- Email: [email protected]
3)O3–PbTiO3 system
December 2002
Materials Letters 57 (2002) 609 – 614 www.elsevier.com/locate/matlet
Structural study of Pb(Fe2/3W1/3)O3 –PbTiO3 system Liliana Mitoseriu a,b,*, Paula M. Vilarinho a, Massimo Viviani c, Joa˜o L. Baptista a a
Department of Ceramics and Glass Engineering/UIMC, University of Aveiro, 3810, Portugal b Department of Electricity, Faculty of Physics, University Al. I. Cuza Iasi, 6600, Romania c ICFAM-CNR, Via de Marini 6, Genoa 16149, Italy Received 28 February 2002; accepted 14 April 2002
Abstract The crystalline structure of the solid solution (1x)Pb(Fe2/3W1/3)O3 – xPbTiO3 (PFW – PT) with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 was analysed using X-ray diffraction (XRD) with comparative cell-refinement methods, in order to detect the change in symmetry with composition. Pure PFW shows pseudo-cubic symmetry for temperatures in the range 85 – 423 K. For the temperatures T=85 K and T=300 K, the solid solutions have cubic or tetragonal structures. The cell constants are functions of PbTiO3 amount. A temperature-dependent Morphotropic Phase Boundary (MPB) separating the pseudo-cubic (relaxor) and tetragonal (ferroelectric) phases might be present. The ranges of compositions for this boundary are: x<0.2 at T=85 K and xa(0.3,0.35) for T=300 K. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Perovskites; Ferroelectric relaxor; X-ray diffraction
1. Introduction Lead – iron –tungstate Pb(Fe2/3W1/3)O3 (PFW) is a ferroelectric relaxor with disordered ABO3 perovskite structure, in which Fe3+ and W6+ ions are randomly distributed in B-centers of the BO6 octahedral positions [1,2]. PFW has a frequency dependent permittivity with a broad maximum at 180 K [2,3]. Its dielectric, ageing and ordering properties were previously studied [4,5]. As for other members of the lead-based relaxor family, the Curie temperature can be easily shifted to higher temperatures by addition of PbTiO3 (PT). Data about the lattice parameters of PFW – PT solid solutions * Corresponding author. Department of Electricity, Faculty of Physics, Univ. Al. I. Cuza Iasi, 6600, Romania. Tel.: +40-32144760; fax: +40-32-213330. E-mail address: [email protected] (L. Mitoseriu).
have been briefly reported [6] and some dielectric properties were previously investigated by the authors [7]. Studies dedicated to Pb(B1B2)O3 – PbTiO3 solid solutions demonstrated the existence of a Morphotropic Phase Boundary (MPB) where two crystalline phases coexist and optimum dielectric and piezoelectric properties of the system were observed. Among them, the structural and dielectric properties of MPB in the binary systems Pb(Mg 1/3 Nb 2/3 )O 3 – PbTiO 3 (PMN – PT) [8,9] and Pb(Zn 1/3 Nb 2/3 )O 3 – PbTiO 3 (PZN – PT) [10,11] were the most studied ones. Because of the compositional and structural analogy with the other Pb(B1B2)O3 –PbTiO3 systems, a similar MPB might also exist in PFW – PT solid solution. There are no reported data in literature about this possibility. Previous dielectric study of (1x)PFW – xPT system with x=0.2, 0.25, 0.3, 0.35, 0.4 has shown a change
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contain three distinct regions: I—at low temperatures and low PT concentration (relaxor behaviour); II—for low temperatures and high PT content (ferroelectric properties); and region III, at high temperatures for all the compositional range (paraelectric). According to our previous results [7], an MPB might separate the relaxor– ferroelectric regions at low temperature and the possible composition is around x=0.3. The aim of the present work is to study the structural change of PFW – PT system with composition and temperature, in order to detect the existence of a superposition of phases and the range of composition for the possible MPB.
