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Stress-induced phase transition and its reversal by low-temperature annealing in Li-doped sodium niobate
Materials Letters 117 (2014) 204–207
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Stress-induced phase transition and its reversal by low-temperature annealing in Li-doped sodium niobate H.W. Joo a, K.-W. Chae a, C.I. Cheon a, J.S. Kim b,n a b
Department of Materials Science & Engineering, Hoseo University, Asan, Chungnam 336-795, Republic of Korea Department of Digital Display Engineering, Hoseo University, 336-795, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 3 September 2013 Accepted 1 December 2013 Available online 7 December 2013
An extensive phase transition of (Na1 xLix)NbO3 (x ¼0.04, 0.1) from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by mechanical stress. A reverse phase transition was induced by thermal annealing at temperatures as low as 300 1C. A sintered pellet was subjected to mechanical stress by grinding to powder form in a mortar. The orthorhombic phase in the ceramic pellet transformed to R3c phase, by as much as 83–89% by grinding. The net cell volume of the R3c increased by 0.4–0.5 Å3 per four formula unit (z ¼4). The reverse transition from the R3c to Pmc21 was 84–97 mass% after annealing at 300 1C. The activation free energy for the reverse transition was calculated to be 2–10 meV using an Arrhenius-type equation. The crystal class of the Pmc21, i.e. mm2, conformed to the measured P–E hysteresis (Ps ffi20 μC/cm2) of the pellet. & 2013 Elsevier B.V. All rights reserved.
Keywords: Stress-induced Phase transition Li-doped NaNbO3 Annealing Phase reversal
1. Introduction The crystal structure of NaNbO3, which provides the matrix phase for a family of lead-free piezoelectric ceramics [1–3], has been a subject of controversy for decades. The orthorhombic structure with the space group (SG) Pbma (a ¼5.56, b¼15.54, c¼ 5.51 Å) was considered admittedly as a room temperature (RT) structure in several papers [4,5]. On the other hand, from a crystallographic point of view, this Pbma structure belongs to the centrosymmetric crystal class, mmm [6]. This crystal class can be neither antiferroelectric nor ferroelectric. The piezoelectric property observed in pure NaNbO3 could be an electric fieldinduced nature. Many years ago, Shuvaevaa et al. [7] reported that the NaNbO3 single crystal underwent a phase transition to the Pmc21 under the external electric field of 60l elect In addition the field-induced ferroelectricity nature of NaNbO3 had been reported in other previous studies [8]. Otherwise, the admittedly known Pbma could be an incorrect structure. Further crystal structural work is necessary for resolving this structural problem. Li-doping into sodium niobate (Na1 xLixNbO3) gave rise to enhanced piezoelectric properties and a ferroelectric phase transition [8–10]. Until recently, the phase evolution at room temperature with x has been controversial considering its single phase region and morphotropic boundaries [10–13]. Despite the many speculations regarding the crystal structures, only a few studies have been performed detailed crystallographic analysis [13–14]. The complexity in the phase behavior originates from the
n
Corresponding author. Tel.: þ 82 41 540 5921; fax: þ82 41 540 5929. E-mail address: [email protected] (J.S. Kim).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.002
unusually smaller ion size of Na þ compared to the Na–O polyhedral cage. Therefore, the space for Na þ off-centering displacement would be large enough to have multiple energy-minimum positions including equilibrium and meta-stable states. We considered that mechanical stress could be another origin leading to the ambiguity in (Na1 xLix)NbO3, which stress could be produced by either internal or external forces. The mechanical stress-induced phase transition has been well documented in some perovskites [15] and zirconian [16]. Recently, many aspects of the temperature–pressure phase diagrams have been studied using 1st principle calculations and experimental tools [4,5,15,17]. In pure zirconia, Klose et al. [16] reported that a tetragonal-tomonoclinic transition could be induced by several methods, such as ball milling, grinding in a mortar and pressing. In this study, the method, ‘grinding in a mortar’ [16], was used to apply mechanical stress to a (Na1 xLix)NbO3 ceramic. With this method, an appreciable level of complex stress can be applied easily, including shear and isostatic as well as tensile and compression [15,16]. To probe the effect of mechanical stress on the phase transition, quantitative phase analysis using a Rietveld refinement was performed on a sintered pellet, pellet pulverized into a powder in a mortar, and annealed powder at 300–1000 1C. 2. Experimental details The samples were prepared in the following steps. Initially, a sintered pellet of (Na1 xLix)NbO3 (x ¼0.04, 0.1) was prepared. The sintered pellet was pulverized to powder form by grinding in a mortar. The pulverized pellet (powder) was annealed at 300– 1000 1C. The sintered pellets were prepared using conventional
