Core-shell nanostructure and its phase transitions in Pb(Mg 1/3Nb2/3)O3 by Raman spectroscopy and photoluminescence

Abstract

Raman scattering, photoluminescence, X-ray and dielectric studies were carried out in phase-pure relaxor ferroelectic Pb(Mg1/3Nb 2/3)O3 (PMN) polycrystalline sample. Low-wavenumber Raman scattering (LWRS) from surface acoustic phonon modes in the 5-40 cm-1 range was studied at various temperatures from 25 to 550°C. These spectra support the hypothesis of the core-shell nanostructure with a 1:1 ordered negatively charged nanocore surrounded by a positively charged nanoshell. These surface acoustic modes exist even up to 550°C, revealing the existence of the charged core-shell nanostructures, which is well beyond the dissolution temperature of the polar nanoclusters in this material. The diameter of the shell is predicted from the LWRS results as 2.9 nm for a 2 nm core. Optical Raman spectra of PMN from 20 to 1000 cm-1 at 10 and 300 K show overlapping, broad peaks. Deconvolution of the spectrum into 12 modes suggests that they may arise from a 1:1 ordered charged nanocore and polar nanoclusters. Temperature-dependent photoluminescence (PL) spectra of PMN reveal structural phase transitions in the nanoshell. The temperature behavior of the 1.57, 1.67 and 1.73 eV PL bands indicates a phase transition at 110 K from one crystalline phase to another of the nanoshell. The anomalous behavior of the 1.73 eV band intensity at 210 K is attributed to a transition from the crystalline to the amorphous phase in the nanoshell at 210 K. Thermal quenching of the 1.67 eV PL emission predicts a phonon mode at 1153 cm-1, which is identified in the Raman measurement. Copyright © 2007 John Wiley & Sons, Ltd.