Solid Solutions of Lead MetaniobateâStabilization of the Ferroelectric. Polymorph and the Effect on the Lattice Parameters, Dielectric, Ferroelectric,.
J. Am. Ceram. Soc., 97 [1] 220–227 (2014) DOI: 10.1111/jace.12628 © 2013 The American Ceramic Society
Journal
Solid Solutions of Lead Metaniobate—Stabilization of the Ferroelectric Polymorph and the Effect on the Lattice Parameters, Dielectric, Ferroelectric, and Piezoelectric Properties Mtabazi G. Sahini,‡ Tor Grande,‡,§ Barbara Fraygola,‡,¶,† Alberto Biancoli,‡ Dragan Damjanovic,‡ and Nava Setter‡ ‡
Ceramics Laboratory, EPFL – Swiss Federal Institute of Technology, Lausanne 1015, Switzerland
§
Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway ¶
Grupo de Cer^amicas Ferroeletricas – GCFerr, Departamento de Fısica, Universidade Federal de S~ ao Carlos, S~ ao Carlos, CEP 13565-670, S~ ao Paulo, Brazil attractive for high-temperature electroacoustic applications due to its high Curie temperature, a large anisotropy in electromechanical coupling factors, and a low mechanical quality factor.6,9,13 Lead metaniobate ferroelectric-based materials have a high potential for high-temperature piezoelectric transducer applications and due their low dielectric proprieties, for high-frequency applications. The origin of the polarization in the two materials differs, but the macroscopic details of the origin have not been a subject of research. Morphotropic phase boundaries (MPB) were found in solid solutions of BaNb2O6 (BN) and PN and in other tungsten bronze systems.15,16 For these two tungsten bronze compounds, the prototype paraelectric phase possesses the tetragonal structure with space group P4/mbm. The polarization in orthorhombic PbNb2O6 has been shown to be related to displacement of Pb (and Nb) along the [010] direction,11,17 perpendicular to the c-axis in the tetragonal prototype tungsten bronze.18 Another peculiar feature of PN besides the polar axis along [010] direction is that the stable thermodynamic ground state of the system is not the ferroelectric phase, but a polymorph with rhombohedral symmetry, which has apparently no attractive properties.7 The transition from the rhombohedral to the paraelectric tetragonal phase has been reported at 1220°C,9 and the orthorhombic ferroelectric PN can only be obtained by quenching from the paraelectric phase above 1220°C.9,19–21 Reheating orthorhombic PN will cause the transformation to the stable rhombohedral phase around 700°C.10 The tungsten bronze is therefore entropy stabilized relative to the stable low temperature polymorph, and one may speculate that the tungsten bronze phase could be further stabilized by small impurities as indicated by a successful synthesis of orthorhombic PN by molten salt synthesis at 700°C–1100°C.22,23 The tetragonal to rhombohedral phase transition promotes grain growth, and dense and finegrained orthorhombic PN has been obtained by starting with a fine powder of orthorhombic PN sintered above 1220°C.9 Still the materials need to be quenched to avoid nucleation of the stable rhombohedral polymorph, which will deteriorate the piezoelectric properties.20 Alternative routes to stabilize the ferroelectric phase are therefore attractive such that quenching no longer is necessary. Several solid solutions of PN have been reported already more than 50 yr ago,24–26 but in those reports the necessity or nonnecessity of quenching the solid solutions of PN were not mentioned specifically. Here, we report on the stabilization of ferroelectric PN materials by solid solution. By simply partly substituting Pb with Ca, Bi, or combination of Bi and K/Na, the ferroelectric phase could be obtained by simple conventional solidstate sintering without the need of quenching. A systematic
Ferroelectric orthorhombic lead metaniobate (PbNb2O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The hightemperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2O6 by minor chemical substitution in the series [(1 − x) PbNb2O6– xBiTiNbO6], [(1 − x)PbNb2O6–xNa0.5Bi0.5Nb2O6], [(1 − x) PbNb2O6–xK0.5Bi0.5Nb2O6], and [(1 − x)PbNb2O6–xCaTiO3]. The high-temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid-state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2b/a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower Tc. However, some improvements seen with 2% CaTiO3 were not accompanied by Tc decrease.
