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Dissertation zugänglich unter
Structural transformations in complex perovskite-type relaxor and relaxor-based ferroelectrics at high pressures and temperatures
Strukturelle Phaenumwandlungen in komplexen perovsite Relaxoren und Relaxor basierten Ferroelektrika unter hohem Druck und Temperatur
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Freie Schlagwörter (Deutsch):
Ferroelektrische Phasenumwandlung , Raman-Spektroskopie , Röntgenbeugung , Relaxor , Perowskitstruktur
Freie Schlagwörter (Englisch):
perovksite-type relaxors , phase transitions , diffraction , Raman spectroscopy, DAC
38.31 , 38.30
Mihaylova, Boriana (Dr. Habil.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Perovskite-type (ABO3) relaxor ferroelectric crystals have perhaps the most simplest and at the same time the most puzzling structure.
Relaxor ferroelectrics are a special type of ferroelectrics possessing remarkably high dielectric permittivity, electrostrictive coefficients, and electrooptical constants related to the complex nanoscale structure of this type of materials.
These properties open a wide field of applications.
Thus, it is of great significance to understand the atomistic mechanism that tailors the desired properties.
Pb-based B-site complex perovskite-type materials commonly show relaxor behavior, which has been attributed to the existence of dynamic polar nanoregions, associated with local electric and/or elastic fields triggered by substitutional disorder. Recently long-range anti-ferrodistortive order has been detected at pressures up to 10GPa, which at ambient conditions coexists with the polar order on the mesoscopic scale. This may also be an important factor for the outstanding relaxor properties.\par
The objective of this study was to analyze in detail the role of the coexisting polar and anti-ferrodistortive coupling for the development of the relaxor state.
For this purpose first the pressure-induced structural changes up to 30GPa in pure and doped PbSc0.5Ta0.5O3 (PST) and PbSc0.5Nb0.5O3 (PSN) were studied by complementary Raman spectroscopy and XRD, in order to determine whether or not the anti-ferrodistortive order persists at high pressure up to 30GPa.
Next, to gain further insights into the competitive behavior between the mesoscopic polar and anti-ferrodistortive order in-situ high-temperature high-pressure Raman spectroscopy at temperatures above the Curie temperature has been applied, at which relaxors are in ergodic state.
Finally, the knowledge obtained from the structural investigations of relaxors was applied to study the ferroelectric structure of the solid solution PbZn1/3Nb2/3O3-xPbTiO3 with x = 0.1 by combined Raman scattering and x-ray diffraction (XRD) at ambient pressure and different temperatures from 800-100 K as well as at ambient temperature and different pressures up to 18GPa. The study intended to elucidate the origin of the giant piezoelectric effect close to the morphotropic phase boundary of this system.
The complementary XRD and Raman scattering analysis on pure PST and PSN up to 30GPa revealed that Pb-based relaxors exhibit four characteristic pressures:
(i) an intermediate pressure p*1 at which the off-centered Pb and B-cations in PNRs decouple while local anti-polar order of Pb cations as well as quasi-dynamical long-range order of anti-phase BO6 tilts is developed;
(ii) a critical pressure pC1 at which a continuous phase transition from relaxor-cubic to non-polar rhombohedral symmetry occurs, which is associated with anti-phase a-a-a- tilt order;
(iii) a second intermediate pressure p*2 at which the octahedral tilts around the three cubic axis become unequal from each other;
(iv) a second critical pressure pC2 at which a transition to a non-polar monoclinic or triclinic phase occurs, involving long-range ordering of anti-phase tilts with unequal magnitudes (a-b-b-) or long-range order of mixed BO6 tilts (a+b-b-) accompanied by long-range anti-polar Pb2+ order.
Doping on the A or B site changes the critical pressures. The enhancement of compositional disorder on the B site shifts pC2 to higher pressures, while the change in pC1 depends on the local structural distortions in the vicinity of the doping element.
Homovalent substitution of Pb2+ with cations having a larger ionic radius (Ba2+) induces local elastic fields and thus considerably shifts pC1 and pC2 to higher pressures and suppresses the development of long-range anti-polar Pb2+ ordering.
Lowering the tolerance factor by A-site doping favors the BO6 tilting and thus reduces pC1 and pC2.
However, in the case of heterovalent substitution of Pb2+ (La3+) the local B-site cation polar shifts persist to higher pressures due to the chemically-induced local electric fields, and thus the variation of the dopant concentration can tune the ratio between polar and anti-ferrodistortive order on the mesoscopic scale in order to vary the relaxor properties.
The in-situ high-pressure high-temperature Raman scattering data allowed me to construct a pT-phase diagram for Pb-based relaxors, which unambiguously confirms the coexistence of mesoscopic polar and anti-ferrodistortive order in perovskite-type relaxors and the proposed ferrielectric nature of the relaxor state. Using temperature and pressure as two separate tuning mechanisms, one can select a structural state with a certain degree of polar order (by changing temperature) or of anti-ferrodistortive order (by applying pressure). At elevated temperatures the first pressure-induced phase transition drops to lower pressures as the polar coupling is suppressed, which in turn facilitates the development of the mesoscopic anti-ferrodistortive order existing at ambient pressure into a long-range ordered anti-ferrodistortive state at high pressure.
Raman spectroscopy at different temperatures on PZN-0.1PT shows that there are two different intrinsic cubic states of Pb ions in regions with a local chemical order of the type Pb(B2+ 2/3B5+ 1/3)1/2B5+ 1/2O3:
(i) less abundant Pb1 ions surrounded by Nb5+ and
(ii) more abundant Pb2 ions surrounded by both Zn2+ and Nb5+.
The temperature dependence of the Raman spectra of PZN-0.1PT indicates that off-centered Pb2 ions induce coherent polar shifts of ferroelectrically active B-cations, which in turn leads to the off-centering of Pb1 ions.
Also, Raman data suggest the predominance of monoclinic-type ferroelectric domains over tetragonal-type domains in the room temperature structure of as-synthesized (unpoled) crystals.
At room temperature and high pressures PZN-0.1PT undergoes several structural transitions. Near 1GPa the preexisting multiphase (monoclinic + tetragonal) multidomain state changes to another, most probably, single-phase multidomain state. Near 2GPa the system reaches a cubic relaxor state, in which the polar order exists only on the mesoscopic scale, as polar nanoregions. At 5.9GPa PZN-0.1PT undergoes a reversible pressure-induced phase transition from a cubic to an anti-ferrodistortive phase comprising long-range ordered antiphase octahedral tilts, similar to all other Pb-based relaxors.
In the low-pressure range up to 1GPa twinning is strongly enhanced, while the ferroic deviation of the atomic structure is reduced.
This region coincides with the pressure range where the piezoelectric and electrochemical coefficients are decreased.
Thus, the large shear piezoelectric and electromechanical responses are related with the intrinsic structural complexity on the local scale, rather than with the domain texture.
Low levels of Ru doping on the B-site (Ru/(Zn+Nb+Ti) ~ 0.002) enlarges the temperature range where the tetragonal state is preferred at ambient pressures, but has a negligible effect on pressure.
The results of this study indicate the coexistence of polar and anti-ferrodistortive order on the mesoscopic scale as well as the doubling of the perovskite unit cell, which is typical of all Pb-based relaxors and relaxor-based solid solutions. The findings imply that the structure of the dynamic polar nanoregions is ferrielectric rather than ferroelectric in nature.
Hence, the structure of polar nanoregions can be chemically tailored by tuning the polar as well as the anti-ferrodistortive coupling.