Abstract
Chemically induced dynamical Jahn-Taller (CDJT), averaging the g-factor on cooling, below T orb, due to topological transformation from the CuO4 complex to CuO5 state, has been shown in several experiments. The change in order parameter E of spin paramagnetism at T orb was observed. Above T orb a typical antiferromagnetic temperature dependence, with Curie--Weiss temperature Θ < 0, was observed. Below T orb the orbital excitation destroys the antiferromagnetic order and gives rise to a Θ of 0 K.