We have studied structural, optical, electronic and magnetic properties of undoped well crystalline hexagonal and non-hexagonal ZnO system. X-ray diffraction (XRD) and high resolution transmission electron micrographs (HRTEM) of air annealed at 550 degrees C (sample-d) confirmed the presence of hexagonal (wurtzite) single phase ZnO. Raman analysis detected the vibrations of fundamental and second order phonons of Zn and oxygen related species. In Photoluminescence (PL) spectra, we observed that the intensity of UV peak decreases as annealing temperature increases. Broad PL visible band of samples shifted towards lower wavelength due to annealing effects. Fitting of broad PL spectra confirmed the existence of Zinc interstitial (Zn-i), Zinc vacancy (V-z(n)) and oxygen vacancy (V-o) defects. To determine the Zn interstitial and vacancy defects Auger peaks were de-convoluted from X-ray photoelectron spectroscopy (XPS) survey scan. Oxygen vacancies are estimated by fitting of asymmetric XPS O-1s spectra. Incomplete oxidation of magnetization versus temperature in the presence of applied field showed the presence of irreversibility in 550 degrees C annealed-sample. Zn clusters and secondary phases are may be reason of room temperature ferromagnetism in non-hexagonal samples (a-c). While, complex defect (zinc and oxygen vacancies) as observed in electronic and PL data may be responsible for inducing, promoting and stabilizing room temperature ferromagnetism in well crystalline hexagonal ZnO nanoparticles (sample-d). (C) 2018 Elsevier B.V. All rights reserved.