SEM, SAED, and TEM Investigations of Domain Structure in PZT Ceramics at Morphotropic Phase Boundary

Transcription

1 Appl. Phys. A 37, (1985) Appli6 t so,., Physics A "" Surfaces 9 Springer-Verlag 1985 SEM, SAED, and TEM Investigations of Domain Structure in PZT Ceramics at Morphotropic Phase Boundary P. G. Lucuta and V. Teodorescu Institute for Physics and Technology of Materials, P.O. Box MG-7, R Bucharest-M~gurele, Romania F. Vasiliu National Institute for Scientific and Technical Creation, Bd. P~cii nr. 220, P.O. Box 5224, R Bucharest, Romania Received 20 November 1984/Accepted 18 January 1985 Abstract. SEM investigations of ferroelectric domain structure in PbZro.s3Tio.4703 are consistent with a model of spatial domain configuration for the piezoelectric ceramics previously proposed for BaTiO3. TEM and SAED results revealed not only the twinning relation of adjacent tetragonal 90 ~ domains but also the simultaneous presence of the ferroelectric rhombohedral phase. A succession model of ferroelectric domains T1RT2RT1... which needs a smaller energy for the rotation of the polarization vector due to the coexistence of a R domain between the two T 90 ~ domains is proposed. This model is also confirmed by the estimated value of elastically stored energy in the mixed wall and by the dependence upon the sintering temperature of T and R unit cell distortions previously measured by x-ray diffraction. PACS: 61.10, 64, 68.55, Lead zirconate titanate (PZT) ceramics have a perovskitic structure above its Curie temperature (T~ = 375 ~ C), while by cooling below the Curie temperature they undergo a phase transition from cubic to tetragonal or rhombohedral structure depending on the composition ratio of Zr/Ti [1, 2]. The ferroelectric F T (tetragonal) and FR (rhombohedral) phases occur simultaneously at the morphotropic phase boundary corresponding to a specific Zr/Ti ratio of 53/47 [2-5]. The strain associated with this phase transformation is accomodated by the formation of twins, the ferroelectric 90 ~ domains being thus twin plates in a tetragonal structure. The domain structure in PZT ceramics has been studied by using SEM on etched bulk specimens [6], replica [7] or TEM of PZT thin foils [8,9]. Both 90 ~ and 180 ~ domains are observed and the predominant 90 ~ domains are shown to be deformation twins with displacement along (110) directions on {110} planes. The structure of domain walls in PZT [8] is similar to that suggested for BaTiO 3 [10]. In the present study, the domain structure observed by SEM in PbZro.53Tio.47Oa is discussed in terms of a spatial domain configuration already proposed for BaTiO 3 [11]. On the basis of obtained SAED and TEM results concerning the crystallography of domain walls, a possible model for the coexistence of both ferroelectric F T and F R phases in the same PZT ceramic grain is proposed. 1. Experimental Technique PZT specimens were prepared by the classical ceramic technology (1200 ~ C, 2 h) starting from oxide powders of 99.9% pure reagent grade. Cross sections of ceramic disks parallel to the polarization field were polished and etched prior to the SEM study. PZT foils were

