SYNTHESIS, TGA AND EPR STUDIES OF PHASE-TRANSITIONS IN CU2+ DOPED (NH4)(4)THF8 AND (NH4)(3)THF7

The paper presents the EPR evidence for the occurrence of phase transitions in (NH4)(4)ThF8 and (NH4)(3)ThF7. In both cases the Cu2+ probe occupies a NH4+ site and predominantly experiences a dynamic coordination, either due to dynamic Jahn-Teller effect, or due to fluxional behaviour of surrounding (ThF8)(4-) units in (NH4)(4)ThF8. It is proposed that the structural phase transition in this compound at 214 K is associated with the transition of the dynamic coordination of Cu2+ into a static one, probably due to freezing of motion of(ThF8)(4-) units below this temperature. In (NH4)(3)ThF7 the dynamic features of the Cu2+ EPR Spectrum are absent and characteristics of a local orthorhombic symmetry are seen down to 77 K. However, in the high temperature range a change from orthorhombic to axial symmetry is observed at 524 K, possibly due to a phase transition.