2. Experimental procedure Fig. 1. Maximum dielectric constant of (1x)PFW – xPT solid solution versus PT concentration (x), measured at f=100 kHz [7].
from relaxor to a ferroelectric behaviour, as the PT content increases [7]. A maximum value of the dielectric constant of 12,000 at 100 kHz was found for x=0.3, as shown in Fig. 1. The temperature corresponding to the maximum of permittivity is shifted towards high temperatures, as PT content increases (Fig. 2) [7]. The phase diagram of PFW – PT system shown in Fig. 2
Fig. 2. Transition temperature (temperature corresponding to the dielectric maxima) of (1x)PFW – xPT solid solution versus PT concentration (x), measured at f=100 kHz [7].
Ceramic (1x)PFW –xPT with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 were prepared using a conventional mixed oxide method, starting with PbO, Fe2O3, WO6 and TiO2 reagents (Merck, minimum purity 99%) that were mixed, milled, dried and calcined [7]. The phase formation during calcination was analysed by X-ray diffraction (XRD) and best parameters for calcination have been found (time: 3 h and temperature 800– 850 jC). The calcined powders were re-milled for 8 –10 h in order to obtain powders with grain size <2 Am (Fig. 3). The structural symmetry of the solid solutions was studied by XRD analysis at the temperatures 85 and 300 K and of PFW at temperatures in the range 85– 423 K, using a high resolution Philips X’Pert MPD diffrac-
Fig. 3. SEM micrograph of 0.70PFW – 0.30PT ceramic powder.
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tometer, operated at (40 kV, 50 mA) with curved graphite monochromator for CuKa radiation (k= 0.154056 nm), having 0.05j step scan and scan rate of 0.05j/s. A nitrogen Anton Paar camera TTK450 was used for the low temperature measurements. In order to find the cell parameters, the recorded diffractograms were fitted using a trial-and-error program for indexing (soft=PowderX/TREOR90, freeware) [12], where the inputs are the Bragg peaks, as output, the phase symmetry and cell constants are obtained and the control parameter is the figure of merit, calculated according to Ref. [13].
3. Results and discussions 3.1. XRD results for PFW at different temperatures Fig. 4 shows the X-ray diffractograms recorded for pure PFW at different temperatures, in the range of 85– 423 K. In whole the analysed range of temperatures, PFW shows a cubic single phase, as other relaxors from the same family Pb(B1B2)O3 [14]. Even for the lowest temperature (85 K), no broadening of the (222) Bragg reflexion characteristic (not shown here) to a possible rhombohedral distortion was detected in the range of diffraction angles 2ha (0,90j). Supposing a rhombo-
Fig. 4. XRD pattern of PFW at different temperatures indicating no change in crystalline symmetry between T=85 K and T=423 K.
Fig. 5. Simulated XRD pattern for PFW in the cubic and rhombohedral phases (by considering an angular distortion of 0.02j), in the range of 2ha(160j,162j).
hedral symmetry (R3m) at low temperatures and a cubic one (Pm3m) at high temperature and using the cell parameters reported in Ref. [2], we simulated the XRD pattern in the two situations. For a low rhombohedral distortion angle of 0.02j, the two simulated spectra are identical, in the range of low diffraction angles 2ha(0,90j). Some small differences could be obtained only for high diffraction angles and very welldefined peaks for both symmetries are present for 2hi161.3j (Fig. 5). In order to check this possibility, we recorded X-ray diffractograms at 85 and 300 K for high diffraction angles in the range 2ha(160,162j), with low speed scan of 0.01j/s and scan step of 0.01j (the last value of diffraction angle of 162j being the experimental limit of the equipment used in this work). The experimental XRD patterns for 85 and 300 K are shown in Fig. 6. The low temperature spectra shows a slightly higher intensity peak and a possible split of the rhombohedral phase peaks. In fact, the two diffractograms are very similar, taking into account the thermal expansion effect. The difference between the two types of symmetries of PFW, the cubic (Pm3m) and the rhombohedral one (R3m) cannot be clearly detected by XRD measurements in the limit of our experimental accuracy. It is known that at low temperature PFW shows relaxor properties with P(E) hysteresis loop and rema-
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Fig. 6. Experimental XRD pattern for PFW at T=85 K (rhombohedral phase) and at T=300 K (cubic phase), in the range of 2ha(160j,162j).
