Magnetic Resonance Studies of (ReO4)2- in Calcium Tungstate

The e.p.r. spectrum of (ReO₄)O^2- in CaWO₄ has been measured in the temperature 4-120 K as a function of orientation. The spectrum including many forbidden lines may be fitted to the spin Hamiltonian H = μ _BB\cdot g\cdot S\cdot +I\cdot tilde{A}\cdot S\cdot +I\cdot tilde{P}\cdot I\cdot in tetragonal form with S = 1/2 and I=5/2. At 4.2 K, endor measurements for the magnetic field oriented parallel to the unique axis, taken in conjunction with the e.p.r. spectrum, yield spin Hamiltonian parameters g_\| = 1.8549 ± 0.0002, g_perp = 1.7164 ± 0.0002, A_perp = (±)(42± 2)× 10^-4 cm ^-1, A_perp = (±)(324± 2)× 10^-4 cm ^-1 and P_\| = (mp)(5± 3)× 10^-4 cm ^-1, with g_N = (±) 1.27 for the ¹⁸⁷Re nuclide. These parameters are consistent with the single electron occupying the |3z² - r²> orbital of a 5d¹ ion. The e.p.r. linewidth, which is strongly sensitive to both orientation and temperature, has been examined in detail. Preliminary studies of the relaxation properties of the centre are discussed in qualitative terms.

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