Structural research of Li doped ZnO powders

In this article, a series of Li-doped ZnO powders was prepared by melt growth from salt mixtures and investigated by means of Mass-Spectrometry (MS), Scanning Electron Microscopy (SEM), Powder X-Ray Diffraction (PXRD), Raman, Photoluminescence (PL) and Nuclear Magnetic Resonance (NMR) spectroscopies. The MS found that the Li concentration fell into the range from 0.87 to 3.31 atomic % for the prepared Li-doped powders. The powders constituted of slightly elongated particles, covering broad range from hundred nanometres to couple micrometres, as shown by SEM. The PXRD showed that the particles of all powders crystallized in the wurtzite (ZnO) structure with no impurities. The Rietveld analysis found that Li occupied octahedral interstitial positions in the ZnO lattice, together with the contraction of the lattice parameters a and c by 0.09 % and 0.13 %, respectively, upon doping with 1 % of Li ions. The Raman spectra exhibited A₁^(LO) and E₁^(LO) peaks absent in the undoped sample, supporting the Li incorporation into the ZnO lattice. The PL spectra of Li-doped samples exhibited a visible (~2.13 eV) and near infrared (~1.62 eV) luminescent bands associated with intrinsic defects and Li–Li nanoclusters, respectively. The presence of octahedral Li site along with multiple scattered Li sites was proved with almost zero electric field gradient (EFG) originating from octahedral voids in the wurtzite and various non-zero EFG in course of NMR measurement. Finally, the conducted structural research confirmed incorporation of Li into octahedral interstitials of the ZnO lattice supporting possible formation of Li–Li nanoclusters luminescent in the near infrared range.