N u c le a r in te r a c tio n s in eu r o p iu m v a n a d a te B y B. B l e a n e y Clarendon Laboratory, Parks Road, Oxford 0X1 3PU, UK The ground configuration of the europium ion, Eu3+, df1 * * * * 6, 7F, is split by spin-orbit interaction into a series of levels J = 0 to 6. There are matrix elements of the electronic Zeeman interaction Ze and hyperfine interaction Zhfs between states of different J . In particular, for the lowest state, the singlet 0, m atrix elements to the first excited

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... state J = 1 produce contributions to the nuclear Zeema quadrupole interactions in J = 0. Formulae for these are listed by (1994) for three tetragonal compounds. One of these is EUVO4, for which existing measurements by optical 'hole-burning' are discussed, together with the possibility of nuclear orientation and dynamic nuclear polarization. Experiments using nuclear acoustic resonance are suggested in a following paper. In tr o d u c tio n Crystals of EUVO4 are tetragonal, with zircon structure, space group ^ (14i/am d); the lowest level of the trivalent Eu3+ ion, df6, is 7F 0, = 0. The first excited level J = 1 lies at approximately 360 cm-1 (520 K). At low temperatures, where only the level J -0 is populated, this compound becomes a 'Van Vleck' paramagne with the electronic magnetic susceptibility independent of temperature. The main interest then lies in the splittings of the nuclear levels, to which there are several contributions. Europium has two stable isotopes, of mass 151 and 153: two radioactive isotopes, of mass 152 and 154, and half-lives 13 years and 8.5 years respectively, have been studied by nuclear orientation. The nuclear spins, magnetic resonance (nm r ) frequencies / in a field B, and electric quadrupole moments Q (all positive) are given in table 1 For the two stable isotopes, the nm r frequencies are quite precise; although they each have nuclear spin I -5/2, their nuclear magnetic moments differ by a factor of 2.265. Also, the electric quadrupole moments differ by a factor 0.374, though the actual values are considerably less certain. In the ground state J = 0 of Eu3+, interactions with the first ex state J = 1 reduce the effective nuclear Zeeman splittings by appreciable amounts. In a compound where the europium ion is at a site with less than cubic symmetry, the nuclear magnetic resonance (nm r ) frequencies become anisotropic, and splittings of the nuclear levels, quadrupolar in form, are produced. Such effects were first discussed by Elliott (1957) for europium ethylsulphate. His calculations have been repeated for Eu3+ ions in three compounds with tetragonal symmetry by Bleaney & Leask (1994) , using more recent values of the parameters. These are summarized below in § 2, as a prelude to a suggestion for nuclear polarization experiments, and the use of dynamic