Large field-induced-strain at high temperature in ternary ferroelectric crystals

The new generation of ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ferroelectric single crystals have potential applications in high power devices due to their surperior operational

stability relative to the binary system. In this work, a reversible, large electric field induced strain of over 0.9% at room temperature, and in particular over 0.6% above 380 K was

obtained. The polarization rotation path and the phase transition sequence of different compositions in these ternary systems have been determined with increasing electric field applied

along [001] direction based on x-ray diffraction data. Thereafter, composition dependence of field-temperature phase diagrams were constructed, which provide compositional and thermal

prospectus for the electromechanical properties. It was found the structural origin of the large stain, especially at higher temperature is the lattice parameters modulated by dual

independent variables in composition of these ternary solid solution crystals.

Over the past century, material scientists have made great efforts to seek and create new materials with significant mechanical response functionalities for applications in sensing and

actuation, ranging from inorganic to organic1, metal to nonmetal2, and bulk to nano materials3,4. Of the many types of actuator materials (including magnetostrictive2,5, photostrictive6,7,

and shape memory alloys2,4), piezoelectric materials3,8,9,10,11,12,13,14 are widely used due to their superior high force generation, high frequency capabilities, displacement accuracy,

and/or device miniaturization8. In particular, relaxor ferroelectric binary single crystals, regarded as generation I15,16,17, i.e., (1 − x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1 − 

x)Pb(Zn1/3Nb2/3)O3-xPbTiO3] (PZN-PT), with compositions near the morphotropic phase boundary (MPB), exhibit ultrahigh electromechanical coefficients that are up to an order of magnitude

larger than that of conventional Pb(ZrxTi1−x)O3 (PZT) ceramics8,18,19. This important breakthrough20 triggered a race for theoretical understanding of the origin of such high responses in

perovskite oxides21,22,23,24,25, and for the experimental identification of the MPB that is a nearly vertical boundary between rhombohedral (R) and tetragonal (T) phases26,27,28.

In spite of the ultrahigh electromechanical properties of these bianary crystals, several obstacles have restricted them from practical applications15,16. These includes (i) deterioration in

performance with increasing temperature, which limits their operational temperature ranges; and (ii) the coercive field is on the order of 2 kV/cm, which restricts their usage to low power

applications. In order to circumvent these difficulties, numerous investigations have been performed, including doping (Mn) binary crystals, crystallization or texture orientation instead of

conventional PZT ceramics, and adding solid solution components to achieve ternary systems15,16.

The challenges associated with property superiority, operational stability, cost and crystal size, have been addressed, in particular after the development of ternary

Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals15,16,29,30. This generation II system combines the advantages of two respective MPBs, determined by the two individual

binary PMN-PT and PIN-PT solid solutions (see Fig. 1(b)). In turn, the MPB zone, rather than a line in the binary systems, offers a higher degree of structural instability and thus provides

the possibility to enhance the electromechanical properties16. Furthermore, due to complexity in its compositional components, the electromechanical property and Curie temperature (TC) can

be simultaneously optimized and enhanced by varying the concentration of the components near the MPB region, rather than by property tradeoffs as in a binary system (i.e., increase of TC at

cost of decrease in piezoelectric properties).

Comparison of piezoelectric and dielectric properties of ternary PIN-PMN-PT and binary PMN-PT systems.

(a) Photograph for an as-grown large-size PIN-PMN-PT ternary crystal (larger and clearer ones are presented in Fig. S1). (b) Phase diagram for these generation II crystals, where the

rhombohedral and tetragonal phases are separated by a MPB zone. (c) Temperature dependent dielectric constant at 1 kHz in the zero-field-heating condition and unipolar E-induced strains for

crystals (d) A, (e) B, (f) C and (g) PMN-30PT. (h) Temperature dependence of the maximum unipolar strains under E = 50 kV/cm for the crystals A, B, C, and PMN-30PT. (h) Piezoelectric

coefficient d33 of various piezoelectric material systems, including the binary crystal PMN-PT, new generation (A-, B- and C-) ternary crystals, and conventional PZT ceramics (refs 40,41) as

a function of Tc. For the PMN-PT system, the d33 of PMN-30PT was measured in this work, whereas that for other compositions was cited from refs 29,45.

