| Relaxor-PT ferroeletrics have attracted considerable attention due to theirexcellent electromechanical properties (piezoelectric coefficient d33>2000pCN-1and very high electromechanical coupling factor k33>0.90). Theserelaxor-PT ferroelectrics have been extensively applied in various fields,including ultrasonic transducer, actuator, transformer, sensor, etc. Pressure, as animportant mean for tunning materials, can be used in relaxor-PT systems toadjust the structure and improve the properties of materials.In this thesis, some relaxor-PT crystals have been studied using in situ X-raydiffraction (XRD) and Raman spectroscopy techniques under high pressure orhigh temperature conditions. The impact of high temperature-pressure on averageand local structure has been investigated and discussed.Pb(Yb1/2Nb1/2)O3-PbTiO3(PYN-PT) crystals have been studied by usingsynchronous x-ray diffraction and Raman spectroscopy under hightemperature-pressure conditions. PYN-PT crystals remain in the tetragonal phaseup to approximately6.5GPa at room temperature, then transforms to aparaelectric cubic phase, which exhibits local disorder. By usingBirch-Murnaghan equation of state, the dependence of volume with pressure wasfitted, and the bulk modulus at low pressure and high pressure phases wereobtained. In addition, as increasing pressure the lattice of high pressure phasedecreases in a slower trend than that of low pressure phase. Based on the highpressure and high temperature XRD results, we present a pressure-temperaturephase diagram of PYN-PT which constraints the stability region of theferroelectric phase.BiScO3-0.66PbTiO3(BS-0.66PT) crystals have been investigated using insitu X-ray diffraction and Raman spectroscopy techniques under high pressure. Aphase transition occurred from P4mm toPm3_m space group, and the phasetransition process has been discussed. The unit cell volumes as a function ofpressure are fitted to a second-order Birch-Murnaghan equation of state, whichobtained the bulk moduli for the low pressure and high pressure phase. Theresults were also used to compare with classic ferroelectrics PbTiO3. In addition,Raman spectra show a new peak emerging at4.4GPa, demonstrating that localstructure has been changed between2.6GPa and4.4GPa. The results are in good agreement with those of XRD, and Raman spectra suggest that high pressurecubic phase could be asscoited with local disorder.Then, BS-PT crystals with MPB composition have been studied by in situhigh resolution X-ray diffraction (XRD) and Raman spectroscopy techniquesunder temperature-field. Mixing phases of monoclinic and tetragonal phases,refined by Rietveld method, were first discovered at the temperature range of100K-613K in BS-PT solid solution. With increasing temperature, two phasetransitions occurred, from mixing phases to pure tetragonal phase at633K thentransforming to cubic phase at713K.The structural evolution behavior of PIN-PMN-PT crystals has beeninvestiaged using synchronous X-ray diffraction and Raman scatteringtechniques under high pressure. PIN-PMN-PT underwent a phase transion at5.0GPa, transforming from tetragonal phase to cubic phase. Based onBirch-Murnaghan equation of state, the unit cell volumes as a fuction of pressurewere fitted and the bulk moduli at low pressure and high pressure phase wareobtained. Raman spectra exhibited the complex structure, which most of peaksremained at the highest pressure for the experiment, suggesting the symmetry ofnano domain exsiting in local structure is lower than that of PYN-PT.In situ structurual study of Mn modified PIN-PMN-PT crystals has beencarried out using synchronous X-ray diffraction and Raman spectroscopytechniques at high temperature and high pressure, respectively. With temperatureincreasing, one phase transition happened and the tetragonal phase transited tothe cubic phae at231°C, which is the very high curie temperature in ternarysysems and thus can be applied for the devices under high-temperature field. Inaddition, Raman spectra demonstrated the local structure is stable during thewhole heating progress. For the structural study under high pressure, the resultsof XRD demonstrated one phase transition occurred in Mn modifiedPIN-PMN-PT at5.3GPa and Raman bands also give evidence that electronicstructure has been changed, including not only new Raman band emerging, butalso some Raman active bands disappearing. By comparing with three relaxor-PTferroelectris, it is found that B-site ions play an important role in controlling thepressure of phase transion and the local structure. |
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