3.1. PbTiO3-based single crystals

PbTiO3 (PT)-based perovskite compounds are important multifunctional materials, which have been investigated in the last half century due to their controllable physical properties. Most recently, the research hotspot for PbTiO3-BiMeO3 ferroelectrics have stimulated much interest [14]. A range of compelling information on thermal expansion behavior and lattice dynamics of novel ferroelectric perovskite-type 0.62PbTiO3-0.38Bi(Mg0.5Ti0.5)O3 (PT-BMT) single crystal has been revealed by means of temperature-dependent X-ray diffraction and polarized Raman scattering. Figure 1 shows the polarized Raman spectra of 0.62PT-0.38BMT single

Figure 1. The polarized VH and VV Raman spectra of 0.62PT-0.38BMT single crystal at 80 K. The peak assignment is based on the individual vibration mode, which is fitted by Lorentz function (Figure reproduced with permission from [14]. Copyright 2016, Royal Society of Chemistry).

crystal at representative temperature of 80 K. It can be found that the Raman spectra is consistent with previous reports of the tetragonal crystal structure. Comparing to the theoretical calculations and experimental results, three main regions with different kinds of vibration in the lattice can be distinguished: (1) Low frequency below 150 cm<sup>1</sup> , which can be associated with vibrations of the perovskite A-site, involving Pb and Bi ions. (2) Frequency range 150–450 cm<sup>1</sup> , which is related to the Ti-O/Mg-O stretching vibrations. (3) High frequency bands above 450 cm<sup>1</sup> have all been associated with BO6 vibrations, namely the breathing and stretching modes of the oxygen octahedra. The Raman spectra suggest a certain degree of local B-site chemical order for the 0.62PT-0.38BMT single crystal with A-site and B-site simultaneously doping.

lead-based ferroelectric materials. Note that the physical properties of the single crystal have some linear changes near phase transition, for example the refractive index n, lattice parameters, and unit cell volume V. Thus, the abnormal increasing for the soft mode indicates some structure changes from the crystal Raman spectra. In the whole temperature range, two different slopes of

Figure 2. Temperature evolution of Raman intensities for 0.62PT-0.38BMT single crystal. The dashed rectangles mark the temperature ranges of the most pronounced changes (Figure reproduced with permission from [14]. Copyright 2016,

soft mode frequency induced by the local spontaneous polarization (PS) inside the PNRs [18–20]. Therefore, the various tendency of the Raman soft mode frequency with temperature agrees well with the PNRs in Pb-based ferroelectrics, in which unit cell volume deviates from linear thermal expansion at Burns temperature according to the XRD results in Figure 3. The current Raman study presents a novel method to characterize the relationship between the NTE and PS of

Figure 3. Temperature dependence of the lattice parameters and unit cell volume of the 0.62PT-0.38BMT crystal. FE(T) and PE(C) mean ferroelectric tetragonal phase and paraelectric cubic phase, respectively. The phonon frequency of soft mode as a function of temperature for 0.62PT-0.38BMT crystal. The solid lines indicate the different decrement of the soft mode with temperature (Figure reproduced with permission from [14]. Copyright 2016, Royal Society of Chemistry).

), which can be attributed to the variation of the

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lines are observed (0.011 and 0.030 cm<sup>1</sup> <sup>K</sup><sup>1</sup>

perovskite ferroelectric materials.

Royal Society of Chemistry).

In order to understand the relationship between negative thermal expansion (NTE) and lattice dynamic behavior for the 0.62PT-0.38BMT crystal, Figure 2 presents the polarized Raman spectra in different temperature point from 80 to 850 K. It can be found that the peak position and intensity of the polarized Raman phonon modes have a decreasing trend with increasing temperature, while the broadening of the Raman modes is observed, which is consist with the other ferroelectrics [17]. It should be pointed that around the temperature range of 600–700 K, the lowest phonon modes occurs (marked by the dashed rectangles). In XRD results, we can also found the similar anomalies range (different coefficient of thermal expansion values in Figure 3). Thus, there are some minor structural changes in the tetragonal PT-BMT crystal, which is corresponding to the change of the Raman phonon modes above 700 K. This phenomenon is the result of interaction by spontaneous volume ferroelectrostriction mechanism and the dynamics of the polar nanoregions (PNRs).

Figure 3 shows the different temperature Raman spectra with different changing trend. As we know, an abnormal phonon softening upon heating can be detected by high resolution Raman spectra if there is any phase transition change. From the 0.62PT-0.38BMT Raman spectra, one clear anomaly is observed at about 670 K, which is quite different from the phase transition temperature. We found the Curie temperature of this single crystal is located at about 800 K through the XRD pattern and dielectric permittivity experiment. The PNRs can play an important role in

Figure 2. Temperature evolution of Raman intensities for 0.62PT-0.38BMT single crystal. The dashed rectangles mark the temperature ranges of the most pronounced changes (Figure reproduced with permission from [14]. Copyright 2016, Royal Society of Chemistry).

crystal at representative temperature of 80 K. It can be found that the Raman spectra is consistent with previous reports of the tetragonal crystal structure. Comparing to the theoretical calculations and experimental results, three main regions with different kinds of vibration in the lattice can be

Figure 1. The polarized VH and VV Raman spectra of 0.62PT-0.38BMT single crystal at 80 K. The peak assignment is based on the individual vibration mode, which is fitted by Lorentz function (Figure reproduced with permission from

the Ti-O/Mg-O stretching vibrations. (3) High frequency bands above 450 cm<sup>1</sup> have all been associated with BO6 vibrations, namely the breathing and stretching modes of the oxygen octahedra. The Raman spectra suggest a certain degree of local B-site chemical order for the 0.62PT-

