DERIVATION AND APPLICATION OF AB-INITIO NB5-O2- SHORT-RANGE EFFECTIVEPAIR POTENTIALS IN SHELL-MODEL SIMULATIONS OF KNBO3 AND KTAO3()

We derive an ab initio Nb5+-O2- short-range pair potential appropriate to KNbO3, which is based on suitable cluster Hartree-Fock calculation s. The cluster to be chosen must necessarily represent a larger portio n of KNbO3 crystals than a lone Nb5+O2- pair embedded in the appropria te Madelung potential. Our final Hartree-Fock pair potential derived f rom (embedded) (NbO6)7- cluster calculations is in good agreement with a corresponding empirical potential obtained from fitting procedures to macroscopic properties of KNb3O8. Both potentials generate the cubi c phase of KNbO3. Finally, the implications of these two Nb5+-O2- pair potentials are discussed by means of shell-model defect simulations i n KNbO3 and KTaO3.