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|Title: ||Light scattering study on phase transition and micro-heterogeneity in relaxor ferroelectrics (1-x)Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ and (1-x)Pb(Zn₁/₃Nb2₂/₃)O₃-xPbTiO₃|
|Other Titles: ||Ying yong guang san she fa yan jiu chi yu xing tie dian ti (1-x)Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ he (1-x)Pb(Zn₁/₃Nb2₂/₃)O₃-xPbTiO₃ de xiang bian yi ji wei guan bu jun yun xing|
應用光散射法研究馳豫型鉄電体 (1-x)Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ 和 (1-x)Pb(Zn₁/₃Nb2₂/₃)O₃-xPbTiO₃ 的相變以及微觀不均勻性
|Authors: ||Cheng, Juan (程娟)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2009|
|Publisher: ||City University of Hong Kong|
Light -- Scattering.
Phase transformations (Statistical physics)
|Notes: ||CityU Call Number: QC596.5 .C45 2009|
xix, 167 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 140-167)
|Abstract: ||(1-x)Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3
(PZN-xPT) single crystals are relaxor ferroelectrics and possess excellent piezoelectric
properties in the compositions near morphotropic phase boundary (MPB). However, the
origins of their outstanding behavior remain unclear up to date. In order to elucidate the
relationship between the complex perovskite structure of relaxor ferroelectrics and their
piezoelectric properties, polarized micro-Raman technique is used to systematically
investigate their temperature induced phase transitions and electric field induced
polarization rotations as well as micro-heterogeneities in PMN-xPT and PZN-xPT single
crystals. In addition, barium titanate BaTiO3 (BTO) is a prototype ferroelectric crystal
and rhodium-doped barium titanate (Rh:BTO) crystal has good photorefractive
performance. Brillouin scattering, similar to Raman scattering, is suitable for acoustic
phonons study and we also use Brillouin spectroscopy to determine a complete set of the
electro-elastic constants of Rh:BTO with low doping concentration.
The results are briefly summarized as follows.
1. Temperature dependence of polarized micro-Raman spectroscopy of -poled
PMN-0.33PT, <001>-cut unpoled PZN-0.08PT and <001>-cut PZN-0.045PT single
crystals are investigated and phase transitions are determined.
The polarized Raman spectra of the single crystal samples are observed in the
temperature range from -196°C to 350°C, even within the paraelectric cubic phase.
Group theory analysis and Raman selection rules are applied to determine the structures,
the observed phase transition sequences and the striking abnormality. Our finding is as
follows. In the case of poled PMN-0.33PT and unpoled PZN-0.045PT single crystals,
the 1:1 chemical order in the B-ion sublattice at low temperature (-196℃) leads to the
appearance of clusters with R3m (Z=2) symmetry. This, in turn, is responsible for the
presence of low temperatures Raman spectra of PMN-0.33PT and PZN-0.045PT single
crystals. With increasing temperature, PMN-0.33PT undergoes three structural phase
transitions, R→M(R)→T(M)→C(T) occurring at about 34℃, 114℃, and 144℃,
respectively; while PZN-0.045PT undergoes two structural phase transitions,
R→T(R)→C(T) occurring at about -15℃ and 140℃ respectively. The symmetry
structure of unpoled PZN-0.08PT is M at low temperatures and phase transitions
M →T(M)→C(T) take place at about 90℃ and 170℃ respectively. Here, in the
phase transition sequences, the symbol inside parenthesis is the symmetry structure of
the local polar-regions. The space group of the paraelectric cubic phase is Fm3m.
2. After the discovery of the M and O phases, it is believed that polarization
rotation through monoclinic distortions plays an important role in high piezoelectric
performance in the relaxor ferroelectrics. According to an eighth-order expansion from
Devonshire theory, there are potentially three types of monoclinic phases MA, MB, and
MC that can “bridge” the tetragonal <001>, rhombohedral <111> and orthorhombic
<110> directions in the polarization rotation paths respectively. In this study,
polarization rotation paths of PMN-0.30PT and PMN-0.33PT single crystals have been
revealed by the polarized Raman spectra under the E-field applied along C and
For the PMN-0.30PT single crystal, under an electric field loop of increasing and
decreasing applied along C, as well as increasing and decreasing applied along
C [1 10] , the detected polarization MA undergoes a rotation pathway of
010 110 010 111 001 M T M O M T M R M T A C C A B → → → → → → → → → which has the
critical fields of 2.85, 5.52, 9.75, 6.79 2.0, -4.67, -10.67 and -0.6kV/cm, respectively.