nent polarization, i.e. a nonzero dipolar moment, compatible with a non-centrosymmetric phase [15]. For T>TC=180 K, PFW turns into a paraelectric cubic material (centrosymmetric). However, the XRD analysis conducted in this study did not detect clearly the transition between these two states. This behaviour is due to a very low rhombohedral distortion (probably less than 0.01j) related to the short range order (SRO) of nanopolar regions existing in the relaxor state of PFW with average cubic symmetry. This state, called in literature pseudo-cubic phase, has been observed in similar Pb-based relaxors, as PMN [14]. By decreasing the temperature, the tendency towards long range ordering (LRO) increases. The system can be changed from relaxor to a LRO ferroelectric by applying an electric field [12]. Further XRD studies of poled PFW, Raman and diffraction neutron methods, more sensitive to SRO change, might show a clearly structural modification from a pseudo-cubic (rhombohedral) to a cubic symmetry.
increases, the splitting of (200) – (002), (201) –(210), (112) – (211), etc. Bragg peaks becomes more pronounced, demonstrating that the average symmetry becomes tetragonal with increasing PT addition (x). The results of the cell refinement show that all the PFW – PT compounds having a PT content in the range between x=0.2 and x=0.35 have single phase tetragonal symmetry ( P4mm), with the cell parameters dependent on the PT amount. The cell parameters and tetragonality obtained for pure PFW and for the (1x)PFW – xPT compounds with x=0.2, 0.25, 0.3 and 0.35 at 85 pffiffiffiand 300 K are shown in Figs. 8– 10, where a0 ¼ 3 is the average pseudo-cubic perovskite cell. At T=85 K, a monotonous decreasing of acell parameter and an increasing of the c-cell parameter of the perovskite cell and, consequently, an increasing of tetragonality c/a with PT concentration take place (Fig. 10). Because PFW at T=85 K is pseudo-cubic and the 0.2 compound is already tetragonal, it means that a possible MPB could exist for a PT amount less than x=0.2, at this temperature. At T=300 K, a similar analysis of diffractograms shown that the samples with x=0.2, 0.25 and 0.3 have a pseudo-cubic single-phase symmetry, with cell parameters almost independent on the PT concentration (Fig. 9). This result is in agreement with the relaxor state of these compositions at this temperature. The samples with x=0.35 and 0.4 show a tetragonal
3.2. XRD results for (1x)PFW– xPT solid solution at T=85 K and T=300 K The XRD pattern of (1x)PFW –xPT solid solution recorded at T=85 K are depicted in Fig. 7, for x=0.2, 0.25, 0.3 and 0.35 compositions. As PT content
Fig. 7. XRD pattern of (1x)PFW – xPT solid solution with x=0, 0.2, 0.25, 0.3, 0.35 and 0.4 at T=85 K.
L. Mitoseriu et al. / Materials Letters 57 (2002) 609–614
Fig. 8. Cell parameters of (1x)PFW – xPT versus PT amount (x) at T=85 K.
symmetry ( P4mm) with a gradual increasing of c-cell parameter and decreasing of a-cell parameter when increasing of PT concentration (Fig. 9). The previous dielectric data show that the samples with these compositions are in the ferroelectric region (II) of the diagram represented in Fig. 2, at this temperature. From Figs. 9 and 10, it results that there is a change of the symmetry of the system from pseudo-cubic to
Fig. 9. Cell parameters of (1x)PFW – xPT versus PT amount (x) at T=300 K.
