Rhombohedral PLZT piezoelectric microfibers: a combined Raman

and X-ray diffraction study

Lucjan Kozielskia*, Elena Buixaderasb and Frank Clemensc

aDepartment of Materials Science, University of Silesia, Sosnowiec, Poland; bInstitute of Physics,Academy of Sciences of the Czech Republic, Prague, Czech Republic; cEMPA, Swiss Federal

Laboratories for Materials Science and Technology, Laboratory for High Performance Ceramics,Duebendorf, Switzerland

(Received 30 April 2014; accepted 29 June 2014)

A combination of micro- and macro-scale structural characterization methods wasimplemented for clarification of the influence of different sintering atmospheres on thestructural properties of Pb1¡xLax(ZryTi1¡y)O3 (PLZT) fibers. Three powders, PbZrO3

and ZrO2 (PZ C Z), PbZrO3 (PZ), and PbZrO3 C PbO (PZ C P), were used for thegeneration of protective atmospheres. Vibrations corresponding to the rhombohedralphase in (Pb0.93La0.07)(Zr0.65Ti0.35)O3 fibers were measured and mapped along thesection of the fibers by micro-Raman spectroscopy. Comparison of the Raman datawith the evolution of the unit cell parameters indicates that the PZ C Z protectiveatmosphere ensures the best properties during the PLZT sintering at the temperatureof 1250 �C for 6 hours.

Keywords: piezoelectric fibers; electro-optical ceramics; extrusion method

1. Introduction

Perovskite, a calcium titanium oxide mineral discovered in 1839, with the chemical for-

mula CaTiO3, lends its name to the class of compounds which have the same type of

ABO3 crystal pattern known as the perovskite structure.[1] Perovskites exhibit many

interesting electrical properties; they can be semiconductors, as doped BaTiO3,[2] or

even superconductors, as YBa2Cu3O6.[3] Furthermore, the variety of physical properties

they can achieve is impressive, starting from ferroelectricity and piezoelectricity in

CaTiO3 and finishing with the discovered ferroelectric (FE) ferromagnetic materials that

could give rise to new technologies in which the low power and high speed of field-effect

electronics are combined with the permanence and routability of voltage-controlled ferro-

magnetism.[4]

Consequently, for many years, along with the application-oriented works, numerous

phase transitions in ABO3 perovskites (A D K, Ba, Sr, Ca, Pb, La, etc., B D Na, Nb, Ti,

Zr, etc.) have been investigated. Generally, ferroics are cubic above the Curie temperature

and undergo at lower temperature ranges a variety of structural phase transitions of differ-

ent nature. There are many different kinds of phase transformations, as well as deviations

and distortions, related to inherent structural defects, impurities, grain sizes, etc. A good

example of this is the phase transformation in transition metal oxides with preferential

occupation of specific d orbitals that leads to the development of a long-range ordered

pattern manifested in the cooperative Jahn�Teller distortions resulting from the orbital

*Corresponding author. Email: lucjan.kozielski@us.edu.pl

� 2014 Taylor & Francis

Phase Transitions, 2014

Vol. 87, Nos. 10�11, 982�991, http://dx.doi.org/10.1080/01411594.2014.953511

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ordering.[5] It is important to indicate that in all these areas of research, the presence of

dopants, in either the A or the B cation sites of the perovskite structure, is one of the main

factors influencing the properties. The system PbZrO3�PbTiO3 (PZT) provides an exam-

ple of a continuous solid solution with cation substitution, Zr for Ti, on the A site. The

PbZr0.65Ti0.35O3 is rhombohedral at room temperature, with space group R3c. Ti ions

present in PZT create a long-range electric field that destroys the antiferroelectric (AFE)

ordering of the PbZrO3 host lattice. The same effect is observed after application of an

external electric field, which decreases the AFE transition temperature.[6]

