Journal
J. Am. Ceram. Soc., 91 [7] 2298–2303 (2008) DOI: 10.1111/j.1551-2916.2008.02416.x r 2008 The American Ceramic Society
Phase-Transformation-Induced Twinning in Orthorhombic BaCeO3 Shun-Yu Cheng, New-Jin Ho, and Hong-Yang Lu*,w Centre for Nanoscience, Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
domains (IBD), are a direct result of solid-state phase transition involving changes in space group symmetry. They are generated upon the loss of symmetry elements, resulting in orientation6–8 and/or translation9–11 variants. Phase-transformation-induced twinning in other perovskites, e.g., CaTiO3,6,7 MgSiO3,8 LaGaO3,9–11 and BaTiO3,12 has been well characterized. Adopting the space group Pbnm (with the choice of conventional axes b a c as abc, taking the c-axis as the orthorhombic long axis), the reflection twins11–15 lying in {112) and {110) were usually observed; the rotation twins6,7,9,11 of the normal type [010]>9 and parallel type /111S//11 were also identified. Twinning produced from symmetry change was derived12 on the basis of symmetry element loss for phases that are related crystallographically by group–subgroup symmetry, e.g., c-t-BaTiO3. Experimental observations12 were consistent with pseudo-merohedral reflection twins of the 901 and 1801 type thus predicted, although not all twins deduced crystallographically from the space group change were identified under the microscope. Unlike BaTiO3 but similar to LaGaO3,10,11 the r-2o1-BaCeO3 phase transition with no group–subgroup relation existing between the high- and low-symmetry phases is first order in nature.13 The o1- and o2- transition phases are of course group–subgroup related; the o-phase refers to the second orthorhombic (o2) phase hereafter, unless specified. The Landau theory14 forbids transformation domains in lower-symmetry phases that exhibit twin relationships when structural coherence is lost upon the first-order transition. It was proposed by Christy15 that a metastable intermediate phase may exist and permit such phase transition (type II) to occur in a diffusionless, continuous, and second-order fashion (type I) when structural continuity is then retained to circumvent the restrictions imposed by the Landau theory. It is also permissible that several metastable intermediate phases exist in succession to facilitate such phase transition (type III15) between crystalline phases related by non-group–subgroup symmetry. Indeed, reflection twins in o-LaGaO310,11 and o-La1xSrMnO316 found to lie in {110) and {112) were ascribed to type II phase transition15 occurring via an imaginary metastable intermediate phase of the lowest common supergroup (Pm 3m). Transformation twins in o-LaGaO3 (Pnma) were derived11 from considering point group symmetry reduction from r-cubic intermediate-o and are shown in Table I. Similarly, not all predicted twins were found9–11 experimentally. Here, we present TEM analysis of twinning in o-BaCeO3 ceramics prepared by pressureless sintering of solid-state-reacted powder in air. Both {110) and {112) twins are identified, their reflection nature ascertained, and fault vectors (R) determined. The formation of transformation twinning as it relates to symmetry reduction upon phase transition is discussed.
Phase-transformation-induced twinning in orthorhombic barium cerate (BaCeO3) has been analyzed using transmission electron microscopy. Reflection twins with boundaries lying in {110) and {112) are generated by loss of point group symmetry elements during solid-state phase transition along the sequence of Pm3m-R3c-o1-Ibmm-o2-Pbnm when pressureless-sintered samples were cooled from 14001C to room temperature. Fault vectors R 5 e/110] and e/021], in which displacements are not related in a simple way to the lattice translation, are determined for the twin boundaries exhibiting d-fringe patterns. The highand low-symmetry phases, rhombohedral and orthorhombic, are not related by group–subgroup symmetry, and the transformation is discontinuous and first order in nature, where twin relationships in the low-symmetry phase are forbidden by the Landau theory. However, the transformation twins observed experimentally are consistent with those predicted directly from assuming a non-disruption condition between transition phases, where the new structure is described in the frame of the old one geometrically. Such a phase transition can be continuous and diffusionless if it occurs via second order to and from a metastable intermediate phase, which is a shared common space group of the high- and low-symmetry phases. Accordingly, two possible intermediate phases of the minimal common supergroup cubic Pm3m (No. 221) and of the maximal common subgroup monoclinic C2/c (No. 15) for the two end phases are identified.