In order to understand the relationship between negative thermal expansion (NTE) and lattice dynamic behavior for the 0.62PT-0.38BMT crystal, Figure 2 presents the polarized Raman spectra in different temperature point from 80 to 850 K. It can be found that the peak position and intensity of the polarized Raman phonon modes have a decreasing trend with increasing temperature, while the broadening of the Raman modes is observed, which is consist with the other ferroelectrics [17]. It should be pointed that around the temperature range of 600–700 K, the lowest phonon modes occurs (marked by the dashed rectangles). In XRD results, we can also found the similar anomalies range (different coefficient of thermal expansion values in Figure 3). Thus, there are some minor structural changes in the tetragonal PT-BMT crystal, which is corresponding to the change of the Raman phonon modes above 700 K. This phenomenon is the result of interaction by spontaneous volume ferroelectrostriction mechanism

Figure 3 shows the different temperature Raman spectra with different changing trend. As we know, an abnormal phonon softening upon heating can be detected by high resolution Raman spectra if there is any phase transition change. From the 0.62PT-0.38BMT Raman spectra, one clear anomaly is observed at about 670 K, which is quite different from the phase transition temperature. We found the Curie temperature of this single crystal is located at about 800 K through the XRD pattern and dielectric permittivity experiment. The PNRs can play an important role in

perovskite A-site, involving Pb and Bi ions. (2) Frequency range 150–450 cm<sup>1</sup>

0.38BMT single crystal with A-site and B-site simultaneously doping.

, which can be associated with vibrations of the

, which is related to

distinguished: (1) Low frequency below 150 cm<sup>1</sup>

[14]. Copyright 2016, Royal Society of Chemistry).

256 Raman Spectroscopy

and the dynamics of the polar nanoregions (PNRs).

lead-based ferroelectric materials. Note that the physical properties of the single crystal have some linear changes near phase transition, for example the refractive index n, lattice parameters, and unit cell volume V. Thus, the abnormal increasing for the soft mode indicates some structure changes from the crystal Raman spectra. In the whole temperature range, two different slopes of lines are observed (0.011 and 0.030 cm<sup>1</sup> <sup>K</sup><sup>1</sup> ), which can be attributed to the variation of the soft mode frequency induced by the local spontaneous polarization (PS) inside the PNRs [18–20]. Therefore, the various tendency of the Raman soft mode frequency with temperature agrees well with the PNRs in Pb-based ferroelectrics, in which unit cell volume deviates from linear thermal expansion at Burns temperature according to the XRD results in Figure 3. The current Raman study presents a novel method to characterize the relationship between the NTE and PS of perovskite ferroelectric materials.

Figure 3. Temperature dependence of the lattice parameters and unit cell volume of the 0.62PT-0.38BMT crystal. FE(T) and PE(C) mean ferroelectric tetragonal phase and paraelectric cubic phase, respectively. The phonon frequency of soft mode as a function of temperature for 0.62PT-0.38BMT crystal. The solid lines indicate the different decrement of the soft mode with temperature (Figure reproduced with permission from [14]. Copyright 2016, Royal Society of Chemistry).

#### 3.2. Tungsten bronze niobate ferroelectrics

Disordered ferroelectric systems are of great interest from the fundamental as well as applications points of view. The uniaxial relaxor ferroelectric SBN, which is of particular interest due to its well-known pyroelectric, electro-optic, and non-linear optical properties, has some potential applications such as holographic data storage, and generation of photorefractive solitons [21]. The aim of this part is to investigate the lattice vibration of the SrxBa1<sup>x</sup>Nb2O6 ceramics with different Sr/Ba ratios studied by Raman spectroscopy. The physical mechanism of the ferroelectric to paraelectric phase transition is found from the soft mode variation with the temperature. Figure 4 shows Raman spectra of the SBN ceramics recorded at 150 K with different Sr compositions. The A1<sup>g</sup> mode corresponding to a stretch type vibration of the NbO6 octahedron decrease with increasing the Sr/Ba ratio. The Raman shift of the modes decreases from 643 and 612 cm<sup>1</sup> to 636 and 605 cm<sup>1</sup> , respectively. Another important feature observed in Figure 4 is that the lowest modes (labeled with "LM") at about 42 cm<sup>1</sup> can be detected, which is inferred to the soft mode of the SBN ceramics. The soft mode is eigenvector approximated the ionic displacements occurring at a crystallographic phase transition in the ferroelectric material.

From the temperature dependence of the Raman spectra for all SBN ceramics in the temperature range from 150 to 750 K, the temperature evolution of the soft mode peak position is presented in Figure 5. The frequency of the soft modes decreases from 42 to 38 cm<sup>1</sup> with the Sr composition at 150 K. The phenomenon can be attributed to the A site substitution originated in the smaller ionic radius of Sr, as compared to Ba element. All frequency of the soft mode first decreases and then disappears in the SBN paraelectric phase. The TC of the SBN ceramics shifts to lower temperature with increasing Sr composition. It can be well expressed by TC (x) = 566 371x, where x is the Sr molar fraction. According to changes of the soft modes, it is clear that increasing Sr composition leads to the shrinking of TC.

3.3. Aurivillius-type bismuth layer-structured ferroelectrics

temperature points where the soft mode vanishes.

