Spectra On Phase Transition Of Na_0.5Bi_0.5TiO₃-BaTiO₃ At Morphotropic Phase Boundary

Relaxors Na_0.5Bi_0.5TiO_3-x%BaTiO₃?NBBTx?,PbZri-xTixO3?PZT?and Pb?Mgi/3Nb2/3?O3-PbTiO3?PMN-PT?show morphotropic phase boundaries?MPB?in the composition phase diagrams,where the crystal structure changes abruptly and the piezoelectric response is maximal due to symmetry-allowed polarization rotation.Therefore,there is an increasing emphasis on lead-free ferroelectric morphotropic phase boundary?MPB?systems,for the reason that systems with MPB between different competing phases are correlated with enhanced piezoelectric properties.Although leadbased ferroelectrics,such as PZT and PMN-PT,are well studied and widely used from medical ultrasound or sonar to energy harvesting,they are environmentally undesirable due to the toxicity of lead.The lone-pair cation Bi³⁺ is regarded as a natural replacement candidate of Pb²⁺ owing to the electronic similarities.Because both of them have large ionic radius,large number of electrons and a lone electron pair,which contribute to the distortion of the structure and piezoelectric response.Therefore,there is a growing interest in Bi-containing perovskites,which has been triggered by the necessity to replace lead-based ferroelectrics.Na_0.5Bi_0.5TiO₃?NBT?adopts complex ABO3 perovskite with A-site being equally shared by two different cations(Na+ and Bi³⁺).The electrical properties of NBT can be improved by the formation of solid solution with BaTiO₃?BT?,that is Na_0.5Bi_0.5TiO₃-x%BaTiO₃?NBBTx?,which exhibits excellent electromechanical properties around the MPB region when 6%<x<8%.In view of the well established correlation between enhanced piezoelectric properties and MPB patterns,there is an increasing emphasis on lead-free ferroelectric MPB systems.As a result,the NBBTx has been explored for its unique structure property as a prototypical bismuth-based piezoceramic in numerous fundamental studies.Accordingly,the physical properties of NBBTx material,such as electrical and optical characteristics,should be thoroughly studied for potential device applications.Optical spectroscopy is a powerful non-destructive means for materials characterizations.With the aid of spectral analysis,one can determine optical constants,lattice dynamics,electronic band structures,electronic transitions of materials.When the materials are under different temperature,we can also conclude the evolution of phonon behavior,interband transitions with temperature and study the phase transformation.The main work and innovations of this dissertation are listed as follows:1.Ultraviolet-visible spectra of Na_0.5Bi_0.5TiO₃-6%BaTiO₃?NBBT6%-1?single crystal have been studied from 298-453 K,because the phase diagram of NBBTx is still uncertain at MPB region from dielectric and piezoelectric data,especially below 453 K.The abnormal changes of and spectral transmission and absorbance are observed at 356 K,379 K,and 423 K,which will be confirmed by Raman spectra and can be correlated with the structural variation.NBBT6%-1 single crystals were grown by Bridgman method.From the transmittance spectra at room temperature,the absorption edge is observed near 400 nm.And,the direct optical band gap is estimated approximately at 3.13 eV.On heating,the transmittance of NBBT6%-1 increases,and the absorption edge exhibits a red shift trend,which means the optical band gap decreasing.Moreover,the transmission at the wavelength of 650 and 800 nm are represented as a function of temperature,respectively,which gradually increases due to the reduction of optical loss caused by scattering.Also,the absorbance spectra was deduced accordingly at the two specific wavelength,which shows a decreasing trend on heating.However,abnormal changes of both the transmission and absorbance are observed at 356 K,379 K,and 423 K,which indicate structural variations may happen in this temperature interval.2.NBBT7%and x wt%Co doped Na_0.5Bi_0.5TiO₃-6%BaTiO₃?x-Co:NBBT6%,x=0%,0.5%and 0.8%?single crystals are studied by temperature-dependent optical ellipsometry.The second derivative of the complex dielectric functions reveals the all the crystals have two interband transitions?Ecp1 and Ecp2?.Depending on the temperature evolution of Ecp1 and Ecp2:as for NBBT7%,structural