Orbital dynamics of the CuXn complex in solids

The change in copper coordination from CuX4 to CuX5 leads to the quantum averaging of local field studied by g-factor in EPR experiment. Observed `liquid-likeʹ EPR spectra are discussed as the frustration between two equivalent CuX5 orbital states: square pyramid |x2-y2> and trigonal bipyramid |3z2-r2> configuration, related to the degeneracy of the ground orbital state. Orbital crossing point presents ideal outline of `chemicallyʹ induced dynamic Jahn–Teller effect related to the fast exchange within the orbital states due to change in topology of the paramagnetic complex from rigid bonded CuX4 in the lattice to CuX5 with internal quantum dynamics.