However, to date, due to the multiplicity of induced phase transitions with E and temperature31, the sequence of temperature dependent field-induced strains is not clear for ternary

PIN-PMN-PT crystals. This presents a significant challenge to end-users on how to select crystals with different compositions for specific application conditions, especially with regards to

operational temperature ranges. In this paper, the electromechanical and piezoelectric properties of PIN-PMN-PT with compositions near the MPB have been systematically characterizated. Large

field-induced strains >0.6% in the high temperature range from 360 K to 380 K have been obtained. Furthermore, the field-temperature phase diagrams were established by a comprehensive x-ray

diffraction study. The results provide a compositional and thermal prospectus for the electromechanical properties, which can assist end-users to choose compositions for actuation

applications at various operational temperature ranges.

Three typical regions of the as-grown crystal with a nominal composition of 33PIN-35PMN-32PT were selected for investigation (designated as A, B and C), as shown in Fig 1(a). The

compositions of the A-, B- and C-crystal were determined to be 31PIN-37.2PMN-31.8PT, 25.4PIN-41.2PMN-33.4PT and 28.8PIN-33.3PMN-37.9PT by quantitative X-ray fluorescence analysis (PHILIPS

PW2404 X-ray fluorescence spectrometer) based the standard of In2O3, MgO and TiO232,33. All the three crystals are located near MPB, but A- and B- crystals in the rhombohedral region, while

C-crystal in the MPB, close to tetragonal region, as schematically illustrated in Fig. 1(b). The ground state of the crystals will be discussed based on the diffraction data. Figure 1(c)

shows the dielectric constant (εr) as a function of temperature for [001] oriented crystals. The results demonstrate that the binary PMN-30PT has a TC near 413 K; whereas the ternary A-, B-

and C-crystals have higher TC values of 445, 452 and 475 K, respectively. The improvement of the Curie temperature provides potential superiority of ternary crystals to be operated in a

wider temperature range. Except for the ferroelectric-paraelectric phase transition peaks, no other additional peaks were apparent for either PMN-30PT or the ternary A-crystal; however,

additional peaks were distinguishable at lower temperatures for the ternary B- and C-crystals near 367 and 315 K, respectively. These additional phase transitions will be discussed in more

detail below.

The E-induced strain (S) for (001) oriented crystals is shown in Fig. 1(d–g). These data were taken at various temperatures under a maximum ac electric field (Em) of 50 kV/cm. For the

ternary A-crystal, it can be seen that the S-E curve is nearly linear and anhysteretic at 300 K. On heating to 420 K, the value of Sm under Em = 50 kV/cm increased, reaching a maximum value

and then subsequently decreasing. Interestingly, the degree of hysteresis also increased and subsequently decreased. For the ternary B-, C- and binary PMN-30PT crystals, the S-E curves

approach a saturation near Em = 50 kV/cm at all the temperatures studied. It is worth noting that similar results in the ternary ferroelectric crystals with compositions near the MPB were

also observed by Zhang et al.34. Interestingly, diffuse inflections, rather than discontinuous jumps (i.e., first-order-like characteristic35) in S were found for all the ternary crystals

studied, especially at room temperature, which is in contrast to the [001]-oriented binary PMN-30PT crystal that has two abrupt jumps due to the previously reported phase transformational

sequence of MA → MC → T26,36,37,38. The atypical S-E curves for ternary systems might be due to a bifurcated polarization rotation path or diffuse phase transitions with increasing E39.

The temperature dependence of Sm at Em = 50 kV/cm is summarized in Fig. 1(h). Among the ternary and binary crystals studied, the B-crystal was found to exhibit the highest value of Sm over

the temperature range from 300 to 420 K amongst the ternary and binary crystals studied. The value of Sm for the B-crystal near linearly decreased with increasing temperature. Strains as

high as Sm = 0.9% were obtained under Em = 50 kV/cm at room temperature for the B-crystal, which was 1.5× higher than that of PMN-30PT. Note that the enhancement was even larger for the

B-crystal if higher fields were applied, as its S-E curve did not completely saturate (see Fig. 1e). In contrast, the other crystals exhibited discontinuous decreases in Sm with increasing

temperature: A-crystal near 315 and 385 K, C-crystal near 320 and 350 K, and the binary PMN-30PT near 320 and 340 K (see Fig. 1h). These abrupt changes are indicative of phase

transformations, as will be discussed based on x-ray diffraction below.

