Ab-initio study of the hyperfine parameters in P21/c, P42nmc and Fm3m zirconia phases doped with Tazr and the vacancy-Tazr complex

 In this work we develop selfconsistent calculations by means of the all-electron method NFP-LMTO. The electronic structure, quadrupolar frequencies and asymmetry parameters of ZrO2 polymorphs doped with Ta placed at substitutional site to Zr (TaZr), with and without vacancies are studied in the monoclinic, tetragonal and cubic phases. The calculated hyperfine parameters in neutral TaZr in the monoclinic phase are in agreement with hypine parameters measured with PAC and assigned to substitutional site in a wide range of temperatures. However, in the case of TaZr in the tetragonal P42nmc phase, the electric field gradient (EFG) is in large disagreement with the experimental assignment. Therefore we explored the incorporation of a near neighbor oxygen vacancy in several charged states. We found that the TaV0 and TaV+1 pairs in the tetragonal symmetry with axis length ratio c/a=1.02 gives electric field gradients Vzz and η in agreement with low-temperature values of the experimentally assigned pure tetragonal, called t-form. Further, the pair Ta-V with a ratio c/a=1 gives EFG in close agreement with reported high-temperature values.