H.W. Joo et al. / Materials Letters 117 (2014) 204–207
solid state reaction method. The calcined raw material with a PVA binder was compacted into discs, 10 mm in diameter and 2 mm in thickness, by cold isostatic pressing. The compacted disc was sintered at 1220 1C for 3 h in air. The surfaces of the sintered pellets were removed using SiC-coated abrasive paper with fine grit size (#2000) because the surface layer might be Na-deficient due to evaporation. The surface-removed pellets (polished pellet) were pulverized to a powder with a mean particle size of approximately 4 μm in a mortar manually for approximately 5 min. The powder was annealed in air (annealed powder) at 300 1C for 10 h 500 1C for 5 h, and 1000 1C for 3 h. The extent of the phase transformation was examined using the Reitveld method with the RIETAN2000 program. X-ray diffraction was performed at RT using Cu Kα radiation from the as-sintered pellet, the polished pellet, the powder (pulverized pellet), and the annealed powder.
3. Results Evolution of XRD patterns: Fig. 1 shows the evolution of the XRD patterns according to the sample treatment. This figure shows the effect of the mechanical stress on the pattern evolution. The XRD patterns can be classified into two groups with apparently different crystal structures: the powder (a-3, b-3) and the remaining samples.
205
The distinctive features of the powder compared to the other samples is found at the peaks marked ● and ❖. The former peak (●) shows ample peak-splitting, whereas the latter (❖) shows peak-merging. This distinction should come from the structural change induced by the grinding process. The marked peaks were characteristic of the low temperature phase of NaNbO3 (R3C, a¼ 7.81 Å, α ¼ 89.171, ICSD #76762). The pattern was indexed to this rhombohedral cell. These rhombohedral features were not observed when the powder was annealed at Z300 1C. The XRD patterns of the other group including the sintered pellet and annealed powders were similar to that of the orthorhombic NaNbO3, Pbma (ICSD # 82420, 8242). This group was indexed temporarily based on the Pbma cell. The x ¼0.1 sample (Fig. 1(b)) also shows the same evolution with the applied stress and annealing. Quantitative phase analysis: The most important step during a Rietveld refinement is to set a structural model. Every possible structural model reported previously that are related to both pure and Li-doped NaNbO3 was tested. For the orthorhombic phase, all the orthorhombic models reported previously were tested, e.g. Pbma [4,5], Pmc21 [7], Pc21b [ICSD #19133], and P2221 [ICSD #55835]. Pmc21 is the peculiar structure, which can be induced by the external electric field of 60 kV/cm on NaNbO3 [7]. The monoclinic cell [12] was also tested. The rhombohedral (R) phase in the powder (Fig. 1) (a-3) and (b-3) was analyzed using the
Fig. 1. Evolution of the XRD patterns of the x ¼0.04 (a) and 0.1 (b). The powder (pulverized pellet) was indexed to the rhombohedral R3C, whereas the others were indexed to the orthorhombic Pbma. Table 1 Summary of the crystal structure analysis using a Rietveld refinement method. Sample
Refined parameters
Compositions x ¼0.04
Sintered pellet (polished surface)
Pulverized powder
a
SG , mass % Lattice parametersb R-factorsc SG, mass % Lattice parameters Cell volumed R-factors
Annealed powder (300 1C)
a
Space group. a, b, c (Å), angle (o). Rwp, Rp, Re, Rb, Rf. d Å3. b c
SG, mass % Lattice parametersb Cell volume R-factors
x¼ 0.1
Pmc21, 100% 7.7531(2), 5.5091(2), 5.5698(2)
R3c, 0% –
Pmc21, 100% 7.7409(3), 5.5005(3), 5.5644(3)
R3c, 0% –
13.9, 10.6, 5.9 5.9, 5.1 Pmc21, 17% 7.7500(19), 5.4957(24), 5.5686 (22) 237.2(1) 9.9, 7.5, 4.2 7.1, 6.5 Pmc21, 97% 7.7586(5) 5.5117(3) 5.5765(3)
– R3c, 83% 7.8047(3), 89.129 (15) 475.2(0) –––– 6.1, 5.8 R3c, 3% 7.8135(–) 89.149(–)
13.6,10.0, 5.7 Pmc21, 11% 7.7445(10), 5.5139(11), 5.5750 (10) 238.1(1) 16.2, 11.7, 4.3 5.2, 5.0 Pmc21, 84 7.7529(4) 5.5062(3) 5.5766(3)
– R3c, 89% 7.8147(5), 89.132(8)
– 16.2, 11.9, 6.3 6.3, 5.0
– –––– 6.2, 4.6
238.1(0) 13.8, 10.8, 7.2, 8.0, 4.5
477.1(0) –––– 4.6, 4.4 R3c, 16% 7.8135(6) 89.149 (24) 476.9(0) –––– 7.7, 4.5
206
H.W. Joo et al. / Materials Letters 117 (2014) 204–207
Fig. 2. Refined profiles of x ¼0.1. (a) powder indexed to the R3c and (b) annealed powder at 300 1C indexed to the Pmc21.
