Atomistic structure and segregation behavior in secondary structure and facet of Pr-doped ZnO [0001] tilt grain boundary

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1 Paper Atomistic structure and segregation behavior in secondary structure and facet of Pr-doped ZnO [0001] tilt grain boundary Ji-Young ROH, *,³ Yukio SATO * and Yuichi IKUHARA *, **, *** *Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo , Japan **Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya , Japan ***World Premier International Research Center Initiative for Advanced Institute for Materials Research, Tohoku University, Sendai , Japan Much attention has been paid to grain boundaries (GBs) in ceramics owing to the impact on material properties. GB atomic scale investigations have so far mostly focused on the major structures. However, actual GB structure is more complex; there could be multiple types of atomistic structure and different morphology such as step and facet. As a case study to characterize these, we report extensive scanning transmission electron microscopy observations for a zinc oxide (ZnO) [0001] 27.8 ( 13, in the framework of coincidence site lattice theory) tilt GB doped with praseodymium (Pr) in this paper. In addition to the major structure that covers most of the (2570) GB plane area [Sato et al., Phys. Rev. B, 87, (2013)], two types of metastable atomistic structure are found. One is the secondary structure for the (2570) GB plane area, which is mostly found near facets. The other is a different type of atomistic structure formed in facets. Pr concentration is lower in the secondary structure than in the major structure. It is thus demonstrated that there is a variety in the atomistic structure and chemical composition within a single GB The Ceramic Society of Japan. All rights reserved. Key-words : Zinc oxide, Grain boundary, Atomistic structure, Segregation, Scanning transmission electron microscopy [Received December 15, 2013; Accepted December 27, 2013] 1. Introduction Much attention has been paid to grain boundaries (GBs) in ceramics owing to the impact on material properties. 1) 7) The atomic-scale structure has been characterized by transmission electron microscopy (TEM) and scanning TEM (STEM). Mostly because of the difficulty in these experiments, atomic-scale analysis has often been limited in small area of GB, and most of the studies have focused only on major structure. On the other hand, GB actually includes not only the major atomistic structure but also metastable ones. Furthermore, the GB plane is not completely flat in large area but could be stepped, curved, or facetted. 8),9) In recent years, stability in electron microscopy observations has been largely improved, which allows us to investigate GB atomistic structure in larger area by a quantitative manner. 10) One of typical electroceramics where GB has an impact on the properties would be zinc oxide (ZnO) ceramics. This is because ZnO ceramics doped with praseodymium (Pr) and some other additives exhibit highly nonlinear current voltage (I V) characteristics that originate from the GBs. 11) Therefore, in order to characterize the atomic-scale GB structure and further to understand the microscopic origin of nonlinear I V characteristics, we have studied some of ZnO GBs doped with Pr with STEM. 11) 13) We have used ZnO [0001] tilt GBs fabricated within bicrystals, since orientation relation of the two crystals can be controlled ³ Corresponding author: J.-Y. Roh; roh@sigma.t.u-tokyo. ac.jp Preface for this article: DOI The Ceramic Society of Japan DOI Fig. 1. (a) Schematic drawings of ZnO [0001] tilt GB and bicrystal. Two crystals have common [0001] axes, and the GB tilt angle (2ª) is made by (1120) of the two crystals. Prior to the bonding, a thin layer of Pr metal was deposited on the surface of one crystal. (b) Alignment of structural units (SUs) of the Pr-doped ZnO [0001] symmetric tilt GBs. 11) 13) There are three kinds of SUs;,, and. The SUs,, and respectively include six- and eight-membered rings, four- and sixmembered rings, and four-, eight- and six-membered rings. [Fig. 1(a)]. We have recently found that the Pr doping and increase of the tilt angle up to 27.8 ( 13, in the framework of coincidence site lattice theory 14) ) cooperatively induce the structural transformation [Fig. 1(b)]. 13) Atomistic structures of the 16.4 ( 49) and the 21.8 ( 7) GBs are described as structural units (SUs) of and/or, while that of the 27.8 GB is characterized as another SU type, the SU [Fig. 1(c)]. This 381

