EPR studies of phase transitions in perchlorates [M 2+(ClO4)2 · 6H2O] at high pressures

The EPR spectra of the Mn²⁺ ion in crystals of the perchlorate hexahydrates Zn(ClO₄)₂ · 6H₂O, Mg(ClO₄)₂ · 6H₂O, and Cd(ClO₄)₂ · 6H₂O were studied in the temperature range 77–320 K under hydrostatic pressure. It is shown that the octahedron of six molecules H₂O surrounding this paramagnetic ion is contracted along the c axis and that pressure decreases this distortion. The second-order phase transition that occurs near 200 K in the perchlorates and in other crystal hydrates is shown to be associated with changes in the bonds in the nearest ligand environment. As the pressure is increased, the phase-transition temperatures shift and the perchlorate crystals tend to a single-phase state. The low-temperature phase is assumed to disappear as the pressure increases, and this phase exists in a closed T-P region in the phase diagram. As the pressure increases, the character of the high-temperature transition in the Cd(ClO₄)₂ · 6H₂O changes: the jumplike transition at T ₁ with a 1-K hysteresis changes into a smooth transition and then disappears as the pressure increases further.