EPR study of a new low-temperature phase transition in Zn(ClO4)2·6H2O

From a temperature variation EPR study of Mn²⁺ doped single crystals of Zn(ClO₄)₂·6H₂O phase transition has been detected at T₂~290 K. The phase relationships in this crystal are as follows. Phase I transforms atT₂~346K to Phase II, which in turn transforms to Phase III at T₂ ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn2₁ both above and below T₂. The water-perchlorate sublattice symmetry below T₂ is found to be lower than the P6₃mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T₂, is reflected through the observed temperature dependence of the rhombic distortion parameter E. It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.