PIN PMN PT piezoelectric crystals with increased rhombohedral-to-tetragonal phase transition temperature

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1 JOURNAL OF ADVANCED DIELECTRICS Vol. 4, No. 1 (2014) (6 pages) The Authors DOI: /S X PIN PMN PT piezoelectric crystals with increased rhombohedral-to-tetragonal phase transition temperature Jun Luo*,, Shujun Zhang, Wesley Hackenberger* and Thomas R. Shrout *TRS Technologies, Inc, 2820 East College Avenue State College, PA 16801, USA Material Research Institute Pennsylvania State University University Park, PA 16802, USA jun@trstechnologies.com Received 17 November 2013; Accepted 23 December 2013; Published 21 March 2014 In this work, crystal growth and characterization of PIN PMN PT (29 59% PIN and 28 35% PT) were conducted to understand how PIN ratio in the PIN PMN PT system impacts its phase stability during crystallization. High-quality PIN PMN PT crystals with 36% PIN were obtained using the self-seeded Bridgman process, even though the cubic phase In 2 O 3 formed at the very beginning of solidification. The melt became more unstable when the PIN ratio in the PIN PMN PT system increased to 49% and above, which affected the composition and quality of the as-grown crystals significantly. By increasing the PIN to 36% in PIN PMN PT crystal, the rhombohedral-to-tetragonal phase transition temperatures and the coercive field reached C and kv/cm, respectively, that greatly expanded the operation domains compared to PMN PT crystals. Keywords: PIN PMN PT; crystal growth; the Bridgman method; piezoelectric property; dielectric property. 1. Introduction It has been known that there is a correlation between the Curie temperature (T C ) of perovskite ABO 3 ferroelectrics and the Goldschmidt tolerance factor. 1 The tolerance factor (t), is given by Eq. (1); t ¼ r a þ r pffiffi o ; ð1þ 2 rb þ r o where r a is the ionic radius of A site cations, r b is the ionic radius of B-site cations and r o is the ionic radius of oxygen anion (perovskite stability is maximized for tolerance factors between 0.80 and 1.05). A correlation between the end member tolerance factor and the T C of the morpho torphic phase boundary (MPB) composition has been proposed based on data for known ABO 3 PbTiO 3 solid solution systems. As t decreases, the T C of the MPB composition increases. This prediction was verified when the BiScO 3 PbTiO 3 system was found to have an MPB composition (at 64% PbTiO 3 ) with a T C of 450 C, 100 C higher than PZT. 2,3 For polycrystalline ceramics in the (1 x)pb(yb 1=2 Nb 1=2 ÞO 3 xpbtio 3 (PYN PT) system, the MPB was shown to occur at x 0:5, with a Curie temperature of 350 C. 4,5 For the single crystals of (1 x)pb(in 1=2 Nb 1=2 ÞO 3 xpbtio 3 (PIN PT), T C as high as 260 C was reported when x is 0.28, which was about 100 C higher than that of Pb(Mg 1=3 Nb 2=3 )O 3 PbTiO 3 (PMN PT) crystals. 6 For relaxor-pt systems near MPB, the ceramics and crystals may experience a rhombohedral-to-tetragonal phase transition at a temperature (T rt ) lower than T C, which usually sets the upper temperature limit for most applications involved in these materials. In general, a relaxor-pt system may possess higher T rt if it has higher T C, but for crystals in the same system, higher T C usually is accompanied by lower T rt, due to its strong curved MPB. Furthermore, it was observed that a relaxor-pt system with higher T C usually exhibit higher coercive field (E C ), which determines the maximum AC field that can be applied to the material without causing depolarization. 6 It was demonstrated that crystals in the Pb(In 1=2 Nb 1=2 ÞO 3 Pb(Mg 1=3 Nb 2=3 ÞO 3 PbTiO 3 (PIN PMN PT) ternary system can be directly grown from their melt by the Bridgman technique. 7 9 More study was preferred to understand how PIN ratio in the PIN PMN PT system impacts its phase stability during crystallization, and how it affects the thermal and electrical field stability of the as-grown crystals. In this work, crystal growth and characterization of PIN PMN PT with a broad range of PIN ratio (29 59%) were conducted. 2. Challenges in Crystal Growth of PIN PMN PT Ternary PIN PMN PT single crystals with 15 mm in diameter and mm in length were grown by the self-seeded Bridgman technique. The starting composition was selected as PIN PMN PT with 29 59% PIN and 28 35% PT. In Bridgman growth process, the cylindrical PT crucibles charged with This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 3.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited

Fig. 1. Photo of as grown PIN PMN PT (29% PIN and 28 35% PT) (crystallization from left to right). PIN PMN PT starting materials were placed in a two-zone furnace.

temperature gradient being on the order of 30 50 C/cm between the two zones.