2 238 P.G. Lucuta et al. prepared from bulk disks by mechanical thinning followed by ion bombardment. The ion thinning is performed using an argon ion beam (E = 10 kev) at a small incident angle (~ 10 ~ with respect to the specimen surface (in order to not disturb the domain configuration existing in the ceramic grains). The PZT thin foils were studied by using a JEOL-TEMSCAN 200 Cx electron microscope operating at 200 kv. 2. Results and Discussion 2.1. SEM Investigations The PZT specimens, prepared for SEM investigations, have been previously characterized by x-ray diffraction, establishing the coexistence of both ferroelectric FT and F R phases and allowing an accurate lattice parameter measurement of the associated unit cells [12]. In the F T phase there are only 90 ~ and 180 ~ reorientations of the polarization vector corresponding to adjacent ferroelectric domains9 In the morphotropic phase boundary region, the 90 ~ domains are predominant [2, 8]. Besides the 90 ~ and 180 ~ domains, the 71 ~ and 109 ~ reofientations of polarization vector corresponding to the angles between the adjacent ceil diagonals can very rarely occur in the FR phase [2]. The different possible orientations of the polarization vectors for the two coexistent ferroelectric phases lead to some complex patterns in SEM images where the adjacent domains are twin-related. A SEM micrograph showing various kinds of ferroelectric domains in etched PZT grains is given in Fig. la, the domain thickness varying in a range of0.1~).5 ~tm. The domain structure contains many parallel bands forming angles of either 90 ~ or 45 ~ A magnified detail image has pointed out angles of about 72 ~ between two parallel line sets (Fig. lb). These angular values can be explained by means of a spatial configuration for domain structure, previously proposed for BaTiO3 [11]9 Figure 2 shows a possible domain configuration in a cubic part of a ceramic grain. All 90 ~ walls are {110} planes and the following possible orientations can occur for domain wall orientation in a cube with tetragonal distortion: A = [011], B = [10l-], C = [150] and their normal counterparts: A = [011], B = [101], and C = [110]. The intersection of these walls with the cube faces leads to the domain configuration, shown in Fig. 2, built up of A and B planes (pure 90 ~ domain walls) and of C planes (alternating stripes of 180 ~ and 90 ~ walls)9 On [001] face, the angles between the intersecting lines of A and B planes have 90 ~ whereas angles of 45 ~ are formed between A, B, and C walls. The tracks of A, B walls upon the [100] and [010] faces form angles of 60 ~ In Fig. lb, the angle of about 72 ~ measured between two parallel line sets, shows up due Fig. 1. (a) SEM image of a domain structure in an etched ceramic Pb(Zr0.s3Tio.47)O 3 specimen; (x 2200) (b)detail from (a) showing a 72 ~ angle between two parallel line stripes (x 10,000) c c B 4 Fig. 2. Spatial domain configuration and patterns of domain walls in a cubic part of a ceramic grain (arrows indicate direction of polarization)

3 Domain Structure in PZT Ceramics at Morphotropic Phase Boundary 239 to the projection of the spatial configuration considered onto the [112] or [021] observation planes, leading thus to the theoretical angular values of 71.5 ~ or 69.9 ~, respectively, between the two parallel stripes. The knowledge of the domain configuration allows us to estimate an approximate value for the elastically stored energy density per unit length WJh, caused by the spontaneous strain in the mixed wall, by using the previously derived relation [11]: W~ C11SZ~d 2 h- 64~' (1) where C~ 1 is the average longitudinal elastic constant, S~ the spontaneous strain of crystal lattice, and d the domain thickness. The spontaneous strain S~ has been calculated as S~ = fir = 1- ale for two tetragonal domains cleft along the C wall [11]. Taking into account the homogeneous parameter of deformation, defined by the term: 6 = (d-do)/do, where d is the cell size in the polar axis direction, and do the length of the section adequate to d (in the same direction) for the regular cell which has the same volume as the considered one, it is very easy to shown that in the case of tetragonal cell 6r-~(2/3) (Cr/ar--1) while for a rhombohedral one 6~-~cos~R 1-13]. Therefore, the spontaneous strains associated with the two kinds of crystal structure are S~ r = 6r and S~ = 6g, respectively. For a domain thickness of about 0.5 gm, we obtain the elastic energy density values of 1.3 x 10-3 erg/cm for a tetragonal distortion and 0.9xl0-4erg/cm for a rhombohedral distortion of crystal lattice, calculated from x-ray diffraction data [12]. The obtained values are smaller than those previously estimated for BaTiO 3 [11], the first value being in a Fig. 3. SEM image of a domain structure for oblique incidence of the electron beam (x 1400) better agreement with the theoretically calculated wall energies for 90 ~ domains [14, 15]. The occurrence of mechanical stresses in the region of domain walls in PZT ceramics could explain the relatively broad range of the coexistence of the two FT and F R phases [16]. Complex domain structures can be seen in SEM for an oblique incidence of electron beam with respect to the surface of PZT specimen (Fig. 3), enabling thus the observation of various domain kinds and of some characteristic features of the domain structure inobservable by other techniques TEM Investigations Recent TEM studies of PZT ceramics [8, 9, 17] have shown the occurrence of deformation twins analogous to 90 ~ ferroelectric domains, the domain walls being the very twinning planes. Twinning is due to the stress in ceramic material arising during paraelectric - ferroelectric phase transition or polarization. In Fig. 4a and b is shown a typical 90 ~ domain structure crossing of a ceramic grain, exhibiting contrast fringes similar to those associated with stacking faults. The domain thickness is in a range from 0.10 to 0.15 gm. Many domains contain a varying number of fine "subdomains" having a thickness between 100 and 300 A (Fig. 4c). The parallel stripes have been identified as twin-related 90 ~ domains. Using selected-area diffraction patterns from several domains, the splitting of spots into doublets along the [110] or [011] directions due to twinning on the (110) or (011) planes is observed. The 90 ~ ferroelectric domain configuration is similar to that reported in PbZro.s2Tio.4sO3 consisting in a displacement twinning along the (i 10) on {110} planes [8]. Another 90 ~ ferroelectric domain structure observed in a non-uniform thickness region of a thin PZT foil and the associated SAD pattern showing splittings into doublets and triplets of spots corresponding to highindex planes are shown in Fig. 5a and b. On the basis of previous lattice parameter measurements by x-ray diffraction for FT (at = A, CT=4.120A) and FR(aR=4.080A, ~R=89.73 ~ coexisting phases, the interpretation of diffraction pattern is given in Fig. 5c, where the splitted spots are designed as 301, 2i3, 225. The triplet splitting appears due not only to the twinning relation between two adjacent 90 ~ domains with tetragonal structure [8] but also to the possible local existence of the rhombohedral phase. In the same specimens having a Zr/Ti ratio placed close to the morphotropic phase boundary, the coexistence of the both ferroelectric FT and FR phases has been demonstrated by x-ray diffraction [12]. The 1% elongation of