Fig. 3. P–E hysteresis loops measured for the x¼ 0.04 (a) and 0.1 (b) which samples were sintered at 1230 1C for 3 h.
low temperature structure R3c [18]. After the structural model tests, satisfactory results were obtained only from the Pmc21 and R3c models. Table 1 lists the refinement results. The sintered ceramic pellets (x¼ 0.4, 0.1) consisted only of an orthorhombic (O) phase (Pmc21). The powder (x ¼0.04 and 0.1) showed remarkable changes in its constituent phase to the R (R3c), which amounted to an 83 and 89 mass% in x ¼0.04 and 0.1 respectively. The remainder was the O phase. The extent of the O-to-R transition would be complete when the pellet undergoes severe grinding to a smaller powder size. Fig. 2(a) shows the refined profile of the powder (x ¼0.1), which was indexed based on the R cell. A striking change occurred after annealing the powder at 300 1C. The R phase mostly returned to the O phase, which amounted to 97 and 84 mass% for x¼0.04 and 0.1, respectively. Fig. 2(b) presents the refined profile of the annealed powder at 300 1C (x ¼0.1), which was indexed based on the O cell. The annealed powders (x ¼0.04, 0.1) transformed completely to the O phase when the annealing temperature was increased to Z500 1C. For a perovskite to be ferroelectric its crystal class should be one of the 10-polar classes [6]. The Pmc21 belongs to the polar class, mm2. P–E hysteresis curves were measured for the pellets sintered at 1230 1C. Fig. 3 shows P–E hysteresis loops of the x ¼0.04 and 0.1. The x ¼0.1 shows higher Pr ( 23 μC/cm2) than that of the x ¼0.04 ( 12 μC/cm2). Coercive field Ec is 3.8–4.7 kV/ mm. The ferroelectric characteristics conform to the analyzed crystal structure Pmc21 in this study and other reports [19].
4. Discussion The cell volume change is a characteristic feature of the O-to-R transition and vice versa. As shown in Table 1, the cell volume per four formula unit (z¼ 4) of R was slightly larger ( 0.4–0.5 Å3) than that of O. The mechanical stress during grinding appears to help expand the net cell volume necessary for the O-to-R transition.
Fig. 4. Schematic variations of Gibbs free energy during the phase transition between the equilibrium orthorhombic and metastable rhombohedral structures.
Fig. 4 presents a schematic diagram of the variation of the Gibbs free energy with a configurational change of the Na–O polyhedron involved with the phase transition. We assumed that there are two off-centering positions for Naþ ion in the Na–O cage: the O position with the equilibrium state and the R position with the metastable state. The figure also shows schematic diagrams of the Na–O cages. Based on the phase reversibility by the mechanical stress and thermal annealing, O and R were placed at the lowest minimum and metastable state, respectively. The activation free energy, ΔGa, for the reverse phase transition was calculated using the quantitative phase analysis results for the powder annealed at 300 1C. A simple Arrhenius type equation was adopted as follows by assuming that the R-to-O transition occurs solely by the displacement of Na þ between the two positions. No ¼ N r exp ð ΔGa =kTÞ;
ð1Þ
where No and Nr are the occupation probability of Naþ ions at the O and R sites, respectively. The calculated ΔGa of x ¼0.04 and
H.W. Joo et al. / Materials Letters 117 (2014) 204–207
0.1 were approximately 2 meV and 10 meV, respectively. The actual activation free energy for the reverse transition would be larger than these values because the cooperative rearrangement of O2 and Nb5 þ should be involved. Nevertheless, this calculated activation energy is similar to the free energy difference ( 2.5 meV) between the antiferroelectric and ferroelectric phases of NaNbO3 calculated by Mishra et al. [4] using the lattice dynamics.
