 (1972) 5432. J.M. Baker, J.R. Chadwick, G. Garton and J.P. Hurrell, Proc. Roy. Lot. (London) A286 (1965) 352. Nuclear Dafa Tables 5 (1969) 459. W.B. Lewis, J.B. Mann, D.A. Liberman and D.F. Cromer, J. Chem. Phys. 53 (1970) 809. E.F. Warden, R.G. Gutmacher, J.G. Conway and R.J. Mehlhorn, J. Opt. Sot. Am. 59 (1969) 1526. J.G. Conway, private communication. C.E. Bemis Jr., private communication.
PHYSICS LETTERS
ELECTRON
6 November 1972
PARAMAGNETIC RESONANCE ThO, SINGLE CRYSTALS*
OF 249Bk4+ IN
L.A. BOATNER, R.W. REYNOLDS Advanced
Technology
Center, Inc., Dallas, Texas 75222,
USA
C.B. FINCH and M.M. ABRAHAM Oak Ridge National Luboratory,
Oak Ridge, Tennessee 37830,
USA
Received 28 August 1972
The EPR spectrum of Bk4’ has been obtained for the fist time. For a ThOa host, a l?e doublet lies lowest. The nuclear spin of 249Bk is confirmed as 7/2 and the estimated nuclear moment is 2.2 fiN.
This work reports
the first observation
by EPR of
tetravalent berkelium which was present as a dilute impurity in a ThO, host crystal. The thorium dioxide
crystals used in this study were grown by a thermal gradient technique [l] and were doped by addition of the oxide of 24gBk to the molten solvent. At a temperature of 1.5 K, the isotropic, eight-line EPR spectrum shown in fig. 1 was observed. At ~9.5 GHz, this spectrum is described by the. conventional spin HamiltonianH=g~BH.StAf.SwithS= l/2,1= 7/2,g = 4.488 + 0.004 and A = 175.2 + 0.5 X 1O-4 cm -l. The observed nuclear spin of 7/2 is consistent with the previous determination [2] made for 249Bk by means of optical spectroscopy. Tetravalent berkelium has the electronic configuration [Rn] 5fl and is the actinide analog of the rareearth ion Tb4+. Previous studies of the electronic properties of actinide ions have shown that the large spin-orbit interaction present in these ions produces considerable departures from pure Russell-Saunders coupling (i.e., intermediate coupling effects are important). In particular, EPR studies of the 5f 7 configuration ions Am2+ and Cm3+ [3-51 have indicated that appreciable admixing of higher-lying states into the pure Russell-Saunders 8s 7,2 ground state occurs. The resulting increase in the ground-state orbital angular momentum of Am2+ and Cm3+ produces crystahine-
* Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation.
ti
!I03
4300
4500
1700
1900
gOUS
Fig. 1. EPR spectrum of ThOa: 249Bk4+ at ‘L 1.5 K. The nuclear spin is equal to 7/2 and the spectrum is isotropic for all crystal orientations at this frequency.
electric-field splittings which are much larger than those of their rare-earth analogs, Eu2+ and Gd3+. Correspondingly, the crystal-field splitting of Bk4+ is expected to be considerably larger than that of Tb4+. For large values of the fourth-order cubic crystal field parameter relative to the electronic Zeeman interaction?, an isotropic EPR transition is observed whose value is either (7/3)gJ or 3gJ depending on whether the ground level is a F6 or F7 doublet. A F6 doublet ground state has been observed previously for Am2+ and Cm3+ in all of the eight-coor+ The sixth-order crystal fieldparameter is also assumed to be somewhat smaller than the fourth-order term.
93
Volume 42 A, number 1
PHYSICS LETTERS
dinated cubic hosts. The g-value of 4.488 determined for berkelium in ThO, results in a value ofg, = 1.923 for a r6 ground state. This value ofgt is in good agreement with all of the values obtained for Am2+ and Cm3+ in g-fold cubic hosts and indicates that the Bk4+ ground state is also a r6 doublet. The sign of the fourth-order crystal field parameter is therefore determined to be negative, as in the case for Tb4+ in ThO, [6]. At sufficiently high microwave frequencies, an admixture of the first excited I’S level by the applied magnetic field will produce an anisotropy in the Bk4+ spectrum. This anisotropy can then be used to determine the r8 - r, separation. An attempt* to make such a determination at 35 GHz was unsuccessful since, within the experimental error, the spectrum remained isotropic. This result does show, however, that for Bk4+ in ThO,, the rs - r6 separation is greater than 50 cm-l. Since the nuclear magnetic moment PI is proportional to the product AZ, where 1 is the nuclear spin, it is possible to determine an approximate value of constant obPI for 249Bk by using a proportional$y of A and I_(~ tained from the known values [3,7]
for the isoelectronic ion 241Am2+. This procedure yields a value of IpI (24gBk) I = 2.2 + 0.4 nuclear magnetons. When this result is modified to account for differences in the values of (r-3) between Am2+
and Bk4+ using the results of Lewis et al. [S] a. value of 1.9 + 0.4 nuclear magnetons is obtained. The value of 5.1 pN obtained from optical data [9] was calculated using assumptions which are no longer believed to be valid [lo]. A preliminary measurement of t5.79 barns for the nuclear quadrupole moment has recently been made [ 1 l] using nuclear coulomb excitation techniques. References [l] C.B. Finch and G.W. Clark, J. Appl. Phys. 36 (1965) 2143.
[ 21 E.F. Warden, E.K. Hulet and R. Lougheed, J. Opt. [ 31 [4]
[ 51 [6]
[ 71 * The experiment at 35 GHz was performed at the Lawrence Radiation Laboratory, Berkeley, with the co ** operation of W. Kolay,andzN; Edelstein. rrI = 1.59 given for Am was used in the estimate of the 249Bk4+ nuclear moment.
94
6November 1972
[8] [9]
[lo] [ 1 l]
SOC.
Am. 57 (1967) 550. N. Edelstein and W. Easley, J. Chem. Phys. 48 (1968) 2110 M.M. Abraham, L.A. Boatner,C.B. Finch, R.W. Reynolds and H. Zeldes, Phys. Rev. B-l (1970) 3555. W. Kolbe, N. Edelstein, C.B. Finch and M.M. Abraham, J. Chem. Phys 56 (1972) 5432. J.M. Baker, J.R. Chadwick, G. Garton and J.P. Hurrell, Proc. Roy. Lot. (London) A286 (1965) 352. Nuclear Dafa Tables 5 (1969) 459. W.B. Lewis, J.B. Mann, D.A. Liberman and D.F. Cromer, J. Chem. Phys. 53 (1970) 809. E.F. Warden, R.G. Gutmacher, J.G. Conway and R.J. Mehlhorn, J. Opt. Sot. Am. 59 (1969) 1526. J.G. Conway, private communication. C.E. Bemis Jr., private communication.