variations can appear near 60 K,150 K,and 240 K,respectively.While for x-Co:NBBT6,two abnormal temperature-points are found between 250 and 650 K,which exhibit an increasing trend with Co dopants.Temperature-dependent optical ellipsometry approach was applied on NBBT7%and Co-doped NBBT6%single crystal?x-Co:NBBT6%,x=0%,0.5%and 0.8%?.Distinguishing from conventional ellipsometry data analysis,deducting the surface roughness layer was considered to derive a more precise complex dielectric constant by a three-layer-model.Based on the second derivation of complex dielectric constant,critical points?CP?was obtained by SCP model fitting.In ultra-low temperature experiment NBBT7%,the variations of CPs?Ecp1 and Ecp2?present three abnormal temperature points,which are 60 K,150 K,and 240 K,respectively.As for x-Co:NBBT6%single crystals,Ecpi and Ecp2 presents discontinuity at 400 K and 500 K,respectively.With increasing the Co-doping?x=0.5%and 0.8%?,the two discontinuous point tend to increasing,too.It is believed that the crystal structure is highly related to these abnormal temperature points.3.Temperature dependent Raman scattering was performed on NBBT6%-1 in the region of 298-453 K.The aforementioned two abnormal temperature points 379 K and 423 K was proven by the variation of the phonon mode that involves the Ti-O vibration.The abnormal temperature points 60 K?150 K and 240 K revealed by ellipsometry on NBBT7%was also proven by Bi-related phonon mode analysis.Also,two critical temperature points of x-Co:NBBT6%are verified by analysing the varition of the phonon modes concerning Ti-O vibrations,which exhibits an increasing trend with Co dopants,and are related to structural variations for ferroelectric to anti-ferroelectric,and anti-ferroelectric to paraelectric transition,respectively.In addition,x-Co:NBBT6%presents Co-valence-related abnormal Raman peak shift in the valid experiment data.It can be ascribed to different relative concentration of Co²⁺ and Co³⁺ in the crystals,which has been confirmed by X-ray Photoelectron Spectroscopy?XPS?data.Two abnormal temperatures are revealed on NBBT6%between the region 298-453 K by analyzing Ti-related phonon mode.The result agrees with transmission spectra.The two temperatures are assigned as de-polar temperature Td-unpoled and transition?from ferroelectric to antiferroelectric?temperature TFE-AFE.Another abnormal temperature revealed by transmission spectra?356 K?could be related to freezing temperature TVF of polar nano domains.The Raman scattering of NBBT7%in ultra-low wavenumber region is corresponding to the vibration of off-centered Bi cation.The variation of frequency and full width at half maximum in region 4.2-300 K proved the result of ellipsometry data.The low-temperature structural variations of NBT-7%BT crystal can be associated with instability of the crystalline lattice driven by off-centered Bi ions,followed by the variations of polarizability of the unit cells.As for x-Co:NBBT6%,the abnormal temperatures on between 250-650 K revealed by ellipsometry were also proven by the variation of Ti-O vibration phonon mode in form of both frequency and relative peak area.In addition,distinguishing abnormal phonon behaviors can be observed from Raman spectra for the crystal of x=0.5%and 0.8%,which can be ascribed to different relative concentration of Co²⁺ and Co³⁺ in the crystals,as proven by the XPS data.In addition,the ceramics of the similar system xCr-BZCT?x=0%,1%,2%and 2.5%?were also studied in approach of temperature-dependent Raman spectra.The results can further confirm the reliability of the conclusion from single crystal Raman data.4.Based on Bruker Dimension Icon,Horiba Jobin Yvon LabRAM HR800,and THMSE 600 cooling/heating stage,one of in-situ AFM/PFM-Raman system was designed for temperature-dependent experiment.Although Raman spectra can provide important structural information by approach of phonon mode,more information,such as surface morphology and domain,is desired in the case that multiple phase coexist in the ferroelectrics of MPB.Unfortunately,the optical microscope in Raman instrument could not meet the requirement.Thus,a system combining AFM/PFM and Raman is on demond.This work proposed a schematic design for the system,which bring it into practice.