References [1] [2] [3] [4] [5] [6] [7]
5. Conclusion At ambient temperature, the phase transition from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by applying mechanical stress to a sintered Na1 xLixNbO3 pellet. The orthorhombic phase of the sintered pellet changed to the rhombohedral by as much as Z83 mass% by grinding in a mortar. The mechanical stress during grinding was considered to help expand the net cell volume. A reverse phase transition occurred by up to 84–97% after annealing the powder (pulverized pellet) at temperatures as low as 300 1C. Such drastic phase reversal at such low temperatures is unique to Li-doped NaNbO3.
207
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Guo Y, Kakimoto K-I, Ohsato H. Mater Lett 2005;59:241–4. Eichel R-A, Kungl H. Funct Mater Lett 2010;03:1–4. Lin D, Xiao D, Zhu J, Yu P, Yan H, Li L. Mater Lett 2004;58:615–8. Mishira SK, Gupta MK, Mittal R, Chaplot SL, Hansen T. Appl Phys Lett 2012; 101:242907-4. Shiratori Y, Magrez A, Kato M, Kasezawa K, Pithan C, Waser R. J Phys Chem C 2008;112:9610–6pp 2008;112:9610–6. Donald Bloss F, editor. Crystallography and Crystal Chemistry: An Introduction. 1st ed. Holt, Rinehart and Winston. (p. 407–408). Shuvaevaa VA, Antipinb MY, Lindemanb RSV, Fesenkoa OE, Smotrakova VG, Struchkovb YT. Ferroelectrics 1993;141:307–11. Arioka T, Taniguchi H, Itoh M, Oka K, Wang R, Fu D. Ferroelectrics 2010:51–5. Nitta T. J Am Ceram Society 1968;51:623–30. Zhong WL, Zhang PL, Zhao HS. Phys Rev B 1992;46:105837. Mitra S, Kulkarni AR, Prakash O. J Appl Phys 2013;114:064106. Pozdnyakova I, Navrotsky A. J Am Ceram Soc 2002;85:379–84. Peel MD, Ashbrook SE, Lightfoot P. Inorg Chem 2013;52:8872–80. Sadel A, Von der Mühll R, Ravez J, Chaminade JP, Hagenmüller P. Solid State Commun 1982;44:345–9. Girshberg Y, Yacoby Y. J Phys: Condens Matter 2012(24():015901. Klose BS, Jentoft RE, Hahn A, Ressler T, Krohnert J, Wrabetz S, et al. J Catal 2003;217:487–90pp 2003;217:487–90. Shen ZX, Wang XB, Tang SH, Kuok MH, Malekfar R. J Raman Spectrosc 2000;31:439–43. Lanfredi S, Lente MH, Eiras JA. Appl Phys Lett 2002;80:2731–3. Jung JH, Lee M, Hong J-I, Ding Y, Chen C-Y, Chou L-J, Wang ZL. ACS Nano 2011;5:10041–6.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Stress-induced phase transition and its reversal by low-temperature annealing in Li-doped sodium niobate H.W. Joo a, K.-W. Chae a, C.I. Cheon a, J.S. Kim b,n a b
Department of Materials Science & Engineering, Hoseo University, Asan, Chungnam 336-795, Republic of Korea Department of Digital Display Engineering, Hoseo University, 336-795, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 3 September 2013 Accepted 1 December 2013 Available online 7 December 2013
An extensive phase transition of (Na1 xLix)NbO3 (x ¼0.04, 0.1) from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by mechanical stress. A reverse phase transition was induced by thermal annealing at temperatures as low as 300 1C. A sintered pellet was subjected to mechanical stress by grinding to powder form in a mortar. The orthorhombic phase in the ceramic pellet transformed to R3c phase, by as much as 83–89% by grinding. The net cell volume of the R3c increased by 0.4–0.5 Å3 per four formula unit (z ¼4). The reverse transition from the R3c to Pmc21 was 84–97 mass% after annealing at 300 1C. The activation free energy for the reverse transition was calculated to be 2–10 meV using an Arrhenius-type equation. The crystal class of the Pmc21, i.e. mm2, conformed to the measured P–E hysteresis (Ps ffi20 μC/cm2) of the pellet. & 2013 Elsevier B.V. All rights reserved.
