EPR detection and study of new phase transitions in Mg(ClO4)2·6H2O

A temperature dependent EPR study of Mn²⁺ impurity-doped crystals of Mg(ClO₄)₂·6H₂O has led to the detection of two critical temperatures T₁ ~ 335K and T₂ ~ 324K where the EPR characteristics undergo distinct changes. The monoclinic distortion of the water octahedron around the metal ion shows little temperature dependence between T₁ and T₂ and T₂ and T₃ ~ 272K, while it is found to be comparatively strongly temperature dependent above T₁ and below T₃, decreasing as T approaches T₁ from the high temperature side and as T approaches T₃ from the low temperature side. At room temperature the water-perchlorate symmetry seems to deviate slightly from the perfect P6₃mc symmetry proposed from previous X-ray measurements by West. Although the space symmetry group of the unit cell appears to remain unchanged at T₁ and T₂, the temperatures T₁ and T₂ are suspected to be related to structural phase transitions in this crystal.