4 240 P.G. Lucuta et al. Fig. 4. (a) Typical 90 ~ domain configuration in a PZT thin foil; (x 37,000). (b) The same zone as in (a) tilted with 28 ~ ; (x 37,000) (c) Fine "subdomain" structure of 90 ~ ferroelectric domains ( 72,000) diffraction spots, which cause an overlapping of the possible splittings for low dhkr is due to the relative rotation of the crystal lattice appearing at the joining of two mismatched adjacent domains in stressed material. ', Z--'" / \ ", \ -'' ',\ -.%,,./.-,, 9., / / \ -- C uh+vk+w[.--1 uh +vk~-wt=o uh+ v k.,-wt =-' Fig. 5. (a) 90 ~ domain structure in a non-uniform thickness of PZT thin foil; ( (b) associated SAD pattern; (c) interpretation of the SAED pattern given in Co). Tilted (021) zone axis (8.2 ~ tilting angle) corresponds to (173) zone axis

5 Domain Structure in PZT Ceramics at Morphotropic Phase Boundary 241 In order to study the nature of diffraction spot splittings, compact size axes {110} associated with the TEM image from Fig. 6a have been selected. The diffraction pattern (Fig.6b) contains two different tetragonal zone axes (li0)t1 and (10i)x2 corresponding to two adjacent twin-related 90 ~ domains. Besides the doublet spot splitting due to the twinning, the triplet splitting of 333 and 224 spots can be attributed to the occurrence of rhombohedral phase (Fig. 6c). Thus, an additional rhombohedral (ll0)r zone axis has also been considered in Fig. 6c. In Fig. 7 a good fitting is obtained between the theoretically calculated pattern for the triplet splitting of the 224 spot from Fig. 6 and the experimental diffraction pattern. However, a small rotation angle of about 0.3 ~ between the (II0)T1 and (10i)x2 zone axis occurs due to the accommodation of crystal lattice strains induced by internal stress (Fig. 6c). The presence of the rhombohedral phase associated with ferroelectric domain structure can be explained by the occurrence of this phase between the 90 ~ domain walls. Thus, the modification of the P polarization vector direction will take place in the following alternative sequence: [001]~[111]~[010]~[111] --*[001]... corresponding to a repetitive domain succession T1RTzRT1... (Fig. 8). This possible model for the stacking succession of ferroelectric domains leads to a lower energy for the rotation of the polarization vector between two adjacent T domains due to the occurrence of an intermediate step in a R domain limited by two pure 90 ~ walls of T domains. Taking into account that the relative rotation of crystal lattice occurs only at the domain walls (R zones), the rotational distortion is not additive at a complete crossing of all domain walls present in the stacking succession T1RT2RT1... existing in a ceramic grain. The same model explains the small value of spot elongation and their displacement against to the theoretical positions. The suggested stacking succession of domain configuration allows also a rough estimation of elastic strain energy associated with a T-R domain wall where the tetragonal and rhombohedral distortions can be added or subtracted for an ideal joining of two different domains. Thus, the lattice distortions and spontaneous strains oscillate between a lower limit Ss mm= ~rnin = ~T -- 6R (2) and an upper limit S~s ax = ~rnax = 6T "~ ~R' (3) On the basis of (1-3) and the definitions of homogeneous parameters for tetragonal and rhombohedral distortions, a minimum and a maximum value of elastic c Y L/ 242T2 Fig. 6. (a) TEM image of a thick region in a PZT thin foil; ( x 30,000); (b) associated SAED pattern; (c) interpretation of the SAED pattern given in (b) (0.3 ~ rotation angle between (1T0)T~ and (10T)x2 zone axis)