2 JCS-Japan Roh et al.: Atomistic structure and segregation behavior in secondary structure and facet of Pr-doped ZnO [0001] tilt grain boundary Fig. 2. (a) Low-magnification BF STEM image of the GB. Arrows indicate the facets. (b) Higher-magnification ADF STEM image of the GB. There are three different segments in the image; major structure in the (2570) GB plane; secondary structure in the (2570) GB plane; and facet that is almost parallel to {1010} and {1120} of the adjacent crystals. implies that atomistic structure in the larger area should be investigated because multiple types of structure are likely to coexist and if so relative stability should be discussed. In this paper, we report atomic-scale STEM observation for a larger area of the Pr-doped ZnO [0001] 27.8 ( 13) tilt GB. 2. Experimental procedure In the present study, the ZnO bicrystal specimen that was used in our previous study 13) is again studied. The bicrystal was fabricated by bonding two ZnO single crystals (Shinkosha Co. Ltd.). In the beginning, the {2570} surface of the crystals was polished to the mirror state. Then, a thin layer of Pr metal with nominal thickness of 5 nm was deposited on the surface of one crystal. One crystal was set on the other so that the bicrystal possess [0001] 27.8 tilt GB [Fig. 1(a)]. The crystal set is heat treated at 1,100 C for 10 h in air under the uniaxial load of about 1.5 MPa for bonding. The heating and cooling rates were 300 C/h. Thin foils for STEM observation were prepared by conventional methods that include mechanical polishing and Ar-ion beam milling processes. STEM observations were carried out using JEM-2100F (JEOL Ltd.) with a spherical-aberration corrector for the electron probe (CEOS Gmbh). The probe forming aperture semiangle was 22 mrad. Bright-field (BF) and annular dark-field (ADF) images were acquired with the detection angle ranges of about 0 13 mrad. and about mrad., respectively. Energy dispersive spectroscopy (EDS) analysis was performed using the EDS system and software attached to the JEM- 2100F microscope. 3. Results and discussion It has been confirmed that the two crystals are directly bonded at the GB [Fig. 2(a)]. The GB plane is flat and parallel to {2570} of the adjacent crystals in most of the area, and there are facets that are almost parallel to {1010} and {1120} of the adjacent crystals. The actual GB tilt angle was identified to be about 27.8 from selected area diffraction pattern, which is quite close to the ideal value for 13 orientation relation. The GB has been further characterized at higher magnification [Fig. 2(b)]. Since contrast of ADF STEM image depends on the atomic number (Z) of the constituting elements, 15) Pr (Z = 59) looks brighter than Zn (Z = 30) does, and therefore the brightest spots indicate the presence of Pr. It is clearly seen that Pr strongly segregates to the GB. GB segments can be characterized by the GB plane orientation and the Pr segregation pattern. Continuous Pr segregation pattern with the brightest contrast is recognized in the {2570} GB plane. Since this segment covers roughly 77% of the {2570} GB plane Fig. 3. ADF STEM images of (a) major and (b) secondary structures in the (2570) GB plane region. Set of circles indicates the alignment of SUs in a GB structural period as shown by dotted lines. The SUs are highlighted on the right-hand side. Circuit mapping analysis for (c) major and (d) secondary structures. Smallest lattice vectors for the upper ( ) and the lower ( ) crystals are shown by a 1,a 2, and a 3. Circuits (S 1 -X-F 1 and S 2 -X-F 2 ) for a GB period are drawn in the GB and bulk regions. Vectors (F 1 -S 1,F 2 -S 2 ) in the bulk regions possess the components of ¹3a 1. area, it will be called major structure hereafter. Pr segregation pattern with less bright contrast is seen in the rest of the {2570} GB plane area, which will be called secondary structure. Our inspection interestingly reveals that the secondary structure mostly forms near facets. Also, there are facets that are almost parallel to {1010} and {1120} of the adjacent crystals. Atomistic structure of major structure is described by the repetition of SUs [Fig. 3(a)] as we have reported. 