Recently, bismuth layer-structured ferroelectrics (BLSFs) have gained increasing attention for the promising applications of non-volatile random access memories (NvRAMs), optical switches and high-temperature piezoelectric devices, because of their relatively high Tc, low dielectric dissipation, and excellent hysteresis behavior [22–25]. Calcium bismuth niobate (CaBi2NbO9, CBN) is an Aurivillius layered material with the ultra-high Tc and relative higher thermal depoling temperature. The different properties induced by doping in BLSFs are always related to the structure distortions. The spectra of vibration modes in different frequencies could be sensitive to describe the structural distortion. As an example, Figure 6 shows temperature-dependent Raman spectra of CaBi2Nb1.97W0.03O9 (W3) ceramic and the CBNW ceramics with different W compositions at 100 K. The results suggest that the peak center of ν5 deviated from the vertical dash line, and the color (in the web version) which represented the intensity of phonon mode faded with increasing temperature. This phenomenon means that the ν5 phonon mode was soften. However, there is no obvious change in the ν6 phonon mode upon heating, as compared to the behaviors of ν5. The similar Raman spectra with different W

Figure 5. The soft mode frequency as a function of the temperature for the SBN ceramics. The dashed arrows show the

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compositions also confirm that CBN doped with tungsten can keep single phase.

To further investigate the evolution of the ν5 and ν6 phonon modes, Raman spectra are wellfitted with multi-Lorentz oscillators. Figure 7 shows Raman scattering results and well-fitted peaks with multi-Lorentz oscillators for all samples at 100, 300 and 800 K. The frequency, intensity and full width at half maximum of each phonon mode at different temperature can be derived from the fitting. The frequencies of the ν5 and ν6 phonon modes for W3 are presented in Figure 8. The shift of phonon modes can be explained by a simplified Klemens model [26].

From the fitting result according to the model, we can conclude that the change of the Raman phonon mode is dominated by the lattice expansion and thermal evolution. However, the intrinsic anharmonic coupling of phonons is quite weak for ν5. We can observe a slightly blue shift for ν6 phonon mode, while an opposite trend for ν5 in the whole temperature range. The

Figure 4. (a) Experimental Raman spectrum (dotted lines) and Lorentzian fitting results (solid lines) of the Sr0.3Ba0.7Nb2O6 ceramic at 150 K. The arrows indicate that the lowest phonon mode is located at about 40 cm<sup>1</sup> (labeled with "LM"). (b) Raman spectra of the SBN ceramics with different Sr composition recorded at 150 K. (c) The A1g Raman-active phonon mode at about 636 and 605 cm<sup>1</sup> as a function of Sr composition.

Figure 5. The soft mode frequency as a function of the temperature for the SBN ceramics. The dashed arrows show the temperature points where the soft mode vanishes.

#### 3.3. Aurivillius-type bismuth layer-structured ferroelectrics

3.2. Tungsten bronze niobate ferroelectrics

decreases from 643 and 612 cm<sup>1</sup> to 636 and 605 cm<sup>1</sup>

mode at about 636 and 605 cm<sup>1</sup> as a function of Sr composition.

ferroelectric material.

258 Raman Spectroscopy

Disordered ferroelectric systems are of great interest from the fundamental as well as applications points of view. The uniaxial relaxor ferroelectric SBN, which is of particular interest due to its well-known pyroelectric, electro-optic, and non-linear optical properties, has some potential applications such as holographic data storage, and generation of photorefractive solitons [21]. The aim of this part is to investigate the lattice vibration of the SrxBa1<sup>x</sup>Nb2O6 ceramics with different Sr/Ba ratios studied by Raman spectroscopy. The physical mechanism of the ferroelectric to paraelectric phase transition is found from the soft mode variation with the temperature. Figure 4 shows Raman spectra of the SBN ceramics recorded at 150 K with different Sr compositions. The A1<sup>g</sup> mode corresponding to a stretch type vibration of the NbO6 octahedron decrease with increasing the Sr/Ba ratio. The Raman shift of the modes

observed in Figure 4 is that the lowest modes (labeled with "LM") at about 42 cm<sup>1</sup> can be detected, which is inferred to the soft mode of the SBN ceramics. The soft mode is eigenvector approximated the ionic displacements occurring at a crystallographic phase transition in the

From the temperature dependence of the Raman spectra for all SBN ceramics in the temperature range from 150 to 750 K, the temperature evolution of the soft mode peak position is presented in Figure 5. The frequency of the soft modes decreases from 42 to 38 cm<sup>1</sup> with the Sr composition at 150 K. The phenomenon can be attributed to the A site substitution originated in the smaller ionic radius of Sr, as compared to Ba element. All frequency of the soft mode first decreases and then disappears in the SBN paraelectric phase. The TC of the SBN ceramics shifts to lower temperature with increasing Sr composition. It can be well expressed by TC (x) = 566 371x, where x is the Sr molar fraction. According to changes of the soft

Figure 4. (a) Experimental Raman spectrum (dotted lines) and Lorentzian fitting results (solid lines) of the Sr0.3Ba0.7Nb2O6 ceramic at 150 K. The arrows indicate that the lowest phonon mode is located at about 40 cm<sup>1</sup> (labeled with "LM"). (b) Raman spectra of the SBN ceramics with different Sr composition recorded at 150 K. (c) The A1g Raman-active phonon

modes, it is clear that increasing Sr composition leads to the shrinking of TC.

