EPR AND DTA STUDIES OF PHASE-TRANSITIONS IN CU2+ DOPED TRIAMMONIUM URANIUM OCTAFLUORIDE

EPR studies of Cu2+ doped (NH4)3UF8 are conducted in the temperature range 77 K to 400 K. At room temperature the spectra consisting of an isotropic quartet with m(I)-dependent line width reveal that the Cu2+ ion experiences a fluctuating crystal field with a characteristic time tau(c) < 1.4 x 10(-7) sec. With lowering of temperature the lines get broadened below 243 K, with associated appearance of an axial spectrum without any hyperfine structure which gets stabilized at 213 K. This is understood as due to Cu2+ at the (NH4)+ site experiencing the (UF8)3- molecular motion. Changes in the spectral features reflect changes in the (UF8)3- dynamics. The correlation time of motion is estimated to be less than 1.4 x 10(-7) sec above 243 K, and of the order of 4.5 x 10(-7) sec below 213 K. The microwave losses from the bulk of the sample in the temperature range 77-400 K show an anomalous change consistent with the change in molecular dynamics observed in EPR. Furthermore, DTA studies in the high temperature region have given evidence for first order phase transitions at 353 K and 382 K.