EPR AND MICROWAVE-ABSORPTION STUDIES OF A NEW PHASE-TRANSITION IN CU2+ DOPED AMMONIUM PLUTONIUM(IV) OCTAFLUORIDE

Electron paramagnetic resonance studies of Cu2+ doped (NH4)(4)PuF8 have shown that the copper ion enters the lattice at NH4+ sites, and that it is subjected to a fluctuating crystal field at room temperature due to the molecular motion of(PuF8)(4-) units. This is inferred from the observation of a isotropic hyperfine quartet with m(I) dependent line widths. On gradual cooling this spectrum transforms into an axial one, with no hyperfine structure at and below 232 K. This is identified as due to sudden slowing down of (PuF8)(4-) units at 232 K. Sharp changes in bulk microwave absorption were observed at this temperature suggesting that the changes in the molecular motion are not probe dependent. This suggests the occurrence of a phase transition in (NH4)(4)PuF8 at 232 K.