, respectively. Another important feature

Recently, bismuth layer-structured ferroelectrics (BLSFs) have gained increasing attention for the promising applications of non-volatile random access memories (NvRAMs), optical switches and high-temperature piezoelectric devices, because of their relatively high Tc, low dielectric dissipation, and excellent hysteresis behavior [22–25]. Calcium bismuth niobate (CaBi2NbO9, CBN) is an Aurivillius layered material with the ultra-high Tc and relative higher thermal depoling temperature. The different properties induced by doping in BLSFs are always related to the structure distortions. The spectra of vibration modes in different frequencies could be sensitive to describe the structural distortion. As an example, Figure 6 shows temperature-dependent Raman spectra of CaBi2Nb1.97W0.03O9 (W3) ceramic and the CBNW ceramics with different W compositions at 100 K. The results suggest that the peak center of ν5 deviated from the vertical dash line, and the color (in the web version) which represented the intensity of phonon mode faded with increasing temperature. This phenomenon means that the ν5 phonon mode was soften. However, there is no obvious change in the ν6 phonon mode upon heating, as compared to the behaviors of ν5. The similar Raman spectra with different W compositions also confirm that CBN doped with tungsten can keep single phase.

To further investigate the evolution of the ν5 and ν6 phonon modes, Raman spectra are wellfitted with multi-Lorentz oscillators. Figure 7 shows Raman scattering results and well-fitted peaks with multi-Lorentz oscillators for all samples at 100, 300 and 800 K. The frequency, intensity and full width at half maximum of each phonon mode at different temperature can be derived from the fitting. The frequencies of the ν5 and ν6 phonon modes for W3 are presented in Figure 8. The shift of phonon modes can be explained by a simplified Klemens model [26].

From the fitting result according to the model, we can conclude that the change of the Raman phonon mode is dominated by the lattice expansion and thermal evolution. However, the intrinsic anharmonic coupling of phonons is quite weak for ν5. We can observe a slightly blue shift for ν6 phonon mode, while an opposite trend for ν5 in the whole temperature range. The

Figure 6. (a) Temperature-dependence of Raman spectra of CaBi2Nb1.97W0.03O9 (W3) ceramics from 100 to 800 K. (b) Raman scattering of CBNW ceramics with different W compositions at 100 K (Figure reproduced with permission from [22]. Copyright 2015, Elsevier).

[(ν5) + I(ν6)] ratio increase with increasing temperature. The result is that the distortion degrees of perovskite (Nb, W)O6 octahedra decrease with increasing temperature [27], and the BO6 octahedra distortion predominates the structure distortion with Aurivillius niobate phase [28]. Thus, it can be concluded that the structure distortion in CBNW ceramics decreases with increasing W composition. The variation tendency is consistent with the XRD result in previous study. From the change of the Raman phonon modes with different temperature, we can see the parameter TC generally decreases with the decrease of structure distortion degree [29, 30]. It was found that the relative peak intensity of I(ν6)/I[(ν5) + I(ν6)] ratio is sensitive for the distortion degree in ferroelectric materials. The Curie temperature of CaBi2Nb2xWxO9 decreases with increasing composition from the Raman results, which is consistent with the results of dielectric permittivity experiments. The results indicate that the Raman spectrum is an effective

compositions (Figure reproduced with permission from [22]. Copyright 2015, Elsevier).

Figure 8. (a) Temperature-dependence of Raman phonon mode of ν5 (squares) and ν6 (trilateral). The inset shows the W3 Raman spectra at 100 K using Lorentz fitting. (b) the ratio of I(ν6)/I[(ν5) + I(ν6)] intensity with different W doping

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tool for detecting structure distortion and phase transition of ferroelectric materials.

In the past few decades, the complex Pb-based ABO3 perovskite materials have attracted much attentions due to the excellent properties obtained in the compositions close to morphotropic phase boundary (MPB) [31]. The A-site substitution plays an important role in phase transition and more studies are requisite. In this part, the A-site substitution effect on the phase transition near MPB is investigated for PLZST ceramics. Transition temperature region and lattice dynamics are systematically discussed according to the temperature dependent Raman scattering spectroscopy. In addition, a new transient phase called the intermediate phase was found to exist between AFE and PE phase, which could be induced by defects through increasing

Figure 9 depicts room-temperature Raman scattering results and well-fitted deconvolution peaks for all samples. A sharp increase of relative strength of E(TO2) mode could be observed

3.4. ABO3 perovskite ceramics

temperature and doping of foreign ions.

Figure 7. Experimental (dots) and the best fitting (solid lines) Raman spectra of CaBi2Nb2-xWxO9 ceramics with (a) 0, (b) 0.01, (c) 0.03, and (d) 0.05 at 100, 300 and 800 K, respectively (Figure reproduced with permission from [22]. Copyright 2015, Elsevier).

different changes between the phonon modes may be ascribed to the unusually strong and positive intrinsic anharmonicity instead of thermal evolution with increasing temperature. On the other hand, for all of the ceramics with increasing W composition, we found that the I(ν6)/I

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Figure 8. (a) Temperature-dependence of Raman phonon mode of ν5 (squares) and ν6 (trilateral). The inset shows the W3 Raman spectra at 100 K using Lorentz fitting. (b) the ratio of I(ν6)/I[(ν5) + I(ν6)] intensity with different W doping compositions (Figure reproduced with permission from [22]. Copyright 2015, Elsevier).

[(ν5) + I(ν6)] ratio increase with increasing temperature. The result is that the distortion degrees of perovskite (Nb, W)O6 octahedra decrease with increasing temperature [27], and the BO6 octahedra distortion predominates the structure distortion with Aurivillius niobate phase [28]. Thus, it can be concluded that the structure distortion in CBNW ceramics decreases with increasing W composition. The variation tendency is consistent with the XRD result in previous study. From the change of the Raman phonon modes with different temperature, we can see the parameter TC generally decreases with the decrease of structure distortion degree [29, 30]. It was found that the relative peak intensity of I(ν6)/I[(ν5) + I(ν6)] ratio is sensitive for the distortion degree in ferroelectric materials. The Curie temperature of CaBi2Nb2xWxO9 decreases with increasing composition from the Raman results, which is consistent with the results of dielectric permittivity experiments. The results indicate that the Raman spectrum is an effective tool for detecting structure distortion and phase transition of ferroelectric materials.