Comparing the results in Fig. 1(h), it was found the significance was not only the enhancement of Sm in the ternary crystals at room temperature, but also the large value of Sm can be

achieved at high temperature range, especially for B-crystal, relative to the binary PMN-30PT ones. Obviously, all the ternary crystals exhibit a large value of Sm > 0.5% over the moderate

high temperature range, while PMN-30PT have a much lower value  8 kV/cm, a single c-domain T phase was obtained. It is interesting to note that this tetragonal single c-domain state was

metastable after removal of E, as no diffraction intensity corresponding to the monoclinic phase was apparent near 44.7°, in contrast to that observed at 300 K (see Fig. S4a). At higher

temperatures (i.e., 360 K), the metastable single c-domain state was partially restored to a multi-domain state upon removal of E, at least a small fraction of a-domains was found near 44.9°

due to depoling (see Fig. S4b). This diffraction feature reveals a structural origin for the noticable decrease in Sm near 320 K (Fig. 1g), and a slight increase with increasing E

thereafter. Changes in Sm with temperature above 350 K was also observed, which can be explained by a tradeoff between depoling effects and a temperature dependent c/a ratio (see Fig. 3c):

i.e., with increasing temperature, the fraction of a-domains increased, but the strain contribution from the unequal crystallographic axis switching decreased.

To clarify and determine the phase transformation sequence in Fig. 2, reciprocal-space mesh scans were performed about the (200)C and (220)C zones at various temperatures with E//[001]C for

A-, B- and C-crystals, as shown in Fig. 5 and S5–S9. For the A-crystal, the R phase was evidenced by a broad peak around the (002)C (see Fig. S5a), and a doublet about (220)C (see Fig. S6a)

under small E. Above 4 kV/cm, the pseudocubic (220) reflection exhibited the characteristic triple splitting of the MA phase, which consists of a (040)MA b-domain and two twined a-domains.

The (200)C zone exhibited the signature doublet splitting consisting of two twined domains. At 330 K, a similar domain configuration evolution and phase transformational sequence was

obtained, but the R → MA transition occurred at a lower field of E = 3 kV/cm, as shown in Fig. 5b. Note that at E = 2 kV/cm, only a broad single peak with a small splitting along the

transverse direction was found around (200)C, while a weak triplet was observed around (220)C. With increasing field to E = 10 kV/cm, a complex domain configuration was found: which

consisted of a strong peak around (220)C accompanied by weak diffraction tails (see Fig. 5c). Moreover, line scans around (002)C revealed a single peak with a weak tail on the low 2θ side.

These abnormal domain configurations can be attributed to coexistence of MA and MC phases by referring to the polarization rotation pathways for binary crystals26,37,38. Complex domain

configurations were also observed under a moderate field of E = 2 kV/cm at 390 K. These results indicate the coexistence of MA and MC phases, in particular in consideration of the

observation of a doublet in the (002)C line scans. At 420 K, a single domain configuration along both (200)C and (220)C became sharp under E > 0.5 kV/cm, and the lattice parameters extracted

from line scans revealed that the structure is tetragonal.

(a–f) Evolution of reciprocal-space mesh scans along the pseudocubic (200) and (220) zones for A- and B-crystals at 330 K with increasing E. (g–i) Evolution of reciprocal-space mesh scans

along the pseudocubic (200) for C-crystals under E = 4 kV/cm at typical temperatures.

For the B-crystal, the R phase was stable under moderate field level, as evidenced by a broad diffraction peak around (002)C and a doublet-like feature about (220)C (see Fig. 5d, S7a and

S8a). On application of higher E, the (200)C diffraction split into two peaks (i.e., E = 4 kV/cm at 300 and 330 K, and E = 2 kV/cm at 360 K, as shown in Fig. 5d, S7a,b). These domain

configurations reveal a R → MA transition with increasing E. It is important to note that atypical domain configurations around (220)C were found in the MA phase (see Fig. 5e): a b-domain

and only a single a-domain was observed, rather than a b-domain containing two a-domains as in previous studies26,37,38. Absence of diffraction peaks can be common in ferroelectric crystals

due to defects, or internal stress, in the crystals that break domain equivalency10. With increasing E to 10 kV/cm, a phase transition of MA → T was found near 360 K, which was manifested as

a sharp single domain with a slight tail around (220)C (see Fig. S7b), where the twined doublet was merged into a sharp single (200)T that had undistinguishable tails. At 390 K, the

reciprocal-space mesh scans provide conclusive evidence of the coexistence of R and T phases at E = 0 kV/cm: strong (200)T and (002)R can be seen in Fig. S7c. Upon application of E > 2 

kV/cm, the two phase coexistence disappeared, and the system transformed into a single domain as evidenced by singlets about both (200)C and (220)C.