13) On the other hand, the secondary structure is described as the SU alignment of - [Fig. 3(b)] which is similar to that of the undoped case. 13) Since contrast for Pr-containing columns seems less bright as compared with the major structure case, occupancy of Pr in these columns may be lower. Relative location of Pr within the SUs and is similar to those in the 16.4 and the 21.8 GB 382

3 JCS-Japan Fig. 5. (a) ADF STEM image of a facet. The atomic configuration is indicated by the set of circles. The SU is shown in the inset. (b) A schematic drawing of the GB plane. (1010), (1120), and (2570) of the adjacent crystals are also shown. The angle made of the (1010) and the (1120) is 2.2. Fig. 4. (a) A Circuit (S 3 -X-F 3 ) drawn to surround a step. GB plane is indicated by a dotted line. (b) Dichromatic pattern with a CSL lattice. Translation vectors are shown by t( ) and t( ), and b indicates the Burgers vector of secondary dislocation. (c) The same pattern with DSC lattice. (d) Schematic illustration of the step structure in Fig. 4 (a). h( ) and h( ) represent the step height for the upper and the lower crystals. cases. 11),12) In addition to SU descriptions, these structures have been analyzed by circuit mapping formalism. 16) GB structure can be characterized with dislocationlike feature with this formalism, which has been used to analyze ZnO and GaN [0001] tilt GBs. 17) 20) Circuits (S 1 -X-F 1 and S 2 -X-F 2 ) are drawn around a GB period and then circuits with identical components are mapped into grain interior [Figs. 3(c) and 3(d)]. Then, the circuits exhibit closure failures, where vectors from the finishing to the starting points (F 1 -S 1,F 2 -S 2 ) correspond to dislocation content of the GB period. Here, both of the structures exhibit the failures with ¹3a 1 = [1210], which is the content of primary dislocations for these structures. Here, a 1 is one of the smallest lattice vectors in the ZnO crystal. It is also found that there is a step in between major and the secondary structures [Fig. 4(a)]. Step can also be analyzed using circuit mapping formalism, which often falls into secondary dislocation character. A circuit (S 3 -X-F 3 ) to surround a step is drawn first. Next, a circuit with identical component is mapped into a dichromatic pattern for the 13 orientation relation [Fig. 4(b)]. Then, the circuit exhibits closure failure with a small Burgers vector of b. The Burgers vector can be understood as the Fig. 6. EDS spectra taken from (a) major and (b) secondary structures and (c) facet. Electron beam was scanned over a 1 nm (in GB normal direction) 2 nm (in GB parallel direction) square box during the measurements. Peaks at ³8.05 kev are artifacts from the TEM specimen holder. 383

4 JCS-Japan Roh et al.: Atomistic structure and segregation behavior in secondary structure and facet of Pr-doped ZnO [0001] tilt grain boundary Table 1. Details for the circuit mapping analysis of a step in Fig. 4 (a). c( ), c( ), and c(, ) denote the vector S 3 -X in the upper crystal, the vector X-F 3 in the lower crystal, and the vector S 3 -X-F 3 for the GB. h( ) and h( ) denote the step height, n is the unit vector normal to the GB plane, b is the Burgers vector of secondary dislocation, and P 13 is a matrix that re-expresses the vectors in the lower crystal ( ) into those in the upper crystal ( ) 21) Upper crystal ( ) Lower crystal ( ) Secondary dislocation c( ) = 1/3[420160] c( ) = [2570] h( ) = ¹n c( ) = ¹4 d(2570) h( ) = n c( ) = ¹3 d(2570) b = ¹c(, ) = c( ) ¹ P 13 c( ) = 1/39[5720] t( ) = ¹2a 2 = 2/3[ 12 10] t( ) = a 3 ¹ a 2 = 