#### 3.4. ABO3 perovskite ceramics

different changes between the phonon modes may be ascribed to the unusually strong and positive intrinsic anharmonicity instead of thermal evolution with increasing temperature. On the other hand, for all of the ceramics with increasing W composition, we found that the I(ν6)/I

Figure 7. Experimental (dots) and the best fitting (solid lines) Raman spectra of CaBi2Nb2-xWxO9 ceramics with (a) 0, (b) 0.01, (c) 0.03, and (d) 0.05 at 100, 300 and 800 K, respectively (Figure reproduced with permission from [22]. Copyright

Figure 6. (a) Temperature-dependence of Raman spectra of CaBi2Nb1.97W0.03O9 (W3) ceramics from 100 to 800 K. (b) Raman scattering of CBNW ceramics with different W compositions at 100 K (Figure reproduced with permission from

[22]. Copyright 2015, Elsevier).

260 Raman Spectroscopy

2015, Elsevier).

In the past few decades, the complex Pb-based ABO3 perovskite materials have attracted much attentions due to the excellent properties obtained in the compositions close to morphotropic phase boundary (MPB) [31]. The A-site substitution plays an important role in phase transition and more studies are requisite. In this part, the A-site substitution effect on the phase transition near MPB is investigated for PLZST ceramics. Transition temperature region and lattice dynamics are systematically discussed according to the temperature dependent Raman scattering spectroscopy. In addition, a new transient phase called the intermediate phase was found to exist between AFE and PE phase, which could be induced by defects through increasing temperature and doping of foreign ions.

Figure 9 depicts room-temperature Raman scattering results and well-fitted deconvolution peaks for all samples. A sharp increase of relative strength of E(TO2) mode could be observed

Figure 9. Raman scattering spectra of Pb11.5xLaxZr0.42Sn0.4Ti0.18O3 ceramics with different La composition recorded at room temperature. The inset shows the frequency variation of A1(TO1) mode and relative strength of E(TO2) mode as a function of La composition (Figure reproduced with permission from [16]. Copyright 2013, American Institute of Physics).

at La composition of 2.8%. This indicates that PLZST3.4 ceramic is more likely to be tetragonal phase rather than the rhombohedral phase in the PLZST2 ceramic. The softing peak upon La composition is assigned as A1(TO1) symmetry, which stems from splitting of T1u in cubic phase. The mode softs from 123.4 (PLZST2) to 116.3 cm<sup>1</sup> (PLZST3.4) with an incommensurate drop at La composition of 2.6%. The fact that both of two dramatic changes occur indicates the phase transition from rhombohedral to tetragonal structure for La composition between 2.6 and 2.8%.

To elucidate the thermal evolution of PLZST ceramics, temperature dependence of Raman spectra from 100 to 650 K are shown in Figure 10. We can see all of the Raman modes for the four PLZST ceramics have a blue shift with increasing temperature. Note that some of them disappear above 400 K during the cubic phase appearing. The results indicate that there is a structure transformation at about 400 K. Similar abnormal decrease in Raman intensity of PLZST ceramics has been detected by the spectra with increasing temperature.

To further investigate the phase transition mechanism, we plot temperature dependence of the wavenumber from the Raman modes in Figure 11. It can be found that some abrupt variations in the whole temperature range, which is easily divided into two phase transition regions. Note that there is a new intermediate phase at about 300 K for all of samples.

Figure 12 shows the phase diagram of PLZST ceramics according to the temperature dependence of Raman spectra. Three different structure phases can be found: AFEO phase, intermediate phase, and PEC phase. We can found that the PLZST ceramics have a transformation from AFEO to the intermediate phase with the composition of Ti exceeds 5.0% at room temperature. However, all of the ceramics remain cubic phase when the temperature upon 450 K. Note that the PLZST ceramics undergo successive phase transitions with increasing

Figure 10. Temperature dependent Raman spectra for PLZST ceramics from 100 to 650 K. The symbol (\*) represents the

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low wavenumber phonon modes (Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

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at La composition of 2.8%. This indicates that PLZST3.4 ceramic is more likely to be tetragonal phase rather than the rhombohedral phase in the PLZST2 ceramic. The softing peak upon La composition is assigned as A1(TO1) symmetry, which stems from splitting of T1u in cubic phase. The mode softs from 123.4 (PLZST2) to 116.3 cm<sup>1</sup> (PLZST3.4) with an incommensurate drop at La composition of 2.6%. The fact that both of two dramatic changes occur indicates the phase transition from rhombohedral to tetragonal structure for La composition between 2.6 and 2.8%. To elucidate the thermal evolution of PLZST ceramics, temperature dependence of Raman spectra from 100 to 650 K are shown in Figure 10. We can see all of the Raman modes for the four PLZST ceramics have a blue shift with increasing temperature. Note that some of them disappear above 400 K during the cubic phase appearing. The results indicate that there is a structure transformation at about 400 K. Similar abnormal decrease in Raman intensity of

La composition (Figure reproduced with permission from [16]. Copyright 2013, American Institute of Physics).

