The properties of CuSiF6 · 6H2O and Cu0.9Zn0.1SiF6 · 6H2O crystals were studied using X-Ray analysis and adiabatic calorimetry. The measurements cover the temperature range from 100 to 310 K. The enthalpy of transition for the phase transitions which occur at 300 K have been obtained and the crystal lattice parameters of the high-temperature phase of Cu0.9Zn0.1SiF6 · 6H2O have been determined. These
copper(II) complexes in the solid state on the basis of crystallographic studies, e.g. hexaaquacopper(II) bromate, [Cu(OH2)6](BrO3)2, hexaaquacopper(II) hexafluorosilicate, [Cu(OH2)6]SiF6, and hexakis(pyridine-1-oxide)copper(II) perchlorate, [Cu(ONC5H5)6](ClO4)2. These results are not consistent with the elongated octahedral configuration expected from the Jahn–Teller theorem for the d9 copper(II) ion