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Fig. 10. Tetragonality c/a of (1x)PFW – xPT solid solution at T=85 K and T=300 K versus PT amount.
tetragonal, in the range of compositions with x=0.3 to 0.35. Consequently, an MPB might be found in this range, at T=300 K. Further studies will analyse in detail this range of compositions, using a higher resolution X-ray diffractograms in order to check the possible superposition of phases. We mention that both the temperature region separating the pseudo-cubic to cubic region (not detected in this study) and the composition boundary between the pseudo-cubic and tetragonal phases are separating similar states with slightly different properties [14]. In the relaxor state with SRO, nanopolar clusters size-distributed give rise to a broad phase transition and deviations from Curie – Weiss law, typical for relaxors. By increasing the temperature, the size of these clusters is progressively reduced and they finally vanish. However, the polar clusters can still be preserved above the ‘‘Curie temperature’’ TC by applying a field (as P(E) loops were observed for temperatures slightly above TC [15]). On the other hand, at fixed temperature, by adding to the relaxor material increasing amounts of a true ferroelectric like PT, stronger dipolar interaction makes the polar nanoregions to join each other and to transform in ferroelectric domains and the system becomes an LRO ferroelectric. The change in size of ordered polar regions with PT concentration is a gradual one and is
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strongly influenced by electrical and thermal history. Consequently, the MPB expected in PFW – PT system will be temperature- and field-dependent, as already pointed in similar solid solutions [9– 11,16]. According to results presented above (Fig. 10), PFW –PT system could have an MPB dependent on temperature, most probably bended towards the rich PT region with increasing the temperature. At low temperature T=85 K, the possible location of the MPB is for compositions with less than x=0.2. At the temperature T=300 K, an MPB might exist in the range between x=0.3 and 0.35. Further investigations are on going in order to find the superposition of phases and to restrict more this range of compositions.
4. Conclusions A study of crystalline symmetry of (1x)PFW – xPT solid solution with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 at T=85 K and T=300 K using XRD analysis has been performed. All the solid solutions show pseudo-cubic or tetragonal single-phase symmetry. The temperature ‘‘boundary’’ between the real cubic (paraelectric) and pseudo-cubic (rhombohedral, relaxor) phases could not be delimited under the present experimental limit of accuracy, even for high Bragg angles around 2hi161.3j. An MPB, separating the pseudo-cubic (relaxor) and tetragonal (ferroelectric) phases, dependent on temperature might exist. The possible ranges of compositions for this MPB are: x<0.2 at T=85 K and xa(0.3,0.35) for T=300 K. Further high resolution XRD analysis and Raman studies are necessary to detect the possible superposition of phases and to restrict the range of compositions. Dielectric and piezoelectric measurements are necessary for a better characterization of the
PFW – PT compounds in the range of structural change.
Acknowledgements One of the authors (LM) thanks NATO Science Fellowship Programme, Portugal, for financial support. The discussions and the permanent assistance in XRD studies of Dr. M.R. Soares from Central Laboratory, University of Aveiro, are highly appreciated.