262 Raman Spectroscopy

Figure 9. Raman scattering spectra of Pb11.5xLaxZr0.42Sn0.4Ti0.18O3 ceramics with different La composition recorded at room temperature. The inset shows the frequency variation of A1(TO1) mode and relative strength of E(TO2) mode as a function of

To further investigate the phase transition mechanism, we plot temperature dependence of the wavenumber from the Raman modes in Figure 11. It can be found that some abrupt variations in the whole temperature range, which is easily divided into two phase transition regions.

Figure 12 shows the phase diagram of PLZST ceramics according to the temperature dependence of Raman spectra. Three different structure phases can be found: AFEO phase, intermediate phase, and PEC phase. We can found that the PLZST ceramics have a transformation from AFEO to the intermediate phase with the composition of Ti exceeds 5.0% at room temperature. However, all of the ceramics remain cubic phase when the temperature upon 450 K. Note that the PLZST ceramics undergo successive phase transitions with increasing

PLZST ceramics has been detected by the spectra with increasing temperature.

Note that there is a new intermediate phase at about 300 K for all of samples.

Figure 10. Temperature dependent Raman spectra for PLZST ceramics from 100 to 650 K. The symbol (\*) represents the low wavenumber phonon modes (Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

Figure 11. The variations of A1(TO1), AFD, E(TO1), and AHC modes as a function of temperature for PLZST ceramics with different Ti composition. Note that the dashed lines show the boundaries of two adjacent phase structures (Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

peak. From the analysis of the mode position with the temperature, the anomalies in phonon

Figure 12. The phase diagram of PLZST ceramics based on the low wavenumber phonon mode variations with the temperature and Ti composition. The phase transformation regions of AFEO, intermediate phase and PEC characteristics can be clearly distinguished by the solid dots (Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

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Figure 14 shows composition dependence of the local structural transition temperature (T\*) and Neel temperature (TN). With the increase in the doping compositions of La and Ti, T\* and TN decrease from 580 to 540 K and from 645 to 630 K, respectively. According to La and Ti-codoped in BFO, an external pressure is induced due to size mismatch of host (Bi, Fe) and substitution (La, Ti) cations, which leads to the variation of tolerance factor and structural distortions. On the other hand, the strength of the antiferromagnetic superexchange interaction relies on the Fe-O-Fe angle. These structural effects weaken the magnetic exchange and decrease TN. Thus, a complicated mechanism induced by the variation of bond length, bond angle, and the exchange interaction between adjacent magnetic moments could substantially contribute to the shrinking of Neel temperature. In summary, the Raman phonon modes have abnormal change around 140 and 205 K, which can be ascribed to the strong magnon-phonon coupling. The structural transition occurred at about 570 K can be detected by the Raman spectra, indicating that the

Recently, KNN based lead-free materials were reported to offer comparable piezoelectric properties to that of PZT [35]. The (K0.5Na0.5)NbO3-0.05LiNbO3-yMnO2 (y = 0 and 1.0%) (KNN-LN-M) single crystals have been studied by the Raman spectra from the temperature 300–800 K. Moreover, the thermotropic phase boundaries are observed, indicating the existence of the mixed-phase region (i.e. PPT) between orthorhombic (O) and tetragonal (T)

modes suggest a strong magnon-phonon coupling of BLFTO in the present work.

Raman modes are sensitive to the structure changes and spin reorientation.

3.6. Lead-free ferroelectrics

phases.

temperature: from antiferroelectric phase at low temperature to intermediate phase and paraelectric cubic phase at high temperature. The Curie temperature for PLZST ceramics is lower than that of pure PZT materials because of some ions doping [32, 33]. Note that the intermediate boundaries in the phase diagram have a downward trend with Ti composition. However, the cubic phase boundaries almost remain the same temperature of about 400 K. The intermediate phase temperature ranges gradually become larger with increasing Ti compositions. The Curie temperature detected by Raman spectra are consist with the XRD and dielectric results, which suggests that the Raman spectrum is an effective tool to distinguish different phase structure in PZT ceramics with different temperature and doping level.

#### 3.5. Multiferroics materials

BiFeO3 (BFO) has been hailed as an important material for magnetoelectric devices due to its room-temperature multiferroic properties, in which the electric polarization is coupled to antiferromagnetic (AFM) order, allowing for manipulation of magnetism by applied electric fields and vice versa [34]. During the last several years, intriguing behavior was found in doped BFO compounds. In this part, we present Raman results on La and Ti codoped BFO ceramics to systematically study phase transitions induced by the chemical substitution and temperature.

Figure 13 demonstrates the spectra recorded at several characteristic temperatures and wellfitted deconvolution peaks at 80 K. The frequencies of the phonon modes shift to low energies and the intensity of all major peaks reduces as the temperature increases from 80 to 680 K. The modes in higher-frequency range become severely widening and merge into a broadening

Figure 12. The phase diagram of PLZST ceramics based on the low wavenumber phonon mode variations with the temperature and Ti composition. The phase transformation regions of AFEO, intermediate phase and PEC characteristics can be clearly distinguished by the solid dots (Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

peak. From the analysis of the mode position with the temperature, the anomalies in phonon modes suggest a strong magnon-phonon coupling of BLFTO in the present work.

Figure 14 shows composition dependence of the local structural transition temperature (T\*) and Neel temperature (TN). With the increase in the doping compositions of La and Ti, T\* and TN decrease from 580 to 540 K and from 645 to 630 K, respectively. According to La and Ti-codoped in BFO, an external pressure is induced due to size mismatch of host (Bi, Fe) and substitution (La, Ti) cations, which leads to the variation of tolerance factor and structural distortions. On the other hand, the strength of the antiferromagnetic superexchange interaction relies on the Fe-O-Fe angle. These structural effects weaken the magnetic exchange and decrease TN. Thus, a complicated mechanism induced by the variation of bond length, bond angle, and the exchange interaction between adjacent magnetic moments could substantially contribute to the shrinking of Neel temperature. In summary, the Raman phonon modes have abnormal change around 140 and 205 K, which can be ascribed to the strong magnon-phonon coupling. The structural transition occurred at about 570 K can be detected by the Raman spectra, indicating that the Raman modes are sensitive to the structure changes and spin reorientation.