The as-grown crystal boules are shown in Figs. 1 4. Fig. 2.

Photo of as-grown PIN PMN PT (59% PIN and 28 35% PT) crystallization from right to left), two small inserts show a crosssection and the top of the crystal, respectively.

ratio in the system is as high as 29% (PT: 28 35%) (Fig. 1). The crystal growth behavior is very similar to that of pure PMN PT crystals.

crystal can still be produced using the self-seeded Bridgman process with no occurrence of any other secondary phases and no significant volatilization from the melt (Fig. 2).

during solidification and covered the surface of the perovskite crystal (Fig. 3).

and PbO, and formation of the pyrochlore phase. For compositions with 59% PIN (PT: 28 35%) (Fig.

2 Fig. 1. Photo of as grown PIN PMN PT (29% PIN and 28 35% PT) (crystallization from left to right). PIN PMN PT starting materials were placed in a two-zone furnace. The temperature of the upper-zone was set C higher than melting point of the ternary compound, while the lower-zone temperature was C lower, with axial temperature gradient being on the order of C/cm between the two zones. After the charge was melted in the upper-zone, the crucible was lowered down slowly through the temperature gradient to accomplish the unidirectional crystallization process. The as-grown crystal boules are shown in Figs Fig. 2. Photo of as-grown PIN PMN PT (36% PIN and 28 35% PT) (crystallization from right to left), the small insert show the bottom portion of the crystal. Fig. 4. Photo of as-grown PIN PMN PT (59% PIN and 28 35% PT) crystallization from right to left), two small inserts show a crosssection and the top of the crystal, respectively. In this work, it was demonstrated that PIN PMN PT crystals can be grown from their melts by the Bridgman method without the occurrence of any secondary phases even if the PIN ratio in the system is as high as 29% (PT: 28 35%) (Fig. 1). The crystal growth behavior is very similar to that of pure PMN PT crystals. It was verified that, for the crystals with 36% PIN (PT: 28 35%), the secondary phase (cubic In 2 O 3 ) formed at the very beginning of solidification, but high-quality single crystal can still be produced using the self-seeded Bridgman process with no occurrence of any other secondary phases and no significant volatilization from the melt (Fig. 2). In Bridgman growth of crystals with 49% PIN (PT: 28 35%), not only the cubic In 2 O 3 phase occurred at the beginning of the growth, but also red-color pyrochlore phases formed during solidification and covered the surface of the perovskite crystal (Fig. 3). Although high-quality single crystal was still obtained from the self-seeded Bridgman process, the composition shifted quite significantly because of volatilization of In 2 O 3 and PbO, and formation of the pyrochlore phase. For compositions with 59% PIN (PT: 28 35%) (Fig. 4) and binary 68% PIN 32% PT, single crystals could not be obtained even in self-seeded Bridgman growth, owing to the formation of a large amount of pyrochlore phase and significant volatilization of the melt. The above crystal growth results indicated that the melt became more and more unstable with the increase of PIN ratio in the ternary system. Formation of the cubic In 2 O 3, volatilization of In 2 O 3 and PbO and formation of the pyrochlore phase, which occurred individually or simultaneously during solidification, became the major obstacles to Bridgman growth of these crystals. Fig. 3. Photo of as-grown PIN PMN PT (49% PIN and 28 35% PT) (crystallization from right to left), two small inserts show a cross-section and the top of the crystal, respectively Nucleation and grain growth of cubic In 2 O 3 As mentioned above, a cubic In 2 O 3 phase was formed in the PIN PMN PT melt at the beginning of solidification in each self-seeded Bridgman crystal growth run with PIN ratio equal

larger, the free energy state of their interfaces in the melt became lower than those of In 2 O 3 grains, so that In 2 O 3 grains disappeared gradually and the melt became stable. Fig. 5.

Since In 2 O 3 has a black color, the bottom of these boules always turned to black (Figs. 2 4).

