The role of ammonia molecules in [Cu(NH 3) 5](ClO 4) 2 structure in the crystal field averaging in Cu(II) EPR powder spectra

Abstract

The EPR results for [Cu(NH 3) 5](ClO 4) 2·0.1NH 3 above and below the phase transition temperature T c =142 K are reported. The comparison to the data reported so far revealed the role of ‘free’ ammonia (i.e. NH 3 molecules non-coordinated to Cu(II) ion) in the averaging process of anisotropic polycrystalline EPR spectra of five-coordinated Cu(II) complex. For T> T C, in the cubic structure, there is a dependence of g av value on the number of free ammonia molecule (0≤ x≤0.2) per one complex. Below T C, in tetragonal phase, for x≈0.1, there are two contributions to the EPR spectrum: the anisotropic one due to the rigid square pyramid coordination and isotropic one due to the centers where the averaging mechanism is efficient. For x=0.2, there is only isotropic spectrum in the tetragonal phase. Additionally, the ‘aging’ process of the freshly made sample has been observed as the increase in the intensity of the isotropic component, relative to the anisotropic one, in the low temperature phase. This behavior has been ascribed to free ammonia ordering process in tetragonal structure.