#### 3.6. Lead-free ferroelectrics

temperature: from antiferroelectric phase at low temperature to intermediate phase and paraelectric cubic phase at high temperature. The Curie temperature for PLZST ceramics is lower than that of pure PZT materials because of some ions doping [32, 33]. Note that the intermediate boundaries in the phase diagram have a downward trend with Ti composition. However, the cubic phase boundaries almost remain the same temperature of about 400 K. The intermediate phase temperature ranges gradually become larger with increasing Ti compositions. The Curie temperature detected by Raman spectra are consist with the XRD and dielectric results, which suggests that the Raman spectrum is an effective tool to distinguish different

Figure 11. The variations of A1(TO1), AFD, E(TO1), and AHC modes as a function of temperature for PLZST ceramics with different Ti composition. Note that the dashed lines show the boundaries of two adjacent phase structures

BiFeO3 (BFO) has been hailed as an important material for magnetoelectric devices due to its room-temperature multiferroic properties, in which the electric polarization is coupled to antiferromagnetic (AFM) order, allowing for manipulation of magnetism by applied electric fields and vice versa [34]. During the last several years, intriguing behavior was found in doped BFO compounds. In this part, we present Raman results on La and Ti codoped BFO ceramics to systematically study phase transitions induced by the chemical substitution and temperature.

Figure 13 demonstrates the spectra recorded at several characteristic temperatures and wellfitted deconvolution peaks at 80 K. The frequencies of the phonon modes shift to low energies and the intensity of all major peaks reduces as the temperature increases from 80 to 680 K. The modes in higher-frequency range become severely widening and merge into a broadening

phase structure in PZT ceramics with different temperature and doping level.

(Figure reproduced with permission from [31]. Copyright 2016, Elsevier).

3.5. Multiferroics materials

264 Raman Spectroscopy

Recently, KNN based lead-free materials were reported to offer comparable piezoelectric properties to that of PZT [35]. The (K0.5Na0.5)NbO3-0.05LiNbO3-yMnO2 (y = 0 and 1.0%) (KNN-LN-M) single crystals have been studied by the Raman spectra from the temperature 300–800 K. Moreover, the thermotropic phase boundaries are observed, indicating the existence of the mixed-phase region (i.e. PPT) between orthorhombic (O) and tetragonal (T) phases.

For a closer inspection, the peak positions of all modes in each phase are plotted in Figure 16 against the temperature between 300 and 800 K. Frequency shifts and transformations of the modes are clear at the O-T and T-C boundaries. The trends within each phase consist primarily of mode softening with increasing temperature, exactly as expected to follow from the lattice thermal expansion. In the light of all the results, displacive and order–disorder mechanisms have to be associated in the description of the phase transitions. Thus, the successive phase transitions in KNN-LN based single crystals derive from competition between a soft phonon

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In order to obtain the phase fraction, we assume that one phase at thermal phase boundaries is a linear superposition of the spectra below and above phase boundaries. As shown in Figure 17, the fitting Raman spectra at 410–440 K in KNN-LN and KNN-LN-1%MnO2, respectively, are displayed in comparison with the observed spectra at the same temperature. The fitting spectra are obtained from the linear superposition of the Raman spectra below and above phase boundaries. The coincidence of the experimental and the fitted Raman spectra are well for both samples, which strongly confirms the coexistence of the O and T phases at the thermal phase boundary from O to T phase. The mixed structure state at the thermal phase boundaries is due to sufficient competing mechanical and dipolar interactions between domains in multi-domain configurations [38]. In single domain case, the system undergoes a series of first-order ferroelectric transitions upon heating, sequentially adopting the O and T ferroelectric phases before

Figure 15. Temperature dependence of Raman scattering for the KNN-LN based single crystals collected in the temperature range from 300 to 800 K and Lorentzian-shaped deconvolution at the temperature of 300 K. The arrows are applied to separate different phase transition (Figure reproduced with permission from [35]. Copyright 2015, American Institute

mode and a relaxation mode.

reverting to the C parent phase.

of Physics).

Figure 13. Temperature dependence of Raman scattering for the BLFTO ceramics collected in the temperature range from 80 to 680 K. As an example, Raman spectra recorded at 80 K, which were fitted with independent damped harmonic oscillators, have been indicated on the bottom (Figure reproduced with permission from [34]. Copyright 2014, American Institute of Physics).

Figure 14. Composition dependence of phase transition temperature T\* and Neel temperature TN for the BLFTO ceramics derived from temperature dependent Raman scattering (Figure reproduced with permission from [34]. Copyright 2014, American Institute of Physics).

Figure 15 depicts temperature dependence of Raman scattering for the KNN-LN and KNN-LN-1%MnO2 crystals, respectively. The phase structure of KNN-LN based crystals transforms from O phase to T phase, then to cubic (C) phase with increasing temperature. The disorder in the O, T, and C phases can be characterized mainly by the Nb central ion allowed positions, and then the framework of the established eight-site model will be adopted [36]. In order to obtain a complete description of the dynamical properties through the successive phase transitions, it is useful to know the classification of the optical modes and their correlations between the various phases [37].

