The Curie point for a rhombohedral piezoelectric thin film was established by in situ micro-Raman spectroscopy. The hysteresis in phase reversal and specific thermal conditions for disrupting such reversal were determined

Adley, D

Bhaskaran, M

Mitchell, A

Perova, T

Sriram, S

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Sriram, S, Bhaskaran, M, Adley, D, Perova, T and Mitchell, A 2011, 'Reversal and pinning of
Curie point transformations in thin film piezoelectrics', CrystEngComm, vol. 13, no. 5, pp.
1280-1282.
http://researchbank.rmit.edu.au/view/rmit:10084
Accepted Manuscript
The Royal Society of Chemistry 2011
http://dx.doi.org/10.1039/c0ce00713g
Reversal and pinning of Curie point transformations in thin film 
piezoelectrics† 
Madhu Bhaskaran,*a Sharath Sriram,a David Adley,b Tatiana S. Perova,b and Arnan Mitchella 
Received (in XXX, XXX) Xth XXXXXXXXX 2010, Accepted Xth XXXXXXXXX 2010 
First published on the web Xth XXXXXXXXX 2010 5 
DOI: 10.1039/b000000x 
The Curie point for a rhombohedral piezoelectric thin film was 
established by in situ micro-Raman spectroscopy. The hysteresis 
in phase reversal and specific thermal conditions for disrupting 
such reversal were determined. 10 
Piezoelectric materials are suitable for a wide range of 
applications such as transducers, actuators, and sensors. The 
operating temperature range of the piezoelectric materials for 
these applications is determined by the Curie point.1 The 
Curie point is the temperature above which the unit cell of 15 
piezoelectric compounds transforms from asymmetric 
tetragonal or rhombohedral arrangement to symmetric cubic 
arrangement; this renders them non-piezoelectric.1,2 The 
determination of the Curie point can be carried out using 
electrical measurements of changes in the dielectric properties 20 
of materials. Kalinin and co-workers have reported on electric 
field induced first-order transformations in oxide thin films.3 
However, techniques that can determine temperature 
dependent changes in crystal structure shed more scientific 
insight into transformations, and also enable accurate 25 
evaluation of the Curie point of materials and hysteresis in 
recovery of the asymmetric piezoelectric crystal states. 
 Probing crystal structure transformations by micro-Raman 
spectroscopy has been shown to be suitable for the 
determination of the Curie point for ceramics.4 Preliminary 30 
work by the authors,5 reported on the first determination of 
phase transformation temperatures for thin films using in situ 
measurements. The determination of Curie point proved to be 
challenging due to weak Raman signatures and limited 
published literature. This indicated the need for further studies 35 
to improve understanding of Raman peak assignments for 
complex perovskite oxides such as the strongly piezoelectric 
ABO3-type lead zirconate titanate (PZT). 
 PZT is the most popular piezoelectric material given its 
large piezoelectric response and wide operating temperature 40 
range.1,6 The addition of strontium as an A-site has been 
shown to enhance the piezoelectric response,7 but is expected 
to decrease the Curie point of the resulting material 
composition. The Curie point for these thin films of Sr-doped 
PZT (PSZT) with composition (Pb0.92Sr0.08)(Zr0.65Ti0.35)O3 is 45 
expected to be around 250-280 ºC.2,5 
 The Curie point induced transformations are considered to 
be reversible with temperature.1,2 This work aimed to study 
the phase transformations in rhombohedral c-axis oriented 
thin films with increase in temperature, determine the thin 50 
film Curie point, and to check the reversibility of the same 
with controlled or uncontrolled cooling of the thin films. 
 Piezoelectric, perovskite-structured PSZT thin films were 
deposited by RF magnetron sputtering onto platinized silicon 
substrates (see ESI† for details). These 700 nm thick films 55 
were preferentially c-axis oriented with rhombohedral 
structure, as verified by X-ray diffraction (XRD) and 
transmission electron microscopy (TEM).8 A micro-Raman 
spectrometer combined with a Linkam hot-stage was used 
extensively for in situ measurements (see ESI† for details). 60 
 In order to determine the Curie point, PSZT thin film 
samples were heated with a controlled ramp rate of 10 ºC/min. 
Heating was carried out to a maximum of 350 ºC, based on the 
estimate of the Curie point being ~280 ºC.2,5 The sample was 
held at maximum temperature for 20 minutes. The sample was 65 
then cooled with controlled ramp-down (10 ºC/min) until 
100 ºC, beyond which the cooling occurred more gradually 