3 larger, the free energy state of their interfaces in the melt became lower than those of In 2 O 3 grains, so that In 2 O 3 grains disappeared gradually and the melt became stable. Fig. 5. SEM photo (BSE image) of In 2 O 3 phase (black) distributed in the PIN PMN PT polycrystalline matrix (white) in the initial part of a PIN PMN PT crystal (49% PIN). to or higher than 36%. Since In 2 O 3 has a black color, the bottom of these boules always turned to black (Figs. 2 4). Cubic or triangular In 2 O 3 grains, formed at the very beginning of the solidification process, were randomly distributed in the polycrystalline PIN PMN PT matrix as shown in the SEM micrograph in Fig. 5. The quantitative composition analysis by electron probe microanalysis (EPMA) indicated that the black grains yielded a signal corresponding solely to indium, while the stoichiometry of the matrix was very close to PIN PMN PT (Table 1). Meanwhile, the powder XRD confirmed that both a PIN PMN PT (perovskite phase) and In 2 O 3 (cubic phase) coexisted in the initial part of a crystal with 49% PIN. With the increase of PIN ratio in PIN PMN PT, the melt might become metastable, which could be the main cause of cubic In 2 O 3 formation. In these self-seeded Bridgman growth runs, spontaneous nucleation of both the perovskite phase PIN PMN PT and the cubic phase In 2 O 3 happened simultaneously and competed with each other at the beginning of solidification. It was quite possible that the solidification process was dominated by the crystallization of PIN PMN PT. When the PIN PMN PT crystal grains grew larger and Table 1. EPMA quantitative chemical analysis of the matrix and black grains shown in Fig Volatilization of the melt In crystal growth of PIN PMN PT with 36% PIN, even though In 2 O 3 nucleation happened at the beginning of solidification, there is no obvious volatilization observed during the crystal growth. However, in the crystal growth run of PIN PMN PT with 49% PIN, a significant amount of melt was volatilized during crystal growth and then solidified on the inner wall of the Pt crucible possibly during cooling. The total amount of volatile was more than 4.2 wt.% in one of these runs (Fig. 6). With higher PIN concentration, even more material was volatilized from the melt. The composition of the volatile from a run was quantitatively analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES), which indicated that it contented major amounts of In and Pb, and minor amounts of Nb, Ti and Mg. Powder XRD pattern verified that it was composed of In 2 O 3 and a type of perovskite compound Formation of pyrochlore phase In the Bridgman growth of PIN PMN PT single crystal with 49% PIN or above, significant amount of red-color Fig. 6. The volatile solidified on the inner wall of Pt crucible. Matrix Black blocks Atomic% Oxide% Atomic% Oxide% In Pb Nb Ti Mg O Total Fig. 7. The pyrochlore crystal grains on the top of the boule

Owing to formation of the pyrochlore phases and volatilization of the melt, the compositions of as-grown PIN PMN PT crystals shifted dramatically from those in the starting material and became hard