For a closer inspection, the peak positions of all modes in each phase are plotted in Figure 16 against the temperature between 300 and 800 K. Frequency shifts and transformations of the modes are clear at the O-T and T-C boundaries. The trends within each phase consist primarily of mode softening with increasing temperature, exactly as expected to follow from the lattice thermal expansion. In the light of all the results, displacive and order–disorder mechanisms have to be associated in the description of the phase transitions. Thus, the successive phase transitions in KNN-LN based single crystals derive from competition between a soft phonon mode and a relaxation mode.

In order to obtain the phase fraction, we assume that one phase at thermal phase boundaries is a linear superposition of the spectra below and above phase boundaries. As shown in Figure 17, the fitting Raman spectra at 410–440 K in KNN-LN and KNN-LN-1%MnO2, respectively, are displayed in comparison with the observed spectra at the same temperature. The fitting spectra are obtained from the linear superposition of the Raman spectra below and above phase boundaries. The coincidence of the experimental and the fitted Raman spectra are well for both samples, which strongly confirms the coexistence of the O and T phases at the thermal phase boundary from O to T phase. The mixed structure state at the thermal phase boundaries is due to sufficient competing mechanical and dipolar interactions between domains in multi-domain configurations [38]. In single domain case, the system undergoes a series of first-order ferroelectric transitions upon heating, sequentially adopting the O and T ferroelectric phases before reverting to the C parent phase.

Figure 15 depicts temperature dependence of Raman scattering for the KNN-LN and KNN-LN-1%MnO2 crystals, respectively. The phase structure of KNN-LN based crystals transforms from O phase to T phase, then to cubic (C) phase with increasing temperature. The disorder in the O, T, and C phases can be characterized mainly by the Nb central ion allowed positions, and then the framework of the established eight-site model will be adopted [36]. In order to obtain a complete description of the dynamical properties through the successive phase transitions, it is useful to know the classification of the optical modes and their correlations

Figure 14. Composition dependence of phase transition temperature T\* and Neel temperature TN for the BLFTO ceramics derived from temperature dependent Raman scattering (Figure reproduced with permission from [34]. Copy-

Figure 13. Temperature dependence of Raman scattering for the BLFTO ceramics collected in the temperature range from 80 to 680 K. As an example, Raman spectra recorded at 80 K, which were fitted with independent damped harmonic oscillators, have been indicated on the bottom (Figure reproduced with permission from [34]. Copyright 2014, American

between the various phases [37].

right 2014, American Institute of Physics).

Institute of Physics).

266 Raman Spectroscopy

Figure 15. Temperature dependence of Raman scattering for the KNN-LN based single crystals collected in the temperature range from 300 to 800 K and Lorentzian-shaped deconvolution at the temperature of 300 K. The arrows are applied to separate different phase transition (Figure reproduced with permission from [35]. Copyright 2015, American Institute of Physics).

4. Conclusions and outlook

with solid state spectroscopy.

physics in the future.

Acknowledgements

Learning.

Author details

This chapter reports a systematic micro-Raman scattering study on A- and B-site doped leadbased, tungsten bronze niobate, Bi-layered, ABO3 perovskites, multiferroics and lead-free ferroelectrics. The processing conditions, substitution, temperature-dependent Raman spectra and the structure–property correlations are discussed in the ceramics and single crystal forms of these materials. It can be concluded that the Raman spectra can provide us with a lot of important physical parameters for application in the future, such as spontaneous polarization, Curie temperature, structure distortion degree, mix-phase region and phase diagram and so on. The excellent agreement between the Raman, XRD or dielectric observations of merging of phase transition temperatures in all of the ferroelectric materials with different doping compositions suggested the powerful tool for detecting phase transition

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It should be emphasized that the domains in these ferroelectric materials and their related phenomena with different conditions, such as electric field, magnetic field and pressure have not been well investigated. The Raman spectra can be used to characterize the species of domain (including a/c domains, 90/180 domains), critical sizes and domain wall in ferroelectrics. Thus, we can check the domain status by recording the high-resolution Raman response. Our next goal is to characterize the domain information in these ferroelectric ceramics and films and identify the factors which can affect the domain structure. We believe that our research results will be of great significance to the development of microelectronic

The authors thank Dr. X. Chen, K. Shi and X. J. Ding for helpful discussions. This work was financially supported by Major State Basic Research Development Program of China (Grant Nos. 2013CB922300), Natural Science Foundation of China (Grant Nos. 61674057, 11374097, 61376129, 61504156, and 61227902), Projects of Science and Technology Commission of Shanghai Municipality (Grant Nos. 15JC1401600 and 14XD1401500), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher

Technical Center for Multifunctional Magneto-Optical Spectroscopy (ECNU), Shanghai and Department of Electronic Engineering, East China Normal University, Shanghai, China

Kai Jiang, Liping Xu, Jinzhong Zhang, Zhigao Hu\* and Junhao Chu

\*Address all correspondence to: zghu@ee.ecnu.edu.cn

Figure 16. Temperature dependence of the phonon frequency from KNN-LN based crystals. (a) and (c): The low frequency range of 10–300 cm<sup>1</sup> , (b) and (d): The high frequency range of 400–900 cm<sup>1</sup> . Note that different shade regions indicate that the crystals are located in diverse phase (Figure reproduced with permission from [35]. Copyright 2015, American Institute of Physics).

Figure 17. Phase fraction of (a) KNN-LN, and (b) KNN-LN-1%MnO2 single crystals as a function of temperature. Note that (c) and (d) is the fitting Raman spectra at 410–440 K for KNN-LN and KNN-LN-1%MnO2, respectively (Figure reproduced with permission from [35]. Copyright 2015, American Institute of Physics).
