Continuous changes of the Jahn–Teller deformation of Cu(H2O)6 complex in ferroelastic Cs2Cu(ZrF6)2·6H2O crystal

Q-band EPR spectra of Cs2Cu(ZrF6)2·6H2O measured in temperature range 4.2–350 K and supported by the X-ray crystal structure analysis, show a gradual transformation of an adiabatic potential surface of the Jahn–Teller (JT) Cu(H2O)62+ complex leading to the switch of its elongation direction. This effect is joined with a ferroelastic phase transition at 320 K. It is shown that gradual movement of minima of the adiabatic potential surface along the φ coordinate for ρ≅ρ0, describing this phenomenon, is a result of a competition between a small lattice deformation at T>Tc and the new deformation appearing at Tc and increasing with decreasing temperature.