References [1] G.A. Smolenskii, A.I. Agranovskaya, V.A. Isupov, Sov. Phys., Solid State 1 (1959) 907. [2] A.I. Agranovskaya, Bull. Acad. Sci. U.S.S.R., Phys. Ser. (1960) 1271. [3] K. Uchino, S. Nomura, J. Phys. Soc. Jpn. 41 (1976) 542. [4] L. Zhou, P.M. Vilarinho, J.L. Baptista, J. Am. Ceram. Soc. 79 (1996) 2436. [5] L. Zhou, P.M. Vilarinho, J.L. Baptista, J. Eur. Ceram. Soc. 18 (1998) 1383. [6] Y.A. Shevchuk, S.K. Korchagina, V.V. Gagulin, V.V. Bogatko, Ferroelectrics 199 (1997) 223. [7] P.M. Vilarinho, L. Zhou, M. Po¨ckl, N. Marques, J.L. Baptista, J. Am. Ceram. Soc. 83 (2000) 1149. [8] J.C. Ho, K.S. Liu, I.N. Liu, J. Mater. Sci. 28 (1993) 4497. [9] S.W. Choi, J.M. Jung, Ferroelectrics 189 (1996) 27. [10] J. Kuwata, K. Uchino, S. Nomura, Ferroelectrics 37 (1981) 579. [11] M.L. Mulvihill, L.E. Cross, K. Uchino, J. Am. Ceram. Soc. 78 (1995) 3345. [12] C. Dong, J. Appl. Crystallogr. 32 (1999) 838. [13] P.M. De Wolff, J. Appl. Crystallogr. 1 (1968) 108. [14] G. Burns, F.H. Dacol, Ferroelectrics 104 (1990) 25. [15] L. Zhou, Relaxor behaviour of PFW ceramics, PhD Thesis, University of Aveiro, 1996. [16] O. Noblanc, P. Gaucher, G. Calvarin, J. Appl. Phys. 79 (1996) 4291.
Materials Letters 57 (2002) 609 – 614 www.elsevier.com/locate/matlet
Structural study of Pb(Fe2/3W1/3)O3 –PbTiO3 system Liliana Mitoseriu a,b,*, Paula M. Vilarinho a, Massimo Viviani c, Joa˜o L. Baptista a a
Department of Ceramics and Glass Engineering/UIMC, University of Aveiro, 3810, Portugal b Department of Electricity, Faculty of Physics, University Al. I. Cuza Iasi, 6600, Romania c ICFAM-CNR, Via de Marini 6, Genoa 16149, Italy Received 28 February 2002; accepted 14 April 2002
Abstract The crystalline structure of the solid solution (1x)Pb(Fe2/3W1/3)O3 – xPbTiO3 (PFW – PT) with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 was analysed using X-ray diffraction (XRD) with comparative cell-refinement methods, in order to detect the change in symmetry with composition. Pure PFW shows pseudo-cubic symmetry for temperatures in the range 85 – 423 K. For the temperatures T=85 K and T=300 K, the solid solutions have cubic or tetragonal structures. The cell constants are functions of PbTiO3 amount. A temperature-dependent Morphotropic Phase Boundary (MPB) separating the pseudo-cubic (relaxor) and tetragonal (ferroelectric) phases might be present. The ranges of compositions for this boundary are: x<0.2 at T=85 K and xa(0.3,0.35) for T=300 K. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Perovskites; Ferroelectric relaxor; X-ray diffraction
1. Introduction Lead – iron –tungstate Pb(Fe2/3W1/3)O3 (PFW) is a ferroelectric relaxor with disordered ABO3 perovskite structure, in which Fe3+ and W6+ ions are randomly distributed in B-centers of the BO6 octahedral positions [1,2]. PFW has a frequency dependent permittivity with a broad maximum at 180 K [2,3]. Its dielectric, ageing and ordering properties were previously studied [4,5]. As for other members of the lead-based relaxor family, the Curie temperature can be easily shifted to higher temperatures by addition of PbTiO3 (PT). Data about the lattice parameters of PFW – PT solid solutions * Corresponding author. Department of Electricity, Faculty of Physics, Univ. Al. I. Cuza Iasi, 6600, Romania. Tel.: +40-32144760; fax: +40-32-213330. E-mail address: [email protected] (L. Mitoseriu).