Keywords: Stress-induced Phase transition Li-doped NaNbO3 Annealing Phase reversal
1. Introduction The crystal structure of NaNbO3, which provides the matrix phase for a family of lead-free piezoelectric ceramics [1–3], has been a subject of controversy for decades. The orthorhombic structure with the space group (SG) Pbma (a ¼5.56, b¼15.54, c¼ 5.51 Å) was considered admittedly as a room temperature (RT) structure in several papers [4,5]. On the other hand, from a crystallographic point of view, this Pbma structure belongs to the centrosymmetric crystal class, mmm [6]. This crystal class can be neither antiferroelectric nor ferroelectric. The piezoelectric property observed in pure NaNbO3 could be an electric fieldinduced nature. Many years ago, Shuvaevaa et al. [7] reported that the NaNbO3 single crystal underwent a phase transition to the Pmc21 under the external electric field of 60l elect In addition the field-induced ferroelectricity nature of NaNbO3 had been reported in other previous studies [8]. Otherwise, the admittedly known Pbma could be an incorrect structure. Further crystal structural work is necessary for resolving this structural problem. Li-doping into sodium niobate (Na1 xLixNbO3) gave rise to enhanced piezoelectric properties and a ferroelectric phase transition [8–10]. Until recently, the phase evolution at room temperature with x has been controversial considering its single phase region and morphotropic boundaries [10–13]. Despite the many speculations regarding the crystal structures, only a few studies have been performed detailed crystallographic analysis [13–14]. The complexity in the phase behavior originates from the
n
Corresponding author. Tel.: þ 82 41 540 5921; fax: þ82 41 540 5929. E-mail address: [email protected] (J.S. Kim).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.002
unusually smaller ion size of Na þ compared to the Na–O polyhedral cage. Therefore, the space for Na þ off-centering displacement would be large enough to have multiple energy-minimum positions including equilibrium and meta-stable states. We considered that mechanical stress could be another origin leading to the ambiguity in (Na1 xLix)NbO3, which stress could be produced by either internal or external forces. The mechanical stress-induced phase transition has been well documented in some perovskites [15] and zirconian [16]. Recently, many aspects of the temperature–pressure phase diagrams have been studied using 1st principle calculations and experimental tools [4,5,15,17]. In pure zirconia, Klose et al. [16] reported that a tetragonal-tomonoclinic transition could be induced by several methods, such as ball milling, grinding in a mortar and pressing. In this study, the method, ‘grinding in a mortar’ [16], was used to apply mechanical stress to a (Na1 xLix)NbO3 ceramic. With this method, an appreciable level of complex stress can be applied easily, including shear and isostatic as well as tensile and compression [15,16]. To probe the effect of mechanical stress on the phase transition, quantitative phase analysis using a Rietveld refinement was performed on a sintered pellet, pellet pulverized into a powder in a mortar, and annealed powder at 300–1000 1C. 2. Experimental details The samples were prepared in the following steps. Initially, a sintered pellet of (Na1 xLix)NbO3 (x ¼0.04, 0.1) was prepared. The sintered pellet was pulverized to powder form by grinding in a mortar. The pulverized pellet (powder) was annealed at 300– 1000 1C. The sintered pellets were prepared using conventional
H.W. Joo et al. / Materials Letters 117 (2014) 204–207
solid state reaction method. The calcined raw material with a PVA binder was compacted into discs, 10 mm in diameter and 2 mm in thickness, by cold isostatic pressing. The compacted disc was sintered at 1220 1C for 3 h in air. The surfaces of the sintered pellets were removed using SiC-coated abrasive paper with fine grit size (#2000) because the surface layer might be Na-deficient due to evaporation. The surface-removed pellets (polished pellet) were pulverized to a powder with a mean particle size of approximately 4 μm in a mortar manually for approximately 5 min. The powder was annealed in air (annealed powder) at 300 1C for 10 h 500 1C for 5 h, and 1000 1C for 3 h. The extent of the phase transformation was examined using the Reitveld method with the RIETAN2000 program. X-ray diffraction was performed at RT using Cu Kα radiation from the as-sintered pellet, the polished pellet, the powder (pulverized pellet), and the annealed powder.
