Relaxation mechanism in mixed iono-covalent systems

Relaxation mechanism in mixed iono-covalent systems 

The oxygen-cation bonds in insulating oxides have a mixed ionic and covalent character. It is thus tempting to merely interpolate the previous results and conclude the existence of an inward relaxation on their surfaces. 

However, great care must be taken in doing so, as revealed by a study of small iono-covalent clusters (Moukouri and Noguera, 1992 1993). 

In oxides, the total energy has three main contributions associated with short-range repulsion, Madelung and covalent interactions  

As demonstrated in Chapter 1, Equation (1.4.63), using the alternating lattice approach and the delta function approximation for M F), the covalent energy Ecov can be expressed in the following way  

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