Electronic structures and abnormal phonon behaviors of cobalt-modified Na0.5Bi0.5TiO3-6%BaTiO3 single crystals

T Huang,C Chen,L P Xu,P Zhang,J Z Zhang,H S Luo,J H Chu

doi:10.1063/1.4966910

Abstract

Optical properties, electronic structures, and structural variations of x wt% cobalt (Co) doped Na0.5Bi0.5TiO3-6%BaTiO3 (x=0%, 0.5%, 0.8%) single crystals have been studied by temperature-dependent optical ellipsometry and Raman spectra from 250 to 650 K. Based on the temperature evolution of electronic transitions (Ecp1 and Ecp2) and the phonon modes involving Ti-O vibrations, two critical temperature points exhibit an increasing trend with Co dopants, which are related to structural variations for ferroelectric to anti-ferroelectric, and anti-ferroelectric to paraelectric transition, respectively. Additionally, distinguishing abnormal phonon behaviors can be observed from Raman spectra for the crystal of x=0.5% and 0.8%, which show reverse frequency shift of the modes involving Ti-O vibration. It can be ascribed to different relative concentration of Co2+ and Co3+ in the crystals, which has been confirmed by X-ray Photoelectron Spectroscopy data.

Highlights

In recent years, lead-free ferroelectrics have attracted considerable attentions for the outstanding advantages in free control of sintering atmosphere and lacking of lead pollution during the process of preparation
The electronic transitions and phonon modes of x wt% Co doped Na0.5Bi0.5TiO36%BaTiO3 (x%-Co:NBBT6, x=0%, 0.5% and 0.8%) single crystals have been investigated by optical ellipsometry and Raman spectra as a function of temperature from 250 to 650 K, respectively
The second derivative of the numerically inverted complex dielectric function (NICDF) was fitted with the standard critical point (SCP) model to obtain the accurate energies Ecp and determine the temperature dependence of critical points (CPs) structures

Summary

INTRODUCTION

Lead-free ferroelectrics have attracted considerable attentions for the outstanding advantages in free control of sintering atmosphere and lacking of lead pollution during the process of preparation In this regard, Na0.5Bi0.5TiO3-based perovskites, which exhibit relatively large spontaneous polarization and high Curie temperature, are leading candidate that have been widely studied.[1,2,3,4,5] Among these, the Na0.5Bi0.5TiO3-x%BaTiO3 (NBBTx%) binary system possess a rhombohedral-tetragonal morphotropic phase boundary in the region of 6

EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

SUMMARY