A fatal drawback in today’s PZT manufacturing is its toxicity due to Pb content,

which may cause environmental and human health problems. However, novel suitable

lead-free FE/piezoelectric materials with properties comparable to those of PZT have not

yet been found because of basic physical limitations. Namely, the higher ferroelectricity

and piezoelectricity of Pb-containing ceramics originated from the high polarizability of

the Pb atom, due to its large radius, high effective number of electrons and also to the

lone electron pair present in the outer shell, hybridized with oxygen ions. Consequently,

transparent Pb1-xLax(ZryTi1-y)O3 (PLZT) in ceramic form is still a leading material in the

most prospective electro-optical applications, for example as a binary optical information

storage, which is the basis for photo-storage effect optical memories.[7] UV light illumi-

nation shifts the electric field threshold of the phase transition between the field-induced

FE phase and the AFE phase. Properties of this photoactivated shift of the FE ! AFE

phase transition, including preliminary photosensitivity measurements and photostorage

mechanisms, are presented by Land.[8]

Electric-field-forced AFE-to-FE phase transitions in PLZT materials are used for

ultra-high-field-induced strain actuator applications. A large field-induced longitudinal

strain of 0.85% and a volume expansion of 0.95% are observed in the ceramic composi-

tion Pb0.97La0.02(Zr0.66Ti0.09Sn0.25)O3 at room temperature. The cause for this is the

nucleation of the FE phase within the AFE one controlled by electric field. Additionally,

the PLZT actuator dynamics is also impressive, providing a switching time below 1 ms.

However, for practical applications, as diesel motor inceptor for instance, one has to take

into account that hydrostatic pressure increases the transition field and the switching

time.[9] The PZLT 2/95/05 ceramic sample emits the main group of energetic electrons

for plasma generation devices 100 ns after the switching of the applied pulse. Another

experiment using different FE materials and experimental methods have found maximum

emissions which are 109 times weaker than in this particular PZLT 2/95/05 composition.

[10]

One of the aims of this paper is to ascertain whether the protective sintering atmo-

sphere can lead to distortions of the perovskite phase in PLZT ceramic fibers. Since the

spatial properties of these fibers have not been deeply investigated so far, the production

process cannot be thoroughly controlled to get a better performance. Therefore, the sec-

ond aim of this work is to investigate if spatial phase degradation maps can be evaluated

effectively using Raman spectroscopy, as it was shown in PZT fibers.[11] It is also impor-

tant to know whether the phase composition at the edge and center of the fiber is similar

or whether there is a gradient. With that in mind, green fibers with the defined phase com-

position (Pb0.93La0.07)(Zr0.65Ti0.35)O3 have been extruded via thermoplastic fiber extru-

sion technique. Those fibers were subsequently sintered using three different sintering

atmospheres. Following this goal, this study has been accomplished by combining two

complementary techniques: X-ray diffraction (XRD) measurements have analyzed the

crystallographic character of the rhombohedral phase distortion, while in situ Raman

spectroscopy has been performed to get spatial distortion maps.

Phase Transitions 983

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2. Experimental

(Pb0.93La0.07)(Zr0.65Ti0.35)O3 composition (PLZT 7/65/35) was prepared from stoichio-

metric amounts of PbO, La2O3, ZrO2, and TiO2 (Aldrich, 99.9%) oxides and was first

dried at 200 �C for three hours and then milled in a planetary ball mill (RETCH PM400)

for 24 hours with zirconia balls and ethanol. The powder mixture was calcined by heating

up to 925 �C at a rate of 200 �C/hour and kept at that temperature for three hours. Analy-

sis by microprobe showed that the sample was close to the nominal composition and was

homogeneous at the 1% level.

The PLZT fibers of 250 mm in diameter were obtained by extrusion � a fiber shaping

method in which ceramics powder with a plastic binder material is forced through the ori-

fice of a rigid die [12] � and, then, sintered in different atmospheres, namely lead zirco-

nate C zirconia (PZ C Z), lead zirconate (PZ) and lead zirconate C lead oxide (PZ C P),

at 1250 �C for six hours to produce three group of fibers for our experiment. Sintering of

the fibers took place in Al2O3crucibles, where the atmosphere was controlled by 3 g of

powder mixtures PZ C Z, PZ, and PZ C P. The powder mixtures were wet coated inside

the Al2O3crucibles, which were used as a lid. Further information can be found in Heiber

et al. [13]. The sintering setup with the three different protective atmospheres, PbZrO3

with ZrO2 (PZ C Z), PbZrO3 (PZ), and PbZrO3 with PbO (PZ C P) powders is schemati-

cally shown in Figure 1.