I.
Introduction
T
HE strontium-barium niobates Sr1 xBaxNb2O6 (SBN) and lead niobate PbNb2O6 (PN) are the most studied among the ferroelectric materials with the tungsten bronze-type crystal structure.1–10 At room temperature, the ferroelectric phases exhibit P4bm (SBN) and Bb21m (PN), symmetries, with the polar axis along, respectively, the [001] and [010] crystallographic directions.11–13 While SBN materials have interesting electrooptical properties,1,14 PN is
D. Viehland—contributing editor
Manuscript No. 33227. Received May 17, 2013; approved August 25, 2013. † Author to whom correspondence should be addressed. e-mail: barbara.fraygola@epfl.ch
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Solid Solutions of Lead Metaniobate
investigation and optimization of solid-state synthesis followed by ceramic processing to obtain dense ceramics were conducted in the following solid solution series PBTN ((1 x)PbNb2O6–xBiTiNbO6), PBNN ((1 x) PbNb2O6– xNa0.5Bi0.5Nb2O6), PBKN ((1 x)PbNb2O6–xK0.5Bi0.5 Nb2 O6), and PCTN ((1 x) PbNb2O6–xCaTiO3). The variation in the unit cell parameters, Curie temperature, ferroelectric, and piezoelectric properties of the PN solid solutions are reported. A plausible explanation of stabilization of the hightemperature paraelectric tungsten bronze crystal structure is outlined.
II.
Experimental Procedure
(1) Materials Synthesis Pure PbNb2O6 (PN) and its solid solutions substituted with BiTiNbO6 (PBTN), Ca2Ti2O6 (PCTN), Bi0.5K0.5Nb2O6 (PBKN), and Bi0.5Na0.5Nb2O6 (PBNN) were prepared by solid-state synthesis from oxide and carbonate precursors. The precursors PbO (99.9%; Sigma-Aldrich Chemie GmbH, Buchs, Switzerland), Nb2O5 (99.9985%; Alfa Aesar GmbH & Co KG, Karlsruhe, Germany ), Bi2O3 (99.975%; Alfa Aesar), and TiO2 (99.8%; Alfa Aesar), Na2CO3 (99.5%; Wako Pure Chemical Industries), K2CO3 (99.997%; Alfa Aesar), CaCO3 (9.99%; Alfa Aesar) were dried at 400°C prior to the synthesis. Stoichiometric amount of the precursor powders were weighed and mixed for 4 h by planetary milling at 180 rpm using 5 mm zirconium balls and isopropanol as a medium. After milling, the slurry was dried at 75°C for 12 h, followed by sieving of the powder mixture. The powdered mixtures of the precursors were then uniaxially pressed into pellets at 98 MPa, followed by calcination for 2 h at 900°C for PN-rich compositions and 850°C for the PN poor. The calcined pellets were crushed into powder using a mortar and pestle and then milled, initially for 4 h using planetary milling at 180 rpm. Preliminary sintering experiments of pure PN revealed that 8 h milling time after calcination was necessary to obtain dense ceramics. Thus, 8 h milling time was used for all the materials, except the PCTN materials where 12 h milling time was necessary. The sintering temperature was optimized for each composition with 1 h sintering time and 5°C/min heating/cooling rate. Stoichiometry control of the materials was achieved by sintering the pellets in alumina crucibles, sealed with alumina cement, and covering the pellets with sacrificial powder with the same composition as the samples. After sintering, the samples were polished or crushed to powder for further analysis. In the pure PN case, orthorhombic phase could be obtained only by rapid cooling synthesis (quenching). (2)Materials Characterization The density of the material were determined by Archimedes method using the ASTM procedure B962-08 and also estimated by the measured weight and volume. Scanning electron microscopy (SEM, XLF 30-FEG; FEI, Hillsboro, OR) was conducted on milled powders as well as on polished and etched ceramics to investigate the particle size and grain size and homogeneity of the materials after synthesis/milling and the final sintering step. Thermal etching on the polished pellets was carried out at a temperature 850°C–1000°C for 15 min. X-ray diffraction analysis (XRD) was carried out on powders as well as pellets of the materials using a Bruker D8 (Bruker-AXS GmbH, Karlsruhe, Germany) DISCOVER X-ray diffractometer with monochromatic CuKa1 radiation and a position sensitive detector in the 2h range (1.540596 A) from 5° to 85°. La Bail Rietveld refinement of the powder diffractograms using the TOPAS 4.2 software (Bruker-AXS GmbH) were performed on data collected with a scan speed of 8 s/step and increments of 0.02°. The materials were refined either using the orthorhombic space group Bb21m, using initially the lattice parameters a = 35.293, b = 17.943, reported for pure PN13 or the paraelectric and c = 7.747 A
tetragonal space group P4/mbm, using initially the lattice cell and c = 3.92 A reported for PN.27 parameters a = 12.56 A The rhombohedral polymorph was refined using the space and group R3m and initial unit cell parameters a = 10.473 A 10 Fundamental parameters were used for the c = 11.534 A. peak profile. Sample displacement, a Chebychev polynomial background function and lattice parameters were refined.
