Abstract
An explanation has been proposed for the unusual behaviour of the asymmetry parameter of NQR spectra of Cu(2) in layered cuprates. When holes are introduced into the plane, Zhang-Rice singlets (ZRS) appear which have a strong vibronic coupling to tetragonal and orthorhombic modes. The equation for the adiabatic potential has been derived. It is shown that its minimum, correspond­ ing to orthorhombic distortions of the oxygen plaquette around the Cu(2) ions, changes under doping to tetragonal character due to the singlet exchange coupling between copper and oxygen holes. A strong interaction of ZRS via the phonon field has been found.

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