4 pyrochlore phases formed during solidification, which was accompanied by the volatilization of the melt. Figure 7 showed that the nucleation and grain growth of pyrochlore phases happened at the end of solidification. Owing to formation of the pyrochlore phases and volatilization of the melt, the compositions of as-grown PIN PMN PT crystals shifted dramatically from those in the starting material and became hard to control even if single crystals still could be grown by Bridgman method. 3. Dielectric and Piezoelectric Properties of PIN PMN PT Crystals 3.1. Longitudinal property distributions Just like PMN PT, PIN PMN PT is a complete solid solution system. It inevitably exhibits an inhomogeneous composition distribution along a crystal grown by Bridgman method, resulting in variation of dielectric and piezoelectric properties along the growth orientation. In a PIN PMN PT crystal, it was identified that distributions of Pb, Mg, Ti and Nb followed similar trends along the crystallization direction as in PMN PT crystal; meanwhile, the In concentration kept nearly constant (only 1% variation) during the whole crystallization process. 10 Since segregation coefficient of In in PIN PMN PT system is close to 1, the composition segregation induced property variation along the crystallization direction in PIN PMN PT crystals is similar to that in PMN PT crystals. Analogous to PMN PT crystals, the PT content varied from 28% to 35% along the growth direction, with low PT content observed at the bottom part of the grown crystal boules, whereas the top portion generally lies in the high PT tetragonal phase region. The longitudinal property distributions of a PIN PMN PT crystal with 36% PIN were characterized in comparison to PMN PT crystals with similar PT content. The T C and T rt were determined from the temperaturedependent dielectric using a multifrequency LCR meter (HP4284A), connected to a computer-controlled temperature chamber. Figure 8 compares the T C and T rt as a function of location in a PIN PMN PT (36% PIN and 28 35% PT) and apmn PT (28 35% PT) crystal boule, which was mainly impacted by the similar Ti segregation along the boules. However, PIN PMN PT possessed consistently higher T C and T rt at the same location of the boule, which was obviously corresponded to In substitution and its nearly even distribution along the boule. The T C and T rt were found to be 230 C and 90 C, respectively at top of the PIN PMN PT crystal boule, corresponding to higher PT content in the crystals, while samples at the bottom part of the crystal boule, with lower PT content, exhibited lower T C (180 C) and higher T rt (135 C). An average T rt increase of 35 Cto 40 C was observed in this PIN PMN PT boule in comparison with the PMN PT boule, which extended the usage temperature to C. Fig. 8. Comparison of the T C and T rt as a function of location in a PIN PMN PT (36% PIN and 28 35% PT) and a PMN PT (28 35% PT) crystal boule. Fig. 9. Illustration of sample cutting from a crystal with 36% PIN. Three h001i-oriented samples cut along the growth direction in rhombohedral portion of the crystal with 36% PIN were studied. The sample cutting from this crystal was illustrated in Fig. 9. Sample 1 has the lowest PT content; while sample 3 has the highest PT content and is more close to the MPB composition. The polarization and strain behavior were investigated at room temperature and elevated temperature using a modified Sawyer-Tower circuit and linear variable differential transducer (LVDT) driven by a lock-in amplifier. Measured from the polarization and butterfly strain behaviors, the coercive fields were found to be on the order of kv/cm for slightly different PT contents. The unipolar strain measured at an electric field of 60 kv/cm indicated that the piezoelectric coefficient (d 33 ) for the crystal with 36% PIN was in the range of pc/n (Fig. 10). Sample 1 and sample 2, located in the lower part of the crystal boule (lower PT content), showed linear strain behavior with no electric field-induced ferroelectric phase transition observed. For sample 3, however, it was found that a field induced phase transition occurred at about 45 kv/cm, above which, the crystal transformed to the tetragonal phase. So with increasing PT content in the ternary PIN PMN PT system, the coercive field and piezoelectric coefficients were found to increase, however, the field-induced phase transition level was decreased. Figure 11 shows the electromechanical coupling factor, k 33, as a function of temperature for [001] oriented longitudinal rods in the temperature range of C, respectively. The

5 Table 2. This effects of PIN content on piezoelectric and dielectric properties of PIN PMN PT. Properties T rt E C K T 33 d 33 PIN% " " " # # Fig. 10. (Color online) The unipolar strain measured at an electric field of 60kV/cm. Fig. 11. The electromechanical coupling as a function of temperature. room-temperature coupling k 33 was found to be 89% to 91%, gradually increasing to about 94% at the phase transition temperature T rt, above which, the coupling value was decreased due to the tetragonal phase. It was confirmed again that the PIN PMN PT crystals can be operated at a temperature C higher than the PMN PT crystals of similar PT content without encountering any phase transition-induced property degradation The relationship between property and composition A binary phase diagram for PIN PT solid solution system was established based on phase transition temperatures of poled ceramics. 11 In this system, the MPB was found to be located at the composition of 62% PIN. It was also revealed that for the PIN PT system near the MPB (PIN between 68% and 78%) the ceramics present T rt above 200 C, which is about C higher than the PMN PT system near its MPB (PMN between 67% and 73%). Furthermore, for the compositions near MPB, PIN-PT also possesses coercive field (E C ) about 10 times higher than PMN PT. 11 It is predictable that, for PIN PMN PT ternary system with the composition near the MPB, both T rt and E C may increase if PIN content increases in the system. These trends were proved by the characterization of PIN PMN PT crystals grown by the Bridgman method in this work for the PIN content up to 36%. As summarized in Table 2, with increasing the PIN content, T rt and E C increase while dielectric permittivity (k T 33) and piezoelectric coefficient (d 33 Þ decreases slightly. As Bridgman growth of the PIN PMN PT crystals with PIN equal to or higher than 49% experienced significant composition shifting due to volatilization of PbO and In 2 O 3 from the melt, and formation of pyrochlore phase, higher T rt and E C have not been demonstrated for them. 4. Conclusions Since it was expected that T rt and E C increase proportionally with the PIN content in PIN PMN PT system, growth of the crystals with higher PIN content becomes a main approach to expand the operation domains of the relaxor-pt piezoelectric single crystals. This work demonstrated that high-quality PIN PMN PT crystals can be grown from its melt directly by the Bridgman method till PIN content reaches certain level. The major obstacles for Bridgman growth of PIN PMN PT crystals with PIN beyond 36% are formation of cubic In 2 O 3, volatilization of In 2 O 3 and PbO and formation of pyrochlore phase, which occurred individually or simultaneously during solidification. A systematic property study was conducted for the PIN PMN PT crystals that consist of 36% PIN. The piezoelectric properties of rhombohedral PIN PMN PT crystals were comparable to binary PMN PT crystals, but the thermal and electric field stability were significantly improved. Of particular interest, the rhombohedral-to-tetragonal phase transition temperatures and the coercive field observed were on the order of C and kv/cm, respectively, that greatly expanded the operation domains compared to PMN PT crystals. It is expected that operation domain can be further enlarged if the composition shifting occurred in Bridgman growth of PIN PMN PT crystals with higher PIN content can be overcome.. References 1 V. M. Goldschmidt, Skrifer Norske Videnskaps-Akad. Oslo, I. Mat.-Nat. Kl. 8, 5 (1926). 2 R. E. Eitel, C. A. Randall, T. R. Shrout, P. W. Rehrig, W. Hackenberger and S.-E. Park, New high temperature morphotropic