XRD diffraction patterns were recorded using an XRD PANalytical X’Pert Pro Multi-

purpose Diffractometer. The wide-angle scan from 5� to 80� was done with a step width

of 0.02� and exposure time of 1250 second per step (CuKa- radiation, 40 kV beam poten-

tial and 40 mA heating current). For phase composition identification of the sintered

fibers, a quantitative analysis was performed by the Rietveld refinement method based on

the respective structural models.[14]

For Raman measurements, pieces of the PLZT fibers were cut and sectioned, and then

embedded in a polymer matrix. This allowed us to perform spatial mapping with easy

manipulation. The micro-Raman equipment was a RM-1000 Renishaw Raman micro-

scope equipped with a grating filter capable of good stray light rejection in the

20�850 cm¡1 range, a Leica microscope and a CCD camera detector. An Ar laser beam

at a power of 20 mW at a wavelength of 514.5 nm was used to illuminate the samples.

The diameter of the laser spot on the sample surface was about 2�3 mm. Additionally, a

video camera and a computer-controlled microscope table enabled us to choose the proper

location on the polished longitudinal section of the ceramic fibers. Raman spectra maps

were collected at constant room temperature with a 3 mm step.

Figure 1. Scheme of placing PLZT fibers on Al2O3 sintering substrate and covering it with anAl2O3 crucible coated with different atmospheric powder compositions.

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3. Results

3.1. Microscopy investigations of the fibers’ surface

In the first sintering variant, the PZC Z powder was used for generating a high-temperature

protective atmosphere. It appears that this preserved only partially the destruction process

of sintered fibers (Figure 2(a) and 2(b)). In the optical micrographs, as well as in the scan-

ning electron microscope (SEM) images, a higher density area in the fiber center and a

highly destroyed area almost at one-third distance between fiber edge and center are dis-

tinctly visible. The final sintering stage leads to an enormous Pb and Zr evaporation in

almost half of the volume, compared with the PZ C P fibers. Heiber et al. [15] observed

Figure 2. Optical micrographs and their respective SEM images of the investigated fibers sinteredwith PZ C Z (a and b), PZ (c and d) and PZ C P (e and f) powder for generation of protective atmo-sphere. Red lines indicate Raman spectra measurements areas.

Phase Transitions 985

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similar results and explained them by the evaporation of PbO in liquid phase, which is pres-

ent during the high-temperature sintering process, and by the associated ZrO dissolution.

For the PZ C Z fibers, there is a visible lowered effect of the protective atmosphere influ-

ence on the fiber microstructure from the surface to a depth of 70 mm. Whereas the grain

size does not change substantially across the fiber radius, the grain structure varies from an

almost fully dense mono-phase PLZT central part, through an at least two-phase area in the

intermediate zone, to a fully destroyed outer part of the fiber (Figure 2(a) and 2(b)).

In the second variant of the processing, with PZ rich sintering atmosphere, the center

as well as the outer region of the fiber are totally dominated by a completely porous struc-

ture and show a mixed phase pattern (Figure 2(c) and 2(d)). For the PZ fibers, there is no

visible influence of the protective atmosphere on the fiber microstructure and the material

is almost entirely porous (Figure 2(c) and 2(d)). Previous investigations together with the

evidence collected by Heiber et al.[15] lead to the conclusion that a higher amount of

PbO liquid phase in the PZ fibers can be expected and evaporated faster under the same

temperature and time conditions.

Finally, we show the measurements on the fibers sintered with the PZ C P atmos-

pheres in Figure 2(e) and2(f). The recorded images reveal that compared to the other sin-

tering atmospheres, these fibers show an exceptionally finished densification process. At

the same time, these fibers possess a narrow grain size distribution: only grains between

1.5 and 2.5 mm were found. Additional porosity measurements confirmed that the densifi-

cation process was almost complete. Consequently, it is clearly visible that the sintering

protective atmosphere PZ C P had been perfectly chosen because the grain growth was

completed in the final sintering stage and only a few pores remained.