3. Results Evolution of XRD patterns: Fig. 1 shows the evolution of the XRD patterns according to the sample treatment. This figure shows the effect of the mechanical stress on the pattern evolution. The XRD patterns can be classified into two groups with apparently different crystal structures: the powder (a-3, b-3) and the remaining samples.
205
The distinctive features of the powder compared to the other samples is found at the peaks marked ● and ❖. The former peak (●) shows ample peak-splitting, whereas the latter (❖) shows peak-merging. This distinction should come from the structural change induced by the grinding process. The marked peaks were characteristic of the low temperature phase of NaNbO3 (R3C, a¼ 7.81 Å, α ¼ 89.171, ICSD #76762). The pattern was indexed to this rhombohedral cell. These rhombohedral features were not observed when the powder was annealed at Z300 1C. The XRD patterns of the other group including the sintered pellet and annealed powders were similar to that of the orthorhombic NaNbO3, Pbma (ICSD # 82420, 8242). This group was indexed temporarily based on the Pbma cell. The x ¼0.1 sample (Fig. 1(b)) also shows the same evolution with the applied stress and annealing. Quantitative phase analysis: The most important step during a Rietveld refinement is to set a structural model. Every possible structural model reported previously that are related to both pure and Li-doped NaNbO3 was tested. For the orthorhombic phase, all the orthorhombic models reported previously were tested, e.g. Pbma [4,5], Pmc21 [7], Pc21b [ICSD #19133], and P2221 [ICSD #55835]. Pmc21 is the peculiar structure, which can be induced by the external electric field of 60 kV/cm on NaNbO3 [7]. The monoclinic cell [12] was also tested. The rhombohedral (R) phase in the powder (Fig. 1) (a-3) and (b-3) was analyzed using the
Fig. 1. Evolution of the XRD patterns of the x ¼0.04 (a) and 0.1 (b). The powder (pulverized pellet) was indexed to the rhombohedral R3C, whereas the others were indexed to the orthorhombic Pbma. Table 1 Summary of the crystal structure analysis using a Rietveld refinement method. Sample
Refined parameters
Compositions x ¼0.04
Sintered pellet (polished surface)
Pulverized powder
a
SG , mass % Lattice parametersb R-factorsc SG, mass % Lattice parameters Cell volumed R-factors
Annealed powder (300 1C)
a
Space group. a, b, c (Å), angle (o). Rwp, Rp, Re, Rb, Rf. d Å3. b c
SG, mass % Lattice parametersb Cell volume R-factors
x¼ 0.1
Pmc21, 100% 7.7531(2), 5.5091(2), 5.5698(2)
R3c, 0% –
Pmc21, 100% 7.7409(3), 5.5005(3), 5.5644(3)
R3c, 0% –
13.9, 10.6, 5.9 5.9, 5.1 Pmc21, 17% 7.7500(19), 5.4957(24), 5.5686 (22) 237.2(1) 9.9, 7.5, 4.2 7.1, 6.5 Pmc21, 97% 7.7586(5) 5.5117(3) 5.5765(3)
– R3c, 83% 7.8047(3), 89.129 (15) 475.2(0) –––– 6.1, 5.8 R3c, 3% 7.8135(–) 89.149(–)
13.6,10.0, 5.7 Pmc21, 11% 7.7445(10), 5.5139(11), 5.5750 (10) 238.1(1) 16.2, 11.7, 4.3 5.2, 5.0 Pmc21, 84 7.7529(4) 5.5062(3) 5.5766(3)
– R3c, 89% 7.8147(5), 89.132(8)
– 16.2, 11.9, 6.3 6.3, 5.0
– –––– 6.2, 4.6
238.1(0) 13.8, 10.8, 7.2, 8.0, 4.5
477.1(0) –––– 4.6, 4.4 R3c, 16% 7.8135(6) 89.149 (24) 476.9(0) –––– 7.7, 4.5
206
H.W. Joo et al. / Materials Letters 117 (2014) 204–207
Fig. 2. Refined profiles of x ¼0.1. (a) powder indexed to the R3c and (b) annealed powder at 300 1C indexed to the Pmc21.
