LOCAL RELAXATION EFFECTS IN THE CRYSTAL-STRUCTURE OF VANADIUM-DOPED ZIRCON - AN AB-INITIO PERTURBED ION CALCULATION

A theoretical description of local relaxation effects-induced by the p resence of impurity centers V4+ at the tetrahedral and dodecahedral si tes of ZrSiO4 is presented. The crystal structure is analyzed via opti mization of local equilibrium: geometries and computation of the force constant associated with the symmetric vibrational mode at each impur ity center. The theoretical study is carried but using an extension of the ab initio perturbed ion method. The relative stability of vanadiu m-substituted vacancies in ZrSiO4 is examined in light of previous res ults reported by the present authors and the vibrational analysis here addressed. Local geometries were optimized by relaxing various sets o f ions around each substituted center. An increased substitution energ y together with a pronounced decrease of the breathing vibrational mod e on the 4-fold-coordinated site shows that this substitution is unsta ble while that on the 8-fold-coordinated ion site is energetically fav orable and the local geometry is minimally relaxed. Selective doping o n each center leads to a decreased force constant as compared to that obtained for the pure crystal structure.