Phase Transitions in Crystalline

Solid polymorphism in five compounds of the type [Me(H20)6](BF4)2 , where Me = Mn, Fe, Co, Ni and Zn, have been studied in the temperature range 120 330 K by DSC. For each substance at least three solid modifications have been found. In addition, for compounds with Me = Mn, Fe and Zn some new low temperature phase transitions have been observed. The transition temperatures for the [Me(H20)6](BF4)2 compounds are distinctly lower than those for the [Me(H20)6](C104)2 compounds. The entropyand enthalpy-changes at the phase transitions of the hexaaquametal(II) tetrafluoroborates are significantly lower than those for the corresponding chlorates(VII). Linear correlations between the highest transition temperature and the crystal density are found for both series of these isomorphous compounds.