have been briefly reported [6] and some dielectric properties were previously investigated by the authors [7]. Studies dedicated to Pb(B1B2)O3 – PbTiO3 solid solutions demonstrated the existence of a Morphotropic Phase Boundary (MPB) where two crystalline phases coexist and optimum dielectric and piezoelectric properties of the system were observed. Among them, the structural and dielectric properties of MPB in the binary systems Pb(Mg 1/3 Nb 2/3 )O 3 – PbTiO 3 (PMN – PT) [8,9] and Pb(Zn 1/3 Nb 2/3 )O 3 – PbTiO 3 (PZN – PT) [10,11] were the most studied ones. Because of the compositional and structural analogy with the other Pb(B1B2)O3 –PbTiO3 systems, a similar MPB might also exist in PFW – PT solid solution. There are no reported data in literature about this possibility. Previous dielectric study of (1x)PFW – xPT system with x=0.2, 0.25, 0.3, 0.35, 0.4 has shown a change
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contain three distinct regions: I—at low temperatures and low PT concentration (relaxor behaviour); II—for low temperatures and high PT content (ferroelectric properties); and region III, at high temperatures for all the compositional range (paraelectric). According to our previous results [7], an MPB might separate the relaxor– ferroelectric regions at low temperature and the possible composition is around x=0.3. The aim of the present work is to study the structural change of PFW – PT system with composition and temperature, in order to detect the existence of a superposition of phases and the range of composition for the possible MPB.
2. Experimental procedure Fig. 1. Maximum dielectric constant of (1x)PFW – xPT solid solution versus PT concentration (x), measured at f=100 kHz [7].
from relaxor to a ferroelectric behaviour, as the PT content increases [7]. A maximum value of the dielectric constant of 12,000 at 100 kHz was found for x=0.3, as shown in Fig. 1. The temperature corresponding to the maximum of permittivity is shifted towards high temperatures, as PT content increases (Fig. 2) [7]. The phase diagram of PFW – PT system shown in Fig. 2
Fig. 2. Transition temperature (temperature corresponding to the dielectric maxima) of (1x)PFW – xPT solid solution versus PT concentration (x), measured at f=100 kHz [7].
Ceramic (1x)PFW –xPT with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 were prepared using a conventional mixed oxide method, starting with PbO, Fe2O3, WO6 and TiO2 reagents (Merck, minimum purity 99%) that were mixed, milled, dried and calcined [7]. The phase formation during calcination was analysed by X-ray diffraction (XRD) and best parameters for calcination have been found (time: 3 h and temperature 800– 850 jC). The calcined powders were re-milled for 8 –10 h in order to obtain powders with grain size <2 Am (Fig. 3). The structural symmetry of the solid solutions was studied by XRD analysis at the temperatures 85 and 300 K and of PFW at temperatures in the range 85– 423 K, using a high resolution Philips X’Pert MPD diffrac-
Fig. 3. SEM micrograph of 0.70PFW – 0.30PT ceramic powder.
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tometer, operated at (40 kV, 50 mA) with curved graphite monochromator for CuKa radiation (k= 0.154056 nm), having 0.05j step scan and scan rate of 0.05j/s. A nitrogen Anton Paar camera TTK450 was used for the low temperature measurements. In order to find the cell parameters, the recorded diffractograms were fitted using a trial-and-error program for indexing (soft=PowderX/TREOR90, freeware) [12], where the inputs are the Bragg peaks, as output, the phase symmetry and cell constants are obtained and the control parameter is the figure of merit, calculated according to Ref. [13].
3. Results and discussions 3.1. XRD results for PFW at different temperatures Fig. 4 shows the X-ray diffractograms recorded for pure PFW at different temperatures, in the range of 85– 423 K. In whole the analysed range of temperatures, PFW shows a cubic single phase, as other relaxors from the same family Pb(B1B2)O3 [14]. Even for the lowest temperature (85 K), no broadening of the (222) Bragg reflexion characteristic (not shown here) to a possible rhombohedral distortion was detected in the range of diffraction angles 2ha (0,90j). Supposing a rhombo-
Fig. 4. XRD pattern of PFW at different temperatures indicating no change in crystalline symmetry between T=85 K and T=423 K.