For the C-crystal, the evolution of the line scans and phase transition sequence with increasing E were easier to distinguish relative to the A-and B- crystals (see Fig. 4c). Therefore, the

reciprocal-space mesh scans were performed under E = 4 kV/cm at various temperatures, as shown in Fig. 5(g–h) and S9. At 300 K, the (200)C and (220)C Bragg peaks split along the longitudinal

direction, demonstrating a coexistence of monoclinic and tetragonal phases (see Fig. 4c). As can be seen in Fig. 5(g) and S9(a), a single strong peak coexisted with a weak triplet, i.e.,

(200)T, (020)MC b-domain and two twined a-domains. This reveals a predominant tetragonal phase with a small fraction of the MC phase. The domain configuration along (220)C was more complex,

however, a single strong peak was obvious, corresponding to the (220)T domain, where the lattice parameter is consistent with that for (200)T. At higher temperatures (i.e., 330 and 360 K), a

pure T phase field was obtained on application of E = 4 kV/cm.

Finally, the structural origin of large strain for ternary crystals at high temperature range can be understood by the electric field-induced phase transitions (see Fig. 2). Based on the

symmetry-allowed polarization rotation theory, a final state of single tetragonal c-domain can be approached under application of large E-filed parallel to the polar vector of tetragonal

phase21. For crystals with compositions near the MPB, the electric field induced strain was mainly attribtued to lattice change due to the phase transition from MA → MC and MC → 

T26,31,36,44. As discussed in Fig. 1h, the value of Sm for PMN-30PT crystals obviously decreased at 330 and 365 K, and was ~0.3% over the temperature above 365 K. Based on the phase diagram,

it can be conjectured that the first dropping of Sm at 330 K was resulted from the temperature induced phase transition of MA → MC, resulting in the vanishing of strain contribution via

E-filed induced unit cell distortion of MA to MC, while the second one at 365 K was due to the temperature induced phase transition of MC → T, leading to the disappearance of strain

contribution from the polarization rotation from monoclinic plane to [001]. In turn, the strain of PMN-30PT crystals at high temperature range (i.e., above 365 K) was mainly from the

polarization extension of tetragonal c-domain. Nevertheless, the B-crystal was still in the R-phase field at temperature around 365 K, indicating the main strain contribution is polarization

rotation, i.e, MA → T phase transition. At temperature above 390 K, even thought the B-crystal in T-phase field, the diffraction data shows that it was in multi-domain state, rather than a

single tetragonal c-domain in PMN-30PT38: a sharp tetragonal c domain was remained, which was coexisted with a weak tetragonal a domain at 390 K (see Fig. S3c). These multi-domain states are

the structural origin of B-crystal exhibiting a much larger strain than PMN-30PT at higher temperature region. In other words, again, under E//[001], the strain in B-crystal was contributed

from the polarization rotation and domain switching, rather than a polarization extension in PMN-30PT at this high temperature region.

In conclusion, a reversible, large E-field induced strain of over 0.9% at room temperature, and in particular over 0.6% above 380 K, was observed in a new generation of ternary piezoelectric

single crystals. The polarization rotation path and the phase transition sequence of different compositions in these ternary systems have been determined with increasing E applied along

[001] direction based on x-ray diffraction data. Electric field-temperature phase diagrams were then constructed. For crystals in the R phase field, a composition-induced bifurcation of the

polarization rotation path was observed, and in particular, in contrast to binary PMN-30PT, no MC phase (or at least no single phase) was found during the E-field induced phase transition

sequence. The large strain in ternary crystals, especially at higher temperature, is attributed to the lattice parameters modulated by dual independent variables in composition of these

ternary solid solution crystals.

High-quality and large size ternary crystals with nominal composition of 33PIN-35PMN-32PT were grown by a modified Bridgman method29 using high purity (better than 99.99%) PbO, In2O3, MgO,

Nb2O5 and TiO2 as starting materials. Due to composition segregation, the real composition of the crystals was varied from those correspond to tetragonal, MPB and rhombohedral regions32.

Three typical compositions from the corresponding regions with dimensions of 4 × 3 × 3 mm3 were selected for investigations. The compositions were determined to be 31PIN-37.2PMN-31.8PT,

25.4PIN-41.2PMN-33.4PT and 28.8PIN-33.3PMN-37.9PT by Energy Dispersive X-ray Spectroscopy. These crystals were designated as A, B and C, respectively. To compare the properties and

structures of this new generation of crystals with previsouly reported binary system, a binary PMN-30PT crystal was also prepared into dimensions of 3 × 3 × 0.5 mm3, which was oriented along

the [001] direction in its thickness. It is well-known to exhibit the highest piezoelectric and electromechanical properties for the binary systems15. All the ternary crystals were oriented

along pseudocubic (100)/(010)/(001) planes, diced into rectangular parallelepipeds with dimensions of 4 × 3 × 3 mm3, and all the faces were polished to 0.25 μm. In case of dielectric

breakdown under application of high E, the corresponding rectangular parallelepipeds were then cut into several pieces along the longitudinal direction with thicknesses ranging from 0.5 to

1.5 mm. Note that these pieces had the same composition as the initial rectangular parallelepipeds due to a negligible composition gradient within such small crystals. The cut faces were

successively re-polished perpendicular to the [001] direction to a roughness of 0.25 μm using a polishing grinder. Prior to the property and structure measurements, all crystals were

annealed at 800 K in air for 1 hour to release any pre-stress induced during the crystal growth and polishing.

