Phase transitions in [Ca(H2O)4](NO3)2 studied by differential scanning calorimetry, X-ray single crystal diffraction and neutron powder diffraction: Part I

Two phase transitions at: TC1h=243.7K and TC2h=203.3K (on heating) and at: TC1c=222.0K and TC2c=201.5K (on cooling) were determined for [Ca(H2O)4](NO3)2 between 90 and 300 K by means of differential scanning calorimetry (DSC). Large thermal hysteresis (∼22 K) of TC1 phase transition is characteristic for similar compounds indicating high degree of dynamical disorder of NO3− anions. Phase transition discovered at TC2 is connected with small change of the crystal structure. Namely, at low temperature phase the unit cell is doubled and there are scant but statistically significant differences between atom positions at 100 and 215 K. The space group (P21/n; No. 14) is the same for high, intermediate and low temperature phase, however, changes of water molecules orientations and changes in the net of hydrogen bonds are distinctly visible.