towards room temperature. Micro-Raman measurements were 
collected at selected set points, during the heating and cooling 
cycle and twice at maximum temperature, during which the 70 
temperature was held constant for 8 minutes for this purpose. 
 PSZT thin films of this composition (with zirconium-
titanium ratio of 65/35) are in the rhombohedral phase at room 
temperature. Room temperature spectra highlight two main 
Raman bands at ~575 and ~744 cm-1 (see ESI†, Fig. S1). 75 
These two bands have been reported in Raman investigations 
of PZT powders and thin films.9 On heating the PSZT thin 
films, these bands decrease in intensity, which can be 
attributed to increased scattering. These two bands are 
asymmetric, with a few pronounced sub-peaks visible at room 80 
temperature. To isolate the constituent peaks, these bands 
Fig. 1  Variation in the ratio of the intensities of the 744 and 575 cm-1 
peaks with temperature during controlled heating and controlled cooling 
of the PSZT sample. 
were fitted with a collection of Gaussian/Lorentzian peaks. 
The band at ~575 cm-1 was fitted with three sub-bands located 
at 565, 575, and 604 cm-1. The broad band centred at 
~744 cm-1 was also fitted with three sub-bands positioned at 
725, 744, and 766 cm-1. In addition a small shoulder is 5 
observed at ~815 cm-1. The position of some of these peaks is 
quite close to that obtained after the deconvolution of the two 
wide bands observed in the region from 450-620 cm-1 and 
620-900 cm-1 for poled polycrystalline composites based on 
PZT.10 It should be noted that our deconvolution produced 10 
three peaks for the ~744 cm-1 band, with only two previously 
reported. This can be attributed to the more pronounced sub-
bands in our spectra as a result of the preferentially textured 
polycrystalline crystallography of PSZT thin films grown 
under optimised conditions; whereas, the sub-bands in 15 
published literature10,11 were not as pronounced. The stronger 
spectral contribution from the preferentially oriented PSZT, 
verified by XRD and TEM results,8 can also be confirmed 
from the results obtained by Foster et al.,12 where the Raman 
spectra of the oblique phonons of single-domain single crystal 20 
PbTiO3 were demonstrated. These results show that a large 
number of different phonon modes can be observed at room 
temperature (300 K) in the region from 420-800 cm-1 in 
platelet geometry for various directions of the phonon wave 
vector k for the PbTiO3 crystal.  25 
 Having established the constituent peaks in the spectra, we 
now analyse the temperature dependence of different phonon 
modes obtained after fitting of the two wide bands described 
above. A comparison of the intensity ratios of the two peaks 
provides accurate evidence of phase transformations. As can 30 
be observed in Fig. 1, the intensity ratio of the 
744 and 575 cm-1 peaks increases at higher temperatures, 
following an apparent change at 275 ºC. The sharp intensity 
ratio variations signify phase transformation, starting at the 
theoretically expected value of ~280 ºC. 35 
 This phase transition observed can be related to the crystal 
structure of the thin film. The 575 cm-1 peak corresponds to 
the rhombohedral phase with a phonon assignment of 
A1(3TO).9,12 The 744 cm-1 peak is a combination of the 
rhombohedral A1(3TO) and cubic E(3LO) phonon 40 
assignments.9,12 As a consequence of this, while both peaks 
should appear at room temperature (for rhombohedral 
structure), on heating the E(3LO) should dominate.12 This 
should result is an increased intensity ratio of the 
744 and 575 cm-1 peaks, as seen in Fig. 1. 45 
 With controlled cooling at 10 ºC/min, the cubic phase 
reverses back to the rhombohedral phase in the same 
temperature window of 275-300 ºC, indicating minimal 
hysteresis in the Curie point phase transformation. This 
reversal occurs with the peak intensity ratio returning to ~1.2. 50 
 In addition to the intensity ratios, the position changes with 
temperature for the two main peaks were also studied, as this 
highlights changes in bond lengths corresponding to crystal 
structure. These results for the ~575 and ~744 cm-1 peaks are 
presented in Fig. 2. After accounting for shifts related to 55 
thermal expansion, clear features of phase transformation can 
be observed. For the ~575 cm-1 peak in Fig. 2(a), the peak 
position gradually decreases from 582 cm-1 (at 23 °C) on 
heating. A sharp change in the slope of the position variation 
occurs beyond 250 °C, corresponding to the intensity ration 60 