6 242 P.G. Lucuta et at. -11o =r~ H<o1~rz "~l~z/~-1,, O O 22~" R 24.2T2 9 catcutated spols o observed spots Fig, 7. Comparison of observed and calculated triplet splitting for the 224 spot (Fig. 6b) t ~-/ ~403 /,2, I ' Fig. 8. A possible stacking succession of ferroelectric domains T1RTzRT~... corresponding to the alternative sequence: [001] -~ [111] --* [010] --* [111] --* [001]... for direction of polarization vector (both T and R phases are coexistent near the morphotropic phase boundary) applied to PZT in agreement with the angular values derived from etch patterns. 2) The oblique incidence of the electron beam against the specimen surface enables the SEM observation of all kinds of ferroelectric domains in a topographic contrast manner. 3) SAED patterns associated with the domain structure observed in TEM have shown spot splittings (especially for high Miller indices) due to the twinning relation between the adjacent tetragonal 90 ~ domains and to the simultaneous occurrence of ferroelectric rhombohedral phase domains. 4) A simple model, assuming the occurrence of a rhombohedral phase between the 90 ~ domains, is proposed for the modification of polarization vector direction in an alternative sequence: [001]--*[111] --* [t310] ~ [111]--, [001]... corresponding to a repetitive domain succession TIRT2RT1... This crystallographic model is obviously related to the coexistence of T and R phases in the same PZT ceramic grain at the morphotropic phase boundary. 5) For the elastically stored energy in a T-R wall an approximate value ranging from 0.6xl0 -a to 2 x 10 -a erg/cm was estimated for the PZT domain configuration associated with the phase coexistence region, energy density per unit length, stored in domain walls, can be found: Wsmin/h=0.6 x 10.3 erg/cm and l't~ax/h = 2.0 x 10- a erg/cm, respectively. The obtained values are lower than those calculated for BaTiOa [11], the upper value being however in good agreement with the wall energies calculated for 90 ~ T-T walls by various authors [14, 15]. The coexistence of the two ferroelectric phases F T and FR in the same PZT grain has been already suggested for the understanding of piezoelectric properties optimization E4] or of variation trends of lattice parameters with the sintering temperature [12], but no microscopic coexistence model has been proposed. 3. Conclusions The present SEM, SAED, and TEM investigations of ferroelectric domains in Pb(Zro.s3Tio.47)Oa ceramics can be summarized as follows: 1) The model of spatial domain configuration in ceramic grains proposed for BaTiO3 can be also References 1. E. Sawaguchi: J. Phys. Soc. Jpn. 8, 615 (1953) 2. B.Jaffe, W. Cook, A. Jaffe : Piezoetectric Ceramics (Academic, London 1971) 3. W. Wersing: Ferroelectrics 7, 163 (1974) 4. V. A. Isupov: Fiz. Tverd. Tela 10, 1244 (1968) 5. P. Ari-Gur, L. Benguigui: J. Phys. DS, 1856 (1975) 6. S.S. Chiang: M.S. Thesis: Univ. of California, Berkeley (1978) LBL No R. Gerson: J. Appl. Phys. 31, 188 (1960) 8. E.K.W. Goo, ILK. Mishra, G. Thomas: J. Appl. Phys. 52, 2940 (1981) 9. Yu-Jin Chang: Appl. Phys. A29, 237 (1982) 10. S.I. Yakunin: Soy. Phys. Solid State 14, 310 (1972) 11. G. Arit, P. Sasko: J. Appl. Phys. 51, 4956 (1980) 12. F. Vasiliu, P.Gr. Lucuta, F. Constantinescu: Phys. Stat. Solidi (a) 80, 637 (1983) 13. E.G. Fesenko, W.S. Filipev, M.F. Kupriyanov: Fiz. Tverd. Tela 11, 466 (1969) 14. V.A. Zhirnov: Soy. Phys. - JETP 35, 822 (1959) 15. L.N. Bulaevski: Sov. Phys. - Solid State 5, 2329 (1964) 16. T. Kala: Phys. Star. Solidi (a) 78, 277 (1983) 17. T. Malis, H. Gleiter: Phys, Stat. Solidi (a) 56, K87 (1979)