Structural investigation of the SrAl2O4-BaAl2O4 solid solution system with unstable domain walls

Author’s Accepted Manuscript Structural investigation of the SrAl2O4-BaAl2O4 solid solution system with unstable domain walls Y. Ishii, H. Tsukasaki, S. Kawaguchi, Y. Ouchi, S. Mori www.elsevier.com/locate/yjssc

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S0022-4596(17)30072-5 http://dx.doi.org/10.1016/j.jssc.2017.03.002 YJSSC19706

To appear in: Journal of Solid State Chemistry Received date: 28 November 2016 Revised date: 25 February 2017 Accepted date: 1 March 2017 Cite this article as: Y. Ishii, H. Tsukasaki, S. Kawaguchi, Y. Ouchi and S. Mori, Structural investigation of the SrAl2O4-BaAl2O4 solid solution system with unstable domain walls, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural investigation of the SrAl2 O4 -BaAl2 O4 solid solution system with unstable domain walls Y. Ishii, H. Tsukasaki, S. Kawaguchia, Y. Ouchi, and S. Mori Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan. a

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo, Hyogo 679-5198, Japan.

Abstract We investigated the structural phase transition of the Ba1−x Sr x Al2 O4 solid-solution system (x ≥ 0.6) via in situ powder √ X-ray diffraction and transmission electron microscopy. The sequence of structural phase transitions P63 22 ↔ P63 ( 3A) ↔ P21 occurs in a composition window of x = 0.8–1.0. Ba substitution suppresses the P63 intermediate and low-temperature P21 phases, which disappear at x = 0.7 and 0.6, respectively. The P21 phase boundary exhibits first-order characteristics, at which the cell parameters change discontinuously and the cell volume slightly contracts as the temperature increases. In the P21 phase, unstable twin walls are observed; the shapes and the crystal axes of each twin domain change as the temperature increases. This instability can be attributed to the latent instabilities that both the BaAl2 O4 and SrAl2 O4 compounds possess. Keywords: Stuffed tridymite-type oxide, Structural phase transition, Transmission electron microscopy

1. Introduction There has been continuous interest in the rich variety of structural chemistry of stuffed-tridymite type oxides AB2 O4 (A = Ca, Sr, Ba; B = Al, Ga, Si, Ge, Mg, Fe, Co, Zn) [1]. Among these, SrAl2 O4 has shown novel luminescent [2, 3, 4], ferroelectric, and ferroelastic properties [5, 6] and has been extensively studied as an environmentally friendly material. Its hightemperature phase above ≈1150 K adopts a hexagonal unit cell (space group P63 22) with ap ≈ 5.2 Å and cp ≈ 8.6 Å [7, 17]. Although the P63 22 phase has long been recognized to directly transform into the mono√ clinic low-temperature P21 phase (am = cp , bm = 3ap , cm = ap , β ≈ 93◦ ), recent in situ powder X-ray diffraction (XRD) experiments revealed √ the presence of an intermediate P63 phase with ah = 3ap and ch = cp between 1130–950 K [7]. The P21 structure is suppressed by Ba substitution at the Sr-site, and a hexagonal phase appears in the Ba-rich composition [8]. Although several structural investigations have been reported, the actual structural phase diagram for the Ba1−x Sr x Al2 O4 solid solution system remains under discussion. S. Ito et al. reported the suppression of the P21 phase by Ba substitution and the development of neighboring P63 22 parent phases in the Ba-rich composition Preprint submitted to Journal of Solid State Chemistry

cp

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bp

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Figure √ 1: Relationships between the P63 22 parent phase, the P63 ( 3A) intermediate phase, and the P21 low-temperature phase of SrAl2 O4 . The subscripts p, h, and m represent the P63 22, P63 , and P21 structures, respectively.

[8]. The structural transformation from the P21 monoclinic phase to the P63 22 parent phase is reported to occur at x = √0.5–0.6 at room temperature. The intermediate P63 ( 3ap ) phase is not mentioned in this literature because of its very weak superlattice reflections. On the other hand, the structural phase diagram that U. Rodehorst et al. proposed [10] contradicts √ previous reports [8, 9] and supports a P63 phase with 3ap for the Ba-rich composition at room temperature. In addition, C.M.B. Henderson et al. [9] reported a coexistence reMarch 1, 2017

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Figure 2: Powder XRD profiles (2θ = 8.6–10.4◦ ) near the 012 peak for (a) x = 1.0, (b) x = 0.9, (c) x = 0.8, (d) √ x = 0.7, and (e) x = 0.6 obtained at various temperatures. The Miller index is based on the P63 22 parent setting. Superlattice reflections of P63 ( 3ap ) and reflections arising from the P21 phase are indicated by filled and open triangles, respectively. The insets show the profiles obtained at lower temperatures. The incident X-ray beam was set to 25 keV (λ = 0.49594 Å). Data were collected on heating.

