Jahn–Teller effect in Ag2+ doped KCl and NaCl: Is there any influence of the host lattice?

Despite spin Hamiltonian parameters measured at T = 77 K for elongated centres in Ag2+ doped KCl and NaCl are nearly identical the transition temperature, Tt, for observing an isotropic EPR spectrum is however quite different. This puzzling situation is solved through CASPT2 calculations which lead to the same value of the equatorial Ag2+–Cl− distance in both systems but to a 15 pm difference between axial distances. This fact implies a strong reduction of the barrier among equivalent minima and Tt on going from KCl to NaCl. The optical absorption spectrum of KCl:Ag2+ is also explained through present calculations.