Phase transitions in [Ca(H2O)4](ClO4)2 studied by differential scanning calorimetry, X-ray single crystal diffraction and neutron powder diffraction

Four phase transitions at: TC1h=267.1K, TC2h=258.8K, TC3h=254.1K and TC4h=195.4K (on heating) and at TC1c=263.8K, TC2c=255.4K, TC3c=246.0K (on cooling) were determined for [Ca(H2O)4](ClO4)2 between 90 and 295 K by means of differential scanning calorimetry (DSC). Phase transitions discovered at TC1, TC2 and TC3 are connected with small change of the crystal structure. Namely, there are scant but statistically significant differences between unit cell parameters (angles: α, β, γ) and torsion angles in the structure of [Ca(H2O)4](ClO4)2 at 293 and 240 K. The space group (P1¯; No. 2) is the same for high, intermediate and low temperature phase, however, changes of perchlorate anions orientations are distinctly visible.