3.2. X-ray diffraction study compared with Raman spectroscopy results

To optimize the sintering process of the PLZT ceramics fibers, microstructure analysis by

SEM is not sufficient because phase composition play a dominant role on the achieved

electromechanical properties. Therefore, XRD structural investigation and spatial map-

ping by Raman spectroscopy were performed.

PLZT 7/65/35 is expected to have rhombohedral (R3c) symmetry, as it lies near the

Zr-rich end-member of the PZT solid solution. Figures 3(a), 4(a), and 5(a) show the

Rietveld refined XRD diffraction patterns of the samples with different sintering

atmospheres.

In the first case, when the PZ C Z powder was used as protective sintering atmo-

sphere, three dominant phases were found (see Figure 3(a)). The XRD analysis reveals

that the sintered material is composed of the PLZT respective perovskite R3c phase

(48%) and two additional phases: monoclinic ZrO2 (37%) and orthorhombic LaTiO3

(15%). By making an analogy with similar systems, we interpret this as the influence of

Pb evaporation with associated structure destruction. The variation of the Raman spectra

with distance from the center to the fiber edge is shown in Figure 3(b) and3(c). There is a

huge difference between the spectra taken inside the fiber and the ones taken in the outer

part. In agreement with the optical findings (Figure 2(a) and 2(b)), the outer shell shows

extra Raman peaks, related to the presence of the additional phases.

In the second case of the PZ-rich sintering atmospheres, the diffraction pattern shows

again three dominant phases (Figure 4(a)), but this time both the center and the fiber’s outer

region are totally dominated by mixed structures that is visible in respective Raman spectra

(Figure 4(b) and 4(c)). Like in the first case, XRD analysis reveals that the sintered material

is composed of the PLZT respective perovskite R3c phase (53%) and two additional

phases: monoclinic ZrO2 (25%) and orthorhombic LaTiO3 (22%). In this sample, some

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small amounts of other phases are also detected (left side in the Figure 4(a)). As expected,

the Pb vapor pressure decrease by changing from PZ to PZ C Z atmosphere, and therefore

Pb can evaporate more easily from the edge of the fiber.

The Raman spectra of the PLZT fibers sintered in the PZ atmosphere (Figure 4(b) and

4(c)) also indicate that there is no sufficient protective atmosphere influence on the fiber

and its microstructure is almost entirely destroyed. Previously mentioned investigations

[11] together with our XRD evidence lead to the conclusion that a lower amount of Pb in

the gas phase results in a faster evaporation from the fiber. Therefore, a deficit of lead is

present in these fibers.

In the third case, fibers sintered in PZ C P atmosphere show an exceptionally pure

R3c perovskite phase (Figure 5(a)) compared to the other sintering atmospheres. The

room-temperature pattern exhibits all the reflections expected for the rhombohedral R3c

space group. The Raman spectra displayed inFigure 5(b) and 5(c) also do not show any

additional phases or defects, and the shape of the spectra between the center and edge of

the fiber is quite homogeneous and very similar to the Raman spectrum obtained in bulk

PLZT 8/65/35 ceramics (also shown for comparison).

3.3. Discussion

The Rietveld refined XRD patterns of the PLZT fibers are shown in Figures 3(a), 4(a), and

5(a). The lattice parameters of the investigated perovskites phases determined by the

Figure 3. XRD diffraction pattern (a) and Raman spectra (c) and (d) of PLZT 7/65/65 ceramicsfibers sintered with/under PZ C Z protective atmosphere.

Phase Transitions 987

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Rietveld refinement are collected in Table 1. It can be stated that the decrease of the effec-

tiveness of the protective atmosphere leads to a small decrease of the ‘a’ and ‘b’ lattice

parameters and to a high decrease of the ‘c’ one; the unit cell becomes ‘flatter’ indicating

that R3c unit cell underwent rhombohedral phase distortion. Finally, after this distortion,

the whole unit cell volume also drops significantly, which gives clear evidence of the

increased level of atomic defects in the structure of not properly protected PLZT fibers.