changes in Fig. 1. However, the peak position behaviour is not 
as sharp as the peak intensity shown in Fig. 1 due to the fact 
that normally the peak intensity is more sensitive to the local 
change in the structure. The ~744 cm-1 peak showed similar 
behaviour during heating as seen in Fig. 2(b). On cooling, 65 
reversal in peak positions of both peaks was observed (Fig. 2), 
with a hysteresis behaviour present. 
 The Curie point phase reversal is a critical contributor to 
high temperature deposition of piezoelectric thin films. Post-
deposition cooling rate is known to greatly influence the final 70 
crystal structure of the piezoelectric materials, with slower 
cooling producing best results (at ~5 ºC/min).6,13 Some 
hypothesise, in a principally intuitive manner, that this is due 
temperature-driven crystallisation given the larger duration 
available for crystallisation.6 This theory can be tested by 75 
rapid cooling of a known rhombohedral film. We carried out 
this investigation, using a fully characterised film, as 
discussed in the earlier sections of this work,. The temperature 
of the PSZT thin films was allowed to ramp up slowly to 
enable phase transformations, following which the effect of 80 
uncontrolled cooling on these films was studied. 
Fig. 2  Variation in the position of the (a) 575 and (b) 744 cm-1 peaks 
during controlled heating and controlled cooling of the PSZT sample. 
 The PSZT thin films produced spectra similar to the heating 
cycle (see ESI†), carried out under identical conditions with a 
ramp rate of 10 ºC/min. The expected Curie point 
transformation was observed beyond 275 ºC. From the 
maximum temperature of 350 ºC, the sample was cooled 5 
naturally. The behaviour of the 575 and 744 cm-1 PSZT 
micro-Raman peaks were studied in detail, with changes in 
intensity and position of the 575 cm-1 peak shown in Fig. 3. 
The behaviour of composite peak areas is similar to that of the 
earlier experiment during the controlled ramp-up of 10 
temperature. However, on rapid cooling of the sample, the 
peak intensities and positions remain locked in the cubic 
phase, indicating a permanent phase change has occurred 
(Fig. 3). Evidence of this pinning to the cubic phase is 
provided by the observation of invariance in peak heights for 15 
the cooling measurements at 350 ºC and 50 ºC, and by 
identical full range micro-Raman spectra (see ESI†, Fig. S2). 
The absence of any relaxation to the room temperature 
rhombohedral phase was verified by a repeated measurement 
of these samples after a period of four days and observing the 20 
same high temperature cubic phases at room temperature, 
indicating that this phase change is permanent. This result 
shows that actual cooling rate induced phase lock-in 
dominates any crystallisation process for these thin films. 
 In summary, we have carried out in situ heating with micro-25 
Raman spectroscopy to study Curie point induced phase 
transformation in piezoelectric PSZT thin films. We have 
identified that Curie point transformation for these room 
temperature rhombohedral films occurs at 275 ºC and is a 
reversible process with minimum hysteresis. The significance 30 
of the rate of cooling post deposition on reversal of Curie 
point transformations was studied. The results show that this 
can strongly disrupt Curie point transformations, by pinning 
the material to the high temperature cubic phase, if the 
cooling process is rapid and uncontrolled. 35 
Acknowledgements 
The authors acknowledge funding support from the Australian 
Research Council Discovery Project DP1092717 (MB); the 
ARC Australian Research Network for Advanced Materials 
(MB); and the IRCSET, Ireland Postgraduate Award (DA). 40 
Notes and references 
a Microplatforms Research Group, School of Electrical and Computer 
Engineering, RMIT University, Melbourne, Victoria 3001, Australia. E-
mail: madhu.bhaskaran@gmail.com 
b Microelectronics Technology Group, Department of Electronic and 45 
Electrical Engineering, University of Dublin – Trinity College, College 
Green, Dublin 2, Ireland. 
† Electronic Supplementary Information (ESI) available: Experimental 
conditions for PSZT thin film deposition and for micro-Raman 
spectroscopy measurements. Additional micro-Raman spectroscopy 50 
measurements results. See DOI: 10.1039/b000000x/ 
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Fig. 3  Intensity and position variations of the 575 cm-1 peak during 
controlled heating and uncontrolled cooling are shown in (a) and (b), 
respectively. 


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