6 J. Luo et al. J. Adv. Dielect. 4, (2014) phase boundary piezoelectrics based on Bi(Me)O 3 -PbTiO 3 ceramics, Jpn. J. Appl. Phys. 40, 5999 (2001). 3 S. J. Zhang, C. A. Randall and T. R. Shrout, High curie temperature piezocrystals in the BiScO 3 -PbTiO 3 perovskite system, Appl. Phys. Lett. 83, 3150 (2003). 4 T. Yamamoto and S. Ohashi, Dielectric and piezoelectric properties of Pb(Yb 1=2 Nb 1=2 ÞO 3 -PbTiO 3 solid solution systems, Jpn. J. Appl. Phys. 34, 5349 (1995). 5 S. Zhang, S. Rhee, C. A. Randall and T. R. Shrout, Dielectric and piezoelectric properties of high curie temperature single crystals in the Pb(Yb 1=2 Nb 1=2 ÞO 3 -xpbtio 3 solid solution series, Jpn. J. Appl. Phys. 41, 722 (2002). 6 S. J. Zhang and T. R. Shrout, Relaxor-PT single crystals: Observations and developments, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57, 2138 (2010). 7 Y. P. Guo, H. S. Luo, T. He and Z. W. Yin, Peculiar properties of a high Curie temperature Pb(In 1/2 Nb 1/2 )O 3 PbTiO 3 single crystal grown by the modified Bridgman technique, Solid State Commun. 123, 417 (2002). 8 G. S. Xu, K. Chen, D. F. Yang and J. B. Li, Growth and electrical properties of large size Pb(In 1/2 Nb 1/2 )O 3 Pb(Mg 1/3 Nb 2/3 )O 3 PbTiO 3 crystals prepared by the vertical Bridgman technique, Appl. Phys. Lett. 90, (2007). 9 S. J. Zhang, J. Luo, W. Hackenberger and T. R. Shrout, Characterization of PIN PMN PT ferroelectric crystal with enhanced phase transition temperatures, J. Appl. Phys. 104, (2008). 10 J. Luo, S. Zhang, W. Hackenberger and T. Shrout, The progress update of relaxor piezoelectric single crystals, Proc. IEEE Int. Ultrasonics Symp., IUS (2009), pp E. F. Alberta and A. S. Bhalla, Piezoelectric properties of Pb (InNb) 1=2 O 3 -PbTiO 3 solid solution ceramics, J. Korean Phys. Soc. 32, S1265 (1998)

Morphotropic PMN-PT system investigated through comparison

Morphotropic PMN-PT system investigated through comparison between ceramic and crystal Gaël Sebald, Laurent Lebrun, Benoît Guiffard, Daniel Guyomar LGEF, INSA de Lyon, Villeurbanne, France Abstract Ferroelectric