Instabilities in Doped Materials Driven by Pseudo Jahn–Teller Mechanisms

Substitution of a host lattice ion by an impurity can produce a spontaneous instability lowering the initial local symmetry of the site. In some cases the impurity remains

on-centre

and the distortion only involves the motion of the nearest ions while in other cases the impurity moves

off-centre

along particular directions of the lattice. In this article experimental and theoretical work on spontaneous instabilities of impurities in ionic solids driven by pseudo Jahn–Teller vibronic coupling mechanisms is reviewed. Particular attention is paid to the results of density functional theory calculations addressed to understand the microscopic origin of the instability and also to quantify the involved distortion. Particularly, we aim to help to overcome a paradigm taken root among many researchers of physics and chemistry of solids: that the instabilities of atoms and ions in pure and doped solids are due to difference of atomic sizes. On the contrary, we present a great quantity of experimental evidences and theoretical results showing that it is an effect of the vibronic coupling.