Mn SUBSTITUTION EFFECTS AND ASSOCIATED DEFECTS IN ZnO NANOPARTICLES STUDIED BY POSITRON ANNIHILATION

摘要

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn werensynthesized by using a modified ceramic route and characterized by X-ray diffraction, transmissionnelectron microscopy, selected area electron diffraction and energy dispersive X-ray analysisnmethods. Positron lifetime and coincidence Doppler broadening measurements were used asnprobes to identify the vacancy-type defects present in them and monitor the changes whilendoping. The predominant positron trapping center in the undoped ZnO is identified as thentrivacancy-type cluster VZn+O+Zn, which is negatively charged, and it transformed to the neutralndivacancy VZn+O on doping with Mn2+ ions. The intensity of the defect-specific positronnlifetime component got reduced initially indicating partial occupancy of the vacancies by thendoped cations but then recovered on further doping due to the additional Zn vacancies creatednas a result of the increasing strain introduced by the Mn ions of larger radius. The creation of annew phase ZnMn2O4 thereafter changed the course of variation of the annihilation parameters,nas the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sitesnof the spinel nanomanganite