Here the minus in the values indicates the direction of E-field is along C [1 10] . When
E-field applied along C [1 10] is 10.67kV/cm, the polarization is rotated to R111
direction. For the PMN-0.33PT single crystal, under increasing and decreasing of the
E-field applied along C direction, the detected polarization T001 undergoes the
rotation path 001 111 010 T M R M T B A → → → → and the critical fields are 1.2, 2.6, and
3.2kV/cm, respectively. When the increasing E-field is 3.2kV/cm, the polarization is
rotated to R111 direction.
3. It is widely accepted that compositional heterogeneity is responsible for the
origin of the relaxor behaviors of the solid solutions PMN-xPT and PZN-xPT, especially
why macroscopically cubic single crystals can show Raman spectra. The local
composition varies in all macroscopically uniform crystals and local polar-regions
appear. The assumed heterogeneous regions in PMN-xPT and PZN-xPT single crystals
are now directly observed by Raman mapping technique. Local polar-regions in micron
dimension were observed and analyzed.
At room temperature, micro-heterogeneity can be observed in either <001>-cut,
<110>-cut, or <111>-cut PMN-xPT and PZN-xPT single crystals, which indicates that
the solid solutions PMN-xPT and PZN-xPT are intrinsically inhomogeneous. The R, M,
and T phases coexist in both PZN-0.08PT and PZN-0.045PT single crystals, and the
proportion area of T phase in PZN-0.08PT is larger than that in PZN-0.045PT single crystal; The R and M phases coexist in both (110)-oriented PMN-0.30PT and
(111)-oriented PMN-0.33PT single crystals, and the proportion area of R phase in
PMN-0.33PT is larger than that in PMN-0.30PT single crystal. In general, the large
proportion of certain phase will affect the observability of heterogeneity in a limited
area. The Ti concentration x-dependence of heterogeneity shows that the local properties
of PMN-xPT and PZN-xPT single crystals with MPB compositions are sensitive to the
relative Ti4+ occupancy of the B site. In addition, since the M phase always exists in the
PMN-xPT and PZN-xPT single crystals, there is no doubt that the M phase is
responsible for the high piezoelectric properties of relaxor ferroelectrics.
The temperature dependence of micro-heterogeneity of structure in PZN-0.08PT
single crystal provides a direct experimental evidence for the changing of heterogeneity
in relaxor ferroelectrics. Such changing is responsible for the broad dielectric relaxation
behavior as well as the broad temperature range of phase transitions. E-field induces a
structural evolution of micro-heterogeneity, which indicates a slow and gradual
polarization process. Under the same E-field strength, the polarization rotation depends
on the distance between the detected position and the electrodes, which indicates the
presence of the boundary effect of E-field and the existence of the multi-domain in the
4. Brillouin scattering is a spectroscopic method for excitations in the GHz region.
We study the anisotropic sound velocity of the high quality tetragonal Rh:BTO single
crystal at room temperature by Brillouin scattering. One sample is enough to provide a
complete set of elastic and piezoelectric coefficients of the (001)-oriented Rh:BTO
single crystal by Brillouin scattering, instead of several samples. The data obtained are
consistent to the previous studies on BTO single crystal, which indicates that small
amount of Rh ions in BTO almost do not change the crystal structure but affects the
electronic structure. The orientation dependencies of both the compressional and the
shear moduli of Rh:BTO in the (100) and (001) planes are presented. The velocity
related to shear (TA1) mode strongly depends on orientation and has a minimum along
(110). In addition, the intensities of TA1 and TA2 modes show a strong dependence on
the scattering angle.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b2374917|
|Appears in Collections:||AP - Doctor of Philosophy |
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