I. Introduction
A
CCEPTOR-DOPED barium cerate (BaCeO3) perovskite is one of the major contenders for solid electrolytes in solid oxide fuel cells (SOFC).1,2 When SOFC operate in the intermediate temperature range of 2001–6001C, the electrolyte experiences a series of solid-state phase transitions3,4 from cubic Pm3m (No. 221) at 9001C-rhombohedral R3c (No. 167) at 4001C-orthorhombic o1-Ibmm (No. 74) at 1541C-o2-Pbnm (No. 62) upon lowering temperature. Antiphase boundary (APB) domains generated by loss of the lattice point at I-center were characterized by TEM, which has confirmed4 the neutron diffraction results that suggested3 the existence of a second orthorhombic phase o1Ibmm at 4001C. Although mode softening occurred, the o1-o2 transition was thought3 to be partly displacive and partly order– disorder. Microstructure analysis of the lower-symmetry phases may therefore provide direct evidence for the solid-state phase transition. Symmetry-breaking phase transitions produce domain microstructure with characteristics that can be predicted by investigating the space group relationships between high- and low-symmetry phases related by group–subgroup symmetry.5 Twin domains, including APB domains and inversion boundary
II. Experimental Procedure
T. Mitchell—contributing editor
Barium carbonate (BaCO3) of499.9% purity, supplied by Nippon Chemical Industry (NCI, Tokyo, Japan), and ceria (CeO2) powder of 499.99% purity, supplied by Meldform Metals Ltd (Royston, UK), were used to prepare BaCeO3 powder via solidstate reaction. The stoichiometric composition of BaCeO3 with BaCO3:CeO2 at a 1:1 molar ratio was attrition milled for 20 h
Manuscript No. 24035. Received November 26, 2007; approved January 25, 2008. This work was financially supported by the National Science Council of Taiwan through contracts NSC 93-2216-E-110-015, 94-2216-E-110-004, and 95-2221-E-110-047. *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail:
[email protected]
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July 2008
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Twinning in Orthorhombic BaCeO3
Table I. Transformation-Induced Twins Derived from Point Group Symmetry Reduction from the r-Cubic Intermediate-o Transition Sequence Rhombohedral ( 32/m)
3 2/m
Cubic (4/m32/m)
!
[111]r r/(1 10)r [110] [01 1]r/(01 1)r [101]r/( 101)r
3 2/m Gain 2/m Gain 4/m
[111]c [01 1]c/(01 1)c [10 1]c/(10 1)c [1 10]c/(1 10)c [011]c/(011)c [101]c/(101)c [110]c/(110)c [1 10]/(1 10) [01 1]/(01 1) [ 101]/( 101)
using a Szegvari attritor (model 01-HD, Union Process, Akron, OH) with yttria-stabilized ZrO2 (YSZ) balls (Toray, Urayasu, Japan) at B400 rpm in ethanol. After drying at 1101C for 48 h, the mixture was ground using an agate mortar and pestle and de-agglomerated by passing through a sieve of B150 mm. It was then calcined at 11001C for 2 h, facilitating the reaction of forming BaCeO3 to completion. The solid-state-reacted BaCeO3 powder was dried, ground, and passed through a similar sieve before it was pressed into disks of 10 mm diameter using a WCinserted steel die with a uniaxial pressure of 100 MPa. Green disks were pressureless-sintered at 14001C for 4 h in air with a 51C/min heating rate and cooled to room temperature by switching off the power. The sintered microstructure was analyzed by TEM using a Tecnait G2 F20 field emission gun (FEG) TEM (Hillsboro, OR) and JEOLt AEM 3010 (Tokyo, Japan), operating at 200 and 300 kV, respectively. Thin foils were prepared from sintered disks by the conventional techniques of cutting, grinding, and polishing successively to a 0.1 mm surface roughness and then ion-beam thinning using a Gatan ion miller (DuoMillt or PIPSt, Pleasanton, CA) to electron transparency.
!