Fig. 3. P–E hysteresis loops measured for the x¼ 0.04 (a) and 0.1 (b) which samples were sintered at 1230 1C for 3 h.
low temperature structure R3c [18]. After the structural model tests, satisfactory results were obtained only from the Pmc21 and R3c models. Table 1 lists the refinement results. The sintered ceramic pellets (x¼ 0.4, 0.1) consisted only of an orthorhombic (O) phase (Pmc21). The powder (x ¼0.04 and 0.1) showed remarkable changes in its constituent phase to the R (R3c), which amounted to an 83 and 89 mass% in x ¼0.04 and 0.1 respectively. The remainder was the O phase. The extent of the O-to-R transition would be complete when the pellet undergoes severe grinding to a smaller powder size. Fig. 2(a) shows the refined profile of the powder (x ¼0.1), which was indexed based on the R cell. A striking change occurred after annealing the powder at 300 1C. The R phase mostly returned to the O phase, which amounted to 97 and 84 mass% for x¼0.04 and 0.1, respectively. Fig. 2(b) presents the refined profile of the annealed powder at 300 1C (x ¼0.1), which was indexed based on the O cell. The annealed powders (x ¼0.04, 0.1) transformed completely to the O phase when the annealing temperature was increased to Z500 1C. For a perovskite to be ferroelectric its crystal class should be one of the 10-polar classes [6]. The Pmc21 belongs to the polar class, mm2. P–E hysteresis curves were measured for the pellets sintered at 1230 1C. Fig. 3 shows P–E hysteresis loops of the x ¼0.04 and 0.1. The x ¼0.1 shows higher Pr ( 23 μC/cm2) than that of the x ¼0.04 ( 12 μC/cm2). Coercive field Ec is 3.8–4.7 kV/ mm. The ferroelectric characteristics conform to the analyzed crystal structure Pmc21 in this study and other reports [19].
4. Discussion The cell volume change is a characteristic feature of the O-to-R transition and vice versa. As shown in Table 1, the cell volume per four formula unit (z¼ 4) of R was slightly larger ( 0.4–0.5 Å3) than that of O. The mechanical stress during grinding appears to help expand the net cell volume necessary for the O-to-R transition.
Fig. 4. Schematic variations of Gibbs free energy during the phase transition between the equilibrium orthorhombic and metastable rhombohedral structures.
Fig. 4 presents a schematic diagram of the variation of the Gibbs free energy with a configurational change of the Na–O polyhedron involved with the phase transition. We assumed that there are two off-centering positions for Naþ ion in the Na–O cage: the O position with the equilibrium state and the R position with the metastable state. The figure also shows schematic diagrams of the Na–O cages. Based on the phase reversibility by the mechanical stress and thermal annealing, O and R were placed at the lowest minimum and metastable state, respectively. The activation free energy, ΔGa, for the reverse phase transition was calculated using the quantitative phase analysis results for the powder annealed at 300 1C. A simple Arrhenius type equation was adopted as follows by assuming that the R-to-O transition occurs solely by the displacement of Na þ between the two positions. No ¼ N r exp ð ΔGa =kTÞ;
ð1Þ
where No and Nr are the occupation probability of Naþ ions at the O and R sites, respectively. The calculated ΔGa of x ¼0.04 and
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0.1 were approximately 2 meV and 10 meV, respectively. The actual activation free energy for the reverse transition would be larger than these values because the cooperative rearrangement of O2 and Nb5 þ should be involved. Nevertheless, this calculated activation energy is similar to the free energy difference ( 2.5 meV) between the antiferroelectric and ferroelectric phases of NaNbO3 calculated by Mishra et al. [4] using the lattice dynamics.
References [1] [2] [3] [4] [5] [6] [7]
5. Conclusion At ambient temperature, the phase transition from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by applying mechanical stress to a sintered Na1 xLixNbO3 pellet. The orthorhombic phase of the sintered pellet changed to the rhombohedral by as much as Z83 mass% by grinding in a mortar. The mechanical stress during grinding was considered to help expand the net cell volume. A reverse phase transition occurred by up to 84–97% after annealing the powder (pulverized pellet) at temperatures as low as 300 1C. Such drastic phase reversal at such low temperatures is unique to Li-doped NaNbO3.
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[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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