Electron paramagnetic resonance (EPR) studies were made on SeO3- radicals created in (NH4)3H(SeO4)2 by gamma irradiation. EPR spectra were recorded of powder and single crystal samples at X-band (~9 GHz) frequencies at three temperatures corresponding to the crystalline phases III, II and I. The g and T tensors of two chemically inequivalent radicals were determined from the rotation data about three mutually perpendicular axes. The g tensors result from the 92.5% abundant spinless isotopes of Se. On the other hand, the T tensors result from the hyperfine splitting of the 7.5% abundant 77Se isotopes.
The nearly identical powder spectra at all three temperatures show the molecular dynamics of the SeO3- radicals to be practically unchanged through the temperature range studied. The nearly equivalent g and T tensors at all three temperatures attest to the stability of the SeO3- radical geometry. At the temperatures corresponding to the superionic phases, the trigonal crystalline symmetry, reported in structural studies of (NH4)3H(SeO4)2, was not evident in the EPR data.
The immobilizing effect of the NH4+ groups through their N-H· · · O hydrogen bonds, which is very effective at low temperatures, is too weak to have much influence on the SeO3- radical geometry at higher temperatures. However, the steady decrease in the intensity of the forbidden dipolar coupling through the region, rather than an abrupt decrease at the III to II phase transition, indicates that the ammonium ions still interact with the SeO3- radical at the higher temperatures.
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