Defects characterization and study of amorphous phase formation in xV2O5-(1-x)Nd2O3 binary glass nanocomposites using positron annihilation and correlated experimental techniques

A glass nanocomposite system with the combination of transition metal oxide and rare earth oxide of the general nomenclature xV2O5-(1-x)Nd2O3 for x = 0, 0.2, 0.4, 0.6 and 0.8 have been synthesized through the rapid quenching process and characterized by X-ray diffraction, transmission electron microscopy and UV-Vis absorption studies. Positron annihilation lifetime and coincidence Doppler broadening measurements are further employed to identify the vacancy or void-type defects existing in the composites of the different V2O5 concentrations (x). The X-ray diffraction patterns indicate distinct diffraction peaks with finite broadening in some of them signifying the co-existence of nanocrystallites and a certain level of amorphous phases in some of them. The crystal morphology, lattice fringes and the selected area electron diffraction patterns identified from the transmission electron microscopy images confirmed the nanocrystallite formation at lower V2O5 concentrations (x) and increasing amorphous character of the composite as x increased. The optical band gap energy has been estimated from the optical absorption spectra and it is found to reduce with increasing x. It is found from positron annihilation studies that a fraction of positrons are getting selectively confined in the interfacial gaps around the nanocrystallites for smaller values of x (i.e., 0, 0.2, 0.4 and 0.6). Porous defects surrounded by an amorphous glassy environment became the dominant trapping centres for the highest value of x (i.e., 0.8). The total free volume fraction decreases with x due to the interfacial gaps gradually disappear and give way to the generation of porous defects within the glassy matrix. The coincidence Doppler broadening spectroscopic measurements also established the transformation of the surroundings of the positron trapping defects.