Raman spectral maps of different PLZT fibers taken from the center to the edge

(shown in Figures 3(b) and 3(c), 4(b) and 4(c), and 5(b) and 5(c), respectively) provide

evidence for the fact that the sintering atmosphere has a huge influence on the microstruc-

ture of the fibers and their homogeneity. Comparing the Raman spectra of the fibers (see

Figures 3�5), it can be clearly seen that the best fibers are obtained when PZ C P atmo-

sphere is used during the sintering process (see Figure 5(b) and 5(c)). The pure perovskite

phase of PLZT 7/65/35 was confirmed by XRD measurements and presented in Figure 5

(a). In contrast, Figure 3(b) and 3(c) shows that the Raman spectra of the fiber sintered in

PZ C Z have additional sharp peaks, visible in the spectra obtained at the edge of the

investigated fiber, which are due to defects. Spectra in the outer part are similar to those

in the fiber sintered with the PZ atmosphere shown in Figure 4(b) and 4(c), but the center

of the fiber displays Raman spectra more similar to the pure perovskite phase fiber sin-

tered in the Pb-rich atmosphere (PZ C P) shown in Figure 5(b) and 5(c). This seems to

indicate that defects reach just the edges of the fiber.

Figure 4. XRD diffraction pattern (a) and Raman spectra (c) and (d) of PLZT 7/65/35 fibers for PZprotective atmosphere.

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The Raman spectra of the fiber sintered in PZ atmosphere depicted in Figure 4(b)

and4(c) indicate spectral homogeneity between the center and the edge. As these defect

peaks are present throughout the whole sample, it seems that the fiber is completely

porous, so that defects can reach the center.

As the best spectra (more similar to the bulk ceramic) are those from the sample sin-

tered in the Pb-enriched atmosphere, it seems natural to associate the defects found with

the Pb evaporation, which is a common effect in the PZT processing.[16] Hence, Pb

vacancies have probably distorted or even destroyed the structure as well as the chemical

bonding, which activates new modes in the Raman spectra. A more detailed study of the

nature of the spatial defects requires other techniques.

Figure 5. XRD diffraction pattern (a) and Raman spectra (b) and (c) of PLZT 7/65/35 fibers forPZ C P protective atmosphere.

Table 1. Rhombohedral unit cell distortion parameters after sintering in three different protectiveatmospheres.

Atmosphere PZ C Z PZ PZ C P

R3c unit cellparameters

a D b D 5.774(2) a D b D 5.671(4) a D b D 5.7713(3)

c D 14.167(8) c D 14.17(1) c D 14.194(1)

V D 408.9 (106 pm3) V D 394.6 (106 pm3) V D 409.4 (106 pm3)

Phase Transitions 989

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4. Conclusions

Structural distortion properties analysis of (Pb0.93La0.07)(Zr0.65Ti0.35)O3 fibers has been

performed by the classical XRD approach, together with Raman spectroscopy, which pro-

vides spatial structural distortion maps. Especially prepared samples of PLZT fibers were

investigated with the programmed pattern from the fiber center to the edge of the fiber.

This gradient sensitive testing method has provided crucial information about the best

sintering atmosphere for the preservation of the piezoelectric fiber structure, and it has

also provided a great insight for material engineers who are generally interested in the

behavior of materials within a wide range of applied temperatures and atmospheres. Since

many engineers may not know exactly how to read or interpret Raman spectroscopy

results, our work shows that such a Raman spatial spectra distribution indicates clearly

and easily how the materials defects structure changes in depth under protective

atmospheres.

Our studies also shows that material science technology design should take into

account the sintering temperature and the effectiveness of the protective atmosphere dur-

ing the manufacturing for the modeling of the diffusion processes. More detailed studies

of the nature of the impurities, as well as better mapping of the defected structure and the

preserved areas, are the next goals of our work.

Acknowledgements

This work was partially supported by the Czech Science Foundation (Project no. 204/13/15110S).

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