Spin-lattice relaxation of tetravalent protactinium in a cubic crystal

The paramagnetic relaxation time is investtigated for Pa 4+ in the cubic matrix Cs2 (Zr"Pa)C16 between 1.4°K and 4".2°K, using the pulse saturation method. A theory, developed for materials in which the electrostatic crystal potential is described by a few crystal field parameters, has been used quite successfully in predicting the relaxation behavior of salts containing rare earth ions. It is the purpose of this study to determine whether, the same theory can be extended to an actinide ion, where the effects of the spin-orbit interaction are comparable to the effects' of the crystal field interaction. Experimental data fit the expression llTl = ADT + CDT9 , with AD = 3.5 + 0.7 sec -1 deg -1 and C~ = (5.1 + 1.5) x 10 -5 sec -1d eg -9 For the purpose of calculating llTl theoretically, static . 0 4> o( 6> crystal fl.eld parameters A4 < rand A6 r are used to predict the corresponding dynamic parameters a: < r 4 > and a~. Second degree coupling coefficients a~are treated in terms of a single parameter a~~r2~ which is consid~red unknown in the calculati6rts, as is the phonon velocity, v. A comparison of the resulting theoretical expressions for AD and C' withithe corresponding experimental values rE:!sults in two non-linear equations in a~ <r2) and v. Thes.e equations are found to be consistent with a measured phonon velocity of 4.5 x 10 5 cm/sec and a value of a0 2> -1 2 <r == 24,000 cm , calculated from a dynamic point chargEl model consistent with the values of a4o 4> 0 < rand a6 r . In Appendix A the relaxation behavior of the rare earth ion, Ho' 3+ , in yttrium ethyl sulphate ~"s treated. Experimental results show a dependence of l/Tl on the concentration of holmium ions. This may be explained by assuming the relaxation to be phonon-limited.U of I OnlyThesi