1/3[0330] = [0110] t( ) ¹ P 13 t( ) = 1/39[5720] Fig. 7. facet). Relation among the atomistic structure (SU), Pr concentration, and morphology (GB plane orientation, step, and difference between the translation vectors for the upper [t( )] and the lower [t( )] crystals, which can be known by mapping circuits S 3 -X and X-F 3 in the dichromatic pattern (Table 1). It turns out that t( ) = ¹2a 2 = 2/3[1210],t( ) = a 3 ¹ a 2 = 1/3[0330] = [0110], and b = 1/39[5720]. It should be also noted that the Burgers vector is identical to that expected for displacement shift complete (DSC) dislocation [Fig. 4(c)]. By using h( ) = n t( )B and h( ) = n t( )B, where n is the unit vector perpendicular to the GB plane, h( ) and h( ) were estimated to be ¹4 d(2570) and ¹3 d(2570) [Fig. 4(d)]. A different type of atomistic structure is found in facets [Fig. 5(a)]. There are wide open spaces in the middle, and there are several four-membered rings. Most of Pr are present at the corner of the four-membered rings that face the open spaces. It should be noted here that (1010) and (1120) of the adjacent crystals are not actually parallel but these make the angle of 2.2 [Fig. 5(b)], which may lead to local strain to accommodate the angle deviation. Furthermore, EDS spectra were measured (Fig. 6) in order to compare the Pr concentration in major and secondary structures, and facet. Zn K, Zn L, O K, and Pr L lines are detected in all the cases [Figs. 6(a) 6(c)]. In this study, eleven spectra were used to quantify the Pr concentration for each case. Average Pr concentration and standard error in cation % were estimated to be 9.5 («0.3), 5.1 («0.2), and 7.1 («0.3) in major and secondary structures and facet, indicating that the major structure has higher Pr concentration than that the secondary structure does. Relationship among atomistic structure, Pr concentration, and morphology of the GB is summarized as shown in Fig. 7. GBis mostly composed of the flat {2570} GB plane area and partly of facets that are almost parallel to {1010} and {1120}of the adjacent crystals. Major structure with SU forms in most of the flat {2570} GB plane area. Secondary structure with SUs and appears in the rest of the area, mostly near facets. There is a step in between the major and the secondary structures, and the step is characterized as secondary dislocation with the Burgers vector of 1/39[5720]. Pr concentration is lower for the secondary structure than that for the major structure. Formation of secondary structure may be related with the facet. It is considered that there is strain field near facets. If the major structure is not preferred under strain, the secondary structure may form instead. 4. Summary Extensive STEM observations were carried out for a larger area in Pr-doped ZnO [0001] 27.8 ( 13) tilt GB. Relation among atomistic structure, segregation behavior of Pr, and morphology has been analyzed and discussed. An important implication is that GB morphology such as step and facet has a potential impact on not only the atomistic structure but also the chemical composition. Acknowledgement A part of this work is supported by the Grant-in-Aid for Young Scientists (B) from Japan Society for Promotion of Science (JSPS), the G-COE program, Nihon Sheet Glass Foundation for Materials Science and Engineering, and Kato Foundation for Promotion of Science. A part of this work was also supported by Nanotechnology Platform (project No ) sponsored by MEXT, Japan. References 1) A. T. Paxton and M. W. Finnis, J. Mater. Sci., 40, 3045 (2005). 2) T. Sakuma, L. Shepard and Y. Ikuhara, Ceram. Trans., 118, (2000). 3) C. Elsässer, A. H. Heuer, M. Rühle and S. M. Wiederhorn, J. Am. Ceram. Soc., 86, 533 (2003). 4) Y. Ikuhara, J. Ceram. Soc. Japan, 109, S110 S120 (2001). 5) J. P. Buban, K. Matsunaga, J. Chen, N. Shibata, W. Y. Ching, T. Yamamoto and Y. Ikuhara, Science, 313, (2006). 6) Z. Zhang, W. Sigle and M. Rühle, Phys. Rev. B, 66, (2002). 7) N. D. Browning, J. P. Buban, P. D. Nellist, D. P. Norton, M. F. Chisholm and S. J. Pennycook, Physica C, 294, (1998). 384

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