Fig. 5. Simulated XRD pattern for PFW in the cubic and rhombohedral phases (by considering an angular distortion of 0.02j), in the range of 2ha(160j,162j).
hedral symmetry (R3m) at low temperatures and a cubic one (Pm3m) at high temperature and using the cell parameters reported in Ref. [2], we simulated the XRD pattern in the two situations. For a low rhombohedral distortion angle of 0.02j, the two simulated spectra are identical, in the range of low diffraction angles 2ha(0,90j). Some small differences could be obtained only for high diffraction angles and very welldefined peaks for both symmetries are present for 2hi161.3j (Fig. 5). In order to check this possibility, we recorded X-ray diffractograms at 85 and 300 K for high diffraction angles in the range 2ha(160,162j), with low speed scan of 0.01j/s and scan step of 0.01j (the last value of diffraction angle of 162j being the experimental limit of the equipment used in this work). The experimental XRD patterns for 85 and 300 K are shown in Fig. 6. The low temperature spectra shows a slightly higher intensity peak and a possible split of the rhombohedral phase peaks. In fact, the two diffractograms are very similar, taking into account the thermal expansion effect. The difference between the two types of symmetries of PFW, the cubic (Pm3m) and the rhombohedral one (R3m) cannot be clearly detected by XRD measurements in the limit of our experimental accuracy. It is known that at low temperature PFW shows relaxor properties with P(E) hysteresis loop and rema-
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Fig. 6. Experimental XRD pattern for PFW at T=85 K (rhombohedral phase) and at T=300 K (cubic phase), in the range of 2ha(160j,162j).
nent polarization, i.e. a nonzero dipolar moment, compatible with a non-centrosymmetric phase [15]. For T>TC=180 K, PFW turns into a paraelectric cubic material (centrosymmetric). However, the XRD analysis conducted in this study did not detect clearly the transition between these two states. This behaviour is due to a very low rhombohedral distortion (probably less than 0.01j) related to the short range order (SRO) of nanopolar regions existing in the relaxor state of PFW with average cubic symmetry. This state, called in literature pseudo-cubic phase, has been observed in similar Pb-based relaxors, as PMN [14]. By decreasing the temperature, the tendency towards long range ordering (LRO) increases. The system can be changed from relaxor to a LRO ferroelectric by applying an electric field [12]. Further XRD studies of poled PFW, Raman and diffraction neutron methods, more sensitive to SRO change, might show a clearly structural modification from a pseudo-cubic (rhombohedral) to a cubic symmetry.
increases, the splitting of (200) – (002), (201) –(210), (112) – (211), etc. Bragg peaks becomes more pronounced, demonstrating that the average symmetry becomes tetragonal with increasing PT addition (x). The results of the cell refinement show that all the PFW – PT compounds having a PT content in the range between x=0.2 and x=0.35 have single phase tetragonal symmetry ( P4mm), with the cell parameters dependent on the PT amount. The cell parameters and tetragonality obtained for pure PFW and for the (1x)PFW – xPT compounds with x=0.2, 0.25, 0.3 and 0.35 at 85 pffiffiffiand 300 K are shown in Figs. 8– 10, where a0 ¼ 3 is the average pseudo-cubic perovskite cell. At T=85 K, a monotonous decreasing of acell parameter and an increasing of the c-cell parameter of the perovskite cell and, consequently, an increasing of tetragonality c/a with PT concentration take place (Fig. 10). Because PFW at T=85 K is pseudo-cubic and the 0.2 compound is already tetragonal, it means that a possible MPB could exist for a PT amount less than x=0.2, at this temperature. At T=300 K, a similar analysis of diffractograms shown that the samples with x=0.2, 0.25 and 0.3 have a pseudo-cubic single-phase symmetry, with cell parameters almost independent on the PT concentration (Fig. 9). This result is in agreement with the relaxor state of these compositions at this temperature. The samples with x=0.35 and 0.4 show a tetragonal
3.2. XRD results for (1x)PFW– xPT solid solution at T=85 K and T=300 K The XRD pattern of (1x)PFW –xPT solid solution recorded at T=85 K are depicted in Fig. 7, for x=0.2, 0.25, 0.3 and 0.35 compositions. As PT content
Fig. 7. XRD pattern of (1x)PFW – xPT solid solution with x=0, 0.2, 0.25, 0.3, 0.35 and 0.4 at T=85 K.
