Abstract
Abstract The paper presents the EPR evidence for the occurrence of phase transitions in (NH4)4ThF8 and (NH4)3ThF7. 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)4ThF8. 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)3ThF7 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.
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