Orthorhombic (2/m2/m2/m)
Three orientation variants along /210S {121} twins (loss of m) {123} twins (loss of 2) {101} twins (loss of m) [010]901 twins (loss of 4)
patterns (SADP) obtained by positioning the diffraction aperture on I, I–II, and II, correspondingly, as indicated in Fig. 1(d). Patterns shown in (a) and (c) are indexed to different zone axes of ZI 5 [ 111] and ZII 5 [1 1 1] to ensure that the shared domain boundary, i.e., twin plane (110), is consistent. Spot splitting6–12 revealed by high-order reflections, e.g., 12 3I and 2 13II (indicated and shown in the inset of Fig. 1(b)), suggests twinning occurs by pseudo-merohedry.5,19 Such reflection spots are commonly addressed as the twin spots. The separation Dg 5 gIgII between them subtending an angle f 0.441, which can be calculated by9–11 o ¼ 2 tan1 ða2 þ c2 Þ1=2 =bÞ ðp=2Þ
(1)
represents twin obliquity (o), where f 5 2o. An unsplit row of reflections (USR) found from Fig. 1(b), as indicated, suggests that the twin plane shared between domains I and II lies in (110). The fact of Dg||USR (or Dg>twin plane) also suggests17 that the (110) domain is a reflection twin. Tilting experiments according to Boulesteix et al.20 further confirm unambiguously that the twins are of the reflection type related by mirror symmetry.
III. Results An average final density at rrel 76.3%, determined by the Archimedes technique, suggests that samples remaining in the second stage are poorly sintered. Only the orthorhombic phase was identified in both calcined powder and sintered samples by X-ray diffractometry, which has been reported.4
(1) {110) Twinning A representative o-BaCeO3 grain from samples of undoped, stoichiometric composition containing transformation twins is shown in Fig. 1(d). Two domain boundaries extending across the grain and segmenting into three twin domains of I, II, and III are observed by the bright-field (BF) image. Residual pores, necking between grains and triple-grain junctions, are clearly discerned, as indicated. A twin boundary inclined to the electron beam exhibits the characteristic d-fringe pattern (summarized in Table II) with extreme fringe contrast (EFC) asymmetric (i.e., dark–bright (D–B) or bright–dark (B–D) about the centerline in the BF image) and simultaneously symmetric (i.e., dark–dark (D–D) or bright–bright (B–B) in the center-dark-field (CDF) image). EFC of the I–II boundary as B–D and B–B is shown representatively in the inset. The corresponding fringe patterns of both I–II and II–III domain boundaries are also shown in juxtaposition with two sets of EFC at d40 and do0,17 together with a schematic illustration depicting the difference in d-factor for two neighboring domains. Forbidden laws18 described for Pbnm state that the reflections (h0l), where h1l is odd, and (0kl), where k is odd, do not appear in the electron diffraction patterns, i.e., 001 and 010 are both forbidden spots. Figures 1(a)–(c) are selected-area diffraction
Fig. 1. Reflection twins lying in (110), SADP corresponding to (a) domain I, (b) boundary I–II, and (c) domain II,(d) BF image, SADP when foil was (e) tilted about USR, and (f) tilted perpendicular to USR (TEM). SADP, selected-area diffraction patterns; BF, bright field; TEM, transmission electron microscopy.
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Table II. Extreme Fringe Contrast of (110) Twin Boundary Exhibits Typical d-Fringe Patterns with Corresponding BF and CDF Image Shown Juxtaposed BF image
DF image
Table III. Fault Vector R 5 e[1 10] Determined for (110) Twins Adopting the Invisibility Criteria of 2pg R 5 0 2pg R 5 0
CDF image ZI/ZII
d40 do0
Vol. 91, No. 7
Journal of the American Ceramic Society—Cheng et al.
Last
First
Last
First
Last
First
D B
B D
B D
B D
D B
D B
½001=½001
½111=½111
½201=½021
gI/gII 020=200 220=220 220/220 ½112=½112 132=312 112=112 204=024
O O X 7enp 7enp O
B, bright fringe; D, dark fringe; CDF, centered-dark-field.
O 7enp
O X 7enp O
R
O 7enp e½1 10
Contrast analysis using the invisibility criteria 2pg R 5 0 with standard TEM procedures. n, integer; O, visible; X, invisible; TEM, transmission electron microscope.
First Last
First Last
First Last
First Last
δ0 viewing direction
I S