L. Mitoseriu et al. / Materials Letters 57 (2002) 609–614
Fig. 8. Cell parameters of (1x)PFW – xPT versus PT amount (x) at T=85 K.
symmetry ( P4mm) with a gradual increasing of c-cell parameter and decreasing of a-cell parameter when increasing of PT concentration (Fig. 9). The previous dielectric data show that the samples with these compositions are in the ferroelectric region (II) of the diagram represented in Fig. 2, at this temperature. From Figs. 9 and 10, it results that there is a change of the symmetry of the system from pseudo-cubic to
Fig. 9. Cell parameters of (1x)PFW – xPT versus PT amount (x) at T=300 K.
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Fig. 10. Tetragonality c/a of (1x)PFW – xPT solid solution at T=85 K and T=300 K versus PT amount.
tetragonal, in the range of compositions with x=0.3 to 0.35. Consequently, an MPB might be found in this range, at T=300 K. Further studies will analyse in detail this range of compositions, using a higher resolution X-ray diffractograms in order to check the possible superposition of phases. We mention that both the temperature region separating the pseudo-cubic to cubic region (not detected in this study) and the composition boundary between the pseudo-cubic and tetragonal phases are separating similar states with slightly different properties [14]. In the relaxor state with SRO, nanopolar clusters size-distributed give rise to a broad phase transition and deviations from Curie – Weiss law, typical for relaxors. By increasing the temperature, the size of these clusters is progressively reduced and they finally vanish. However, the polar clusters can still be preserved above the ‘‘Curie temperature’’ TC by applying a field (as P(E) loops were observed for temperatures slightly above TC [15]). On the other hand, at fixed temperature, by adding to the relaxor material increasing amounts of a true ferroelectric like PT, stronger dipolar interaction makes the polar nanoregions to join each other and to transform in ferroelectric domains and the system becomes an LRO ferroelectric. The change in size of ordered polar regions with PT concentration is a gradual one and is
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strongly influenced by electrical and thermal history. Consequently, the MPB expected in PFW – PT system will be temperature- and field-dependent, as already pointed in similar solid solutions [9– 11,16]. According to results presented above (Fig. 10), PFW –PT system could have an MPB dependent on temperature, most probably bended towards the rich PT region with increasing the temperature. At low temperature T=85 K, the possible location of the MPB is for compositions with less than x=0.2. At the temperature T=300 K, an MPB might exist in the range between x=0.3 and 0.35. Further investigations are on going in order to find the superposition of phases and to restrict more this range of compositions.
4. Conclusions A study of crystalline symmetry of (1x)PFW – xPT solid solution with x=0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 at T=85 K and T=300 K using XRD analysis has been performed. All the solid solutions show pseudo-cubic or tetragonal single-phase symmetry. The temperature ‘‘boundary’’ between the real cubic (paraelectric) and pseudo-cubic (rhombohedral, relaxor) phases could not be delimited under the present experimental limit of accuracy, even for high Bragg angles around 2hi161.3j. An MPB, separating the pseudo-cubic (relaxor) and tetragonal (ferroelectric) phases, dependent on temperature might exist. The possible ranges of compositions for this MPB are: x<0.2 at T=85 K and xa(0.3,0.35) for T=300 K. Further high resolution XRD analysis and Raman studies are necessary to detect the possible superposition of phases and to restrict the range of compositions. Dielectric and piezoelectric measurements are necessary for a better characterization of the
PFW – PT compounds in the range of structural change.
Acknowledgements One of the authors (LM) thanks NATO Science Fellowship Programme, Portugal, for financial support. The discussions and the permanent assistance in XRD studies of Dr. M.R. Soares from Central Laboratory, University of Aveiro, are highly appreciated.
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