Systematics of hyperfine interactions in actinyl compounds

— Hyperfine interactions in several actinyl complexes are presented. The electric quadrupole and magnetic hyperfine interaction constants are calculated within a simple model whereby non-bonding electrons, perturbed by an axial crystal field, are added to a uranyl core. This approach provides reasonable agreement for all cases except the quadrupole interaction in 5f systems, where the measured values are about three times smaller than those calculated. The electronic properties of the actinyl ions have been frequently discussed [1-3]. Although details of various models differ, especially with regard to the correct molecular orbitals in uranyl ions, the following results for the actinyls as a whole are generally accepted : the uranyl complex ( U O / * contains twelve electrons which exactly fill the available bonding orbitals. This then forms a core for other actinyl species, with the additional electrons falling into non-bonding orbitals. These electrons in turn are treated in a framework of Russell-Saunders coupling which is perturbed by the strong axial crystal-field interactions due to the axial oxygen ions. Crystalline field and ligand field effects with equatorial ions are considered to be much less influential than those with the axial ions. At present the most extensive discussions have been with regard to (UOj)*' and (Np0 2 ) ' ' complexes. Except for spectroscopic data, very little work is available which attempts to systematically discuss physical properties of a number of species [1-3]. Recently, Mossbauer effect measurements in U, Np, Pu and Am have made it possible to obtain hyperfine interaction data in the complexes (U02) ++ , (Np02f + , (Np02) , (Pu0 2 )^ and ( A m O / [6-7]. These include the electronic configurations 5f", n = 0 to 4, with the exception of n = 3 , where data are not, yet available. Representative data obtained at 4.2 K for the principal component of the electric field gradient tensor, eq, and for the magnetic hyperfine field, HM, are given in table I. Elementary considerations for the calculation of these quantities, along (*) Work supported by the U.S. Department of Energy, (t) Permanent Address : Physik Department, Technische Universitat Munchen, D8046 Garching, Germany. with the tabulation of appropriate electronic parameters, have been given previously [6]. Within the framework for the actinyls discussed above, the electric field gradient can be written [6, 8] eq = eqmtvl ea, < 3 J\ J ( J + 1)>< r" ) (1 R„) (1) where eq uranyl is the value observed in the uranyl ion, ctj is a reduced matrix element, RQ =» 0.35 is a Sternheimer factor, and ( r~) is a 5f electron radial average. At present, no calculation for equranyl has been performed which both explains the observed magnitude as well as the sign of the interaction [9]. Therefore we must currently adopt this simply as an empirical value. Assuming a strong crystal field within a single / manifold, the crystal field ground states I J, Jz ) in the remaining cases are taken as I 5/2, ± 5 / 2 ) for 5f, 14, ± 4 ) for 5f, and 14,0) for 5f. In the last case, one requires J2 = 0 rather than J2 = ± J due to a change in the signs of the Stevens factors occurring in the crystal field Hamiltonian. The results of the calculations are shown in table I. The agreement is seen to be satisfactory in all cases except for the 5f ions. Magnetic hyperfine interactions are also seen in some cases (table I). The magnetic hyperfine field is given by HM = [ 2 M B N, ( r" ) + X{g] 1)] ( I >. (2) The first term arises from the orbital momentum of electrons in unfilled valence shells, and the second term is due to polarization of the core electrons by the outer electrons. The quantity N} is a reduced matrix element, and x is approximately a constant with the value 600 kOe. Calculated values are Article published online by EDP Sciences and available at http://dx.doi. org/10.1051/jphyscol:1979461 SYSTEMATICS OF HYPERFINE INTERACTIONS IN ACTINYL COMPOUNDS C4-193 Table I. Experimental and calculated values for the principal component of the electric field gradient tensor, eq, and the magnetic hyperfine field, H,, in various actinyl ions. eq (10'' V/cm2) H,, (kOe) Conf i y ra t ion Ion Measured Calculated Measured Calculated sf" (U02)++ 7.7 0 0 Sf' (NPO,)" 12.0 11.0 2 500 3000 Sf2 (NPO,)' 4.7 10.8 6000 5 100 (PuO,)" 4.1 11.3 5f" ( AmO,)' 9.5 8.7 0 0 given in table I. For the 5f' and 5f2 configurations, I J, ) was estimated using g values measured by EPR [lo, 1 11. The hyperfine field vanishes for the 5f0 ions due to the filled f shell. Similarly one has H , = 0 for the 5f" case since the crystal field establishes a J, = 0 ground state. From an investigation of table I, one sees that in general the simple approach described here reproduces the observed hyperfine interactions to within 15 %. The predominant exception is the case of Sf2 ions. Here the calculated magnetic hyperfine field is acceptable, while the calculated quadrupole interaction is about three times larger than that observed. This does not appear to be an experimental error, since the same value is obtained both for (NpO,)' and (PuO,)" complexes. While it is admitted that the approach taken here is very simple, the agreement in all other cases speaks for its essential correctness. Therefore the inadequacy of the calculation in this one isolated case is not presently understood. The most likely explanation is that the assumption of a purely axial crystal field with only the ground state occupied is inadequate.