Gold electrodes were deposited on the top and bottom surfaces by sputtering, then the macro-properties were measured along the thickness direction of the specimens. The temperature-dependent

dielectric constant of crystals A, B and C, together with PMN-30PT, were characterized using a LCR meter (HP 4284A) in the zero-field-cooling condition. Unipolar strain vs E curves were

measured in the temperature range of 300 to 420 K at 5 K temperature steps at a frequency of 1 Hz using a modified Sawyer-Tower circuit and linear variable differential transducer (LVDT)

driven by a lock-in amplifier (Stanford Research SR850). The data was recorded under E = 50 kV cm−1 after at least two runs, thus the unipolar strain curves in Fig. 1(c–f) were recoverable

(i.e., not a one-time effect from poling), and consistent with the way actuators are used. The specimens were poled under E = 10 kV/cm at 400 K for 15 mins, then cooled to 300 K. The

piezoelectric coefficient d33 for various crystals were measured using a quasi-static Berlincourt d33 meter.

X-ray diffraction studies were performed using a Philips MPD high-resolution system. Facility details can be found in a previous publication31. Diffraction measurements for the ternary

crystals were performed along pseudocubic (002) and (220) zones in the zero-field-heating condition from the annealed states. Line scans were performed over the corresponding angular regions

with a 2θ step interval of 0.002°, whereas reciprocal-space mesh scans were taken with a 2θ interval of 0.005°. Studies were performed under electric field by coating gold electrodes on the

appropriate faces, and attaching thin wires on the electroded sides using a small drop of conducting epoxy. Silicon grease with a high dielectric breakdown strength was used to prevent

arcing. We designated the electroded faces as (001). First, to determine the phase transition under E, comprehensive line scans along the (00L) direction (i.e., electrodes was on top and

bottom faces) were performed on the A-, B- and C-crystals at various temperatures by first increasing and subsquently decreasing E. The E was increased until arching or even dielectric

breakdown occurred, or until a sharp single peak was observed that was insensitive to further increase in E (i.e., saturation state). At each temperature, measurements were began from the

annealed condition. Upon finishing the measurements along (001), the electrodes were removed using a polishing blanket with 0.25 μm aluminum powder. Then, the specimen was re-electroded on

one pair of side faces perpendicular to the original (001) by sputtering. Thereafter, diffraction measurements were carried out along the (H00) and (HH0) zones. Next, to determine the domain

configuration and stable phases, reciprocal-space mesh scans were obtained around the (200) Bragg peaks in the (H, 0, L) zone; and around (220) in the (H, H, L) zone. The mesh scans were

recorded by a sequence of 2θ-ω scans at different ω offsets, and were shown in plots of intensity as a function of reciprocal lattice units (rlu). In this study, we fixed the value of rlu at

a* = 2π/a = 1.550 Å−1, and all reciprocal-space mesh scans of PIN-PMN-PT were plotted in reference to this reciprocal unit.

How to cite this article: Wang, Y. et al. Large field-induced-strain at high temperature in ternary ferroelectric crystals. Sci. Rep. 6, 35120; doi: 10.1038/srep35120 (2016).

The Chinese part of this collaborative work was supported by the National Science Foundation of China (51602156 and 11134004), Natural Science Foundation of Jiangsu Province, China

(BK20160824), the Fundamental Research Funds for the Central Universities (30916011208), the Opening Project of Key Laboratory of Inorganic function material and device, and Chinese Academy

of Sciences (KLIFMD-2015-01). The US part of this collaborative work was sponsored by the Office of Naval Research (N00014-13-1-0049).

School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, Jiangsu, China

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Jiading, 201800, Shanghai, China

Materials Science and Engineering, Virginia Tech, Blacksburg, 24061, Virginia, USA

Y.W. conceived this work and designed the experiments. Y.W. and J.L. performed the experiments. Y.W. and H.L. prepared the single crystals. The data analysis was performed by Y.W., L.C.,

G.Y. H.L. D.V. and Y.W. wrote the manuscript with D.V. assistance. All authors reviewed and commented on the manuscript.

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