(3)Electrical Characterization Parallel surfaces of polished ceramic disks used for electrical measurements were sputtered with gold (or platinum for dielectric measurements at temperatures above 550°C). Dielectric constants were measured at various frequencies from 1.0 kHz to 1.0 MHz and a heating rate of 2°C/min up to a temperature ≤620°C. Measurements were conducted using an L-C-R (L is for inductance, C is for Capacitance and R for Resistance) high precision HP 428A (HewlettPackard Company, Palo Alto, CA) and a computerized automatic measuring system. Piezoelectric properties were measured using gold-coated polished ceramic disks, which were poled with a high voltage 9065-DC power supply, using a DC field of 4.5 kV/mm for 90 min at 125°C in a silicon oil. Measurement of charge constant d33 was conducted using a Berlincourt d33-meter. Other piezoelectric constants were determined by resonance– antiresonance method following the IEEE Standards, using an impedance analyzer model Agilent 4294A (Agilent Technologies, Santa Clara, CA). The polarization-electric field behavior was characterized using a computer controlled modified Sawyer-Tower circuit. Measurements were taken at a frequency of 50 Hz and various electric fields with amplitudes from 20 to 85 kV/cm. III.
Results and Discussion
(1)Phase Purity, Microstructure, and Lattice Cell Parameters The calcined PN solid solutions were confirmed to correspond to the rhombohedral polymorph by XRD in agreement with previous reports. Sintering the milled powder above 1220°C, followed by quenching, resulted in ceramics containing only the ferroelectric orthorhombic polymorph, whereas the stable rhombohedral phase appeared after sintering followed by slow cooling (5°C/min), again in agreement with literature.19 The density of PN solid solutions samples, prepared from the rhombohedral powder milled for 8 h before sintering, was 94.3%, while increasing the milling time to 12 h gave only a minor increase in the density. Lee and Kimura obtained similar densities of pure PN by sintering fine-grained powders with orthorhombic structure, followed by quenching.9 XRD patterns of the rhombohedral and orthorhombic polymorphs of PN and the solid solutions are shown in Figs. 2(a) and (b). The calcined powders of the solid solutions were also indexed to the rhombohedral space group, see Fig. 2(a) for the series PBKN. The lattice parameters found by Rietveld refinement demonstrated that the unit cell parameters for the rhombohedral PN remained almost constant and independent of the nominal composition, whereas the intensity of the Bragg reflections not assigned to rhombohedral PN increased with increasing substitution level. This was observed for all the solid solution series. It can therefore be concluded that the solid solubility in rhombohedral PN is strongly limited at the calcination temperature 850°C–900°C, and that the content of a second phase increased with increasing substitution level in all the series. The sintering temperature was optimized for each solid solution, and a density >92% was obtained for all materials. The resulting optimized sintering temperature is displayed in Fig. 1. The density dropped significantly by reducing the sintering temperature below the optimal value. For example,
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(b)
Fig. 1. The optimized sintering temperature for the solid solutions of PN.
(c)
the PBNT with x = 0.1 the density was as