gion of the P21 and P63 22 phases in a compositional window of x = 0.575–0.7 at room temperature. We previously investigated the crystal structure of the Ba1−x Sr x Al2 O4 solid solution system with low Sr concentrations in terms of the low-energy phonons and their softening behavior [11]. The paraelectric phase of BaAl2 O4 crystallites have the same structure as the high-temperature phase of SrAl2 O4 and possesses the energetically competing low-energy phonon modes at the M- and K-points. The characteristic honeycombtype diffuse scatterings observed in several electron diffraction experiments [12, 13] are ascribed to these low-energy phonons. In BaAl2 O4 , the M-point mode condenses at T C = 451 K, giving rise to the lowtemperature ferroelectric P63 phase with 2ap × 2ap × cp , whereas the K-point mode is electrostatically unfavorable and disappears below T C . The P63 (2ap ) phase is significantly suppressed by a small amount of Sr substitution. As a result, the P63 22 structure is retained through a wide range of temperatures and compositions up to x = 0.5 [14, 15, 16]. These data suggest that similar structural instability is expected in the Sr-rich compositions of Ba1−x Sr x Al2 O4 . In this study, we investigated the structural phase transitions of the Ba1−x Sr x Al2 O4 system with a Sr-rich composition via in situ XRD and transmission electron microscopy (TEM). We found an unstable twin domain structure with changeable crystal axes in each domain

of the Sr-rich Ba1−x Sr x Al2 O4 upon heating. 2. Material and methods Polycrystalline samples of Ba1−x Sr x Al2 O4 (x = 0.6– 1.0) were synthesized using a conventional solid-state reaction method. BaCO3 (99.9%), SrCO3 (99.9%), and Al2 O3 (99.99%) powders were mixed with a 1:1 molar ratio and calcined at 1200◦C for 10 h and at 1300◦C for 12 h in air with intermediate grinding. Samples were stored under a vacuum. Synchrotron XRD experiments were conducted at the BL02B2 beamline at SPring-8 over a temperature range of 90–1100 K. Data were collected on heating. The incident X-ray beam was monochromatized to 25 keV using a Si (111) double crystal. The TEM experiments were carried out in a temperature range of 298–773 K using a doubletilted heating holder. After the desired temperature was reached, diffraction patterns and high-resolution TEM images were collected with a time interval of more than 30 min, until no changes in the diffraction patterns were observed. Indices are based on the P63 22 parent phase throughout this paper unless otherwise stated. 3. Results and Discussions Figures 2 (a)–(e) show the powder XRD profiles of Ba1−x Sr x Al2 O4 with x = 1.0–0.6 collected on heating. 2

As reported in reference [7], SrAl2 O4 (x = 1.0) exhibits √ the superlattice reflections arising from the P63 ( 3ap ) intermediate phase above 1000 K, as shown by a filled triangle in Fig. 2(a). Below 975 K, the crystal exhibits prominent P21 peaks, which grow at 100 K, as shown in the inset. The first structural phase transition tem√ perature (T s1 ) from P63 22 to P63 ( 3ap ) and the second √ transition temperature (T s2 ) from P63 ( 3ap ) to P21 are determined as the temperatures at which the superlattice reflections appear and the P21 peaks develop, respectively. In SrAl2 O4 , the T s1 and T s2 are reported to be 1133–1153 K and 950 K, respectively [7, 17]. Although the former temperature exceeds our experimental upper limit, the observed T s2 = 975 K is in good agreement with the previous reports. Notably, the P21 peaks co√ exist with the P63 ( 3ap ) superlattice reflection at 1000 K, marked by open triangles, due to the first-order phase transition [7, 9, 17, 18].

down to 100 K. T s2 also decreases with decreasing x and is 850 K, 700 K, and 525 K for x = 0.9, 0.8, and 0.7, respectively. In these compositions, the weak peaks of P21 are observed above T s2 due to the first-order character. Interestingly, weak peaks of P21 are observed for x = 0.6, although it does not experience temperature history across T s2 ; P63 22 is the main phase for x = 0.6 at the lowest temperature, 90 K. Additionally, this coexistence is observed over a considerable range of temperature, namely, from 90 K to 360 K. This behavior is apparently different from the first-order-like coexistence that the other compositions exhibit. Such coexistence is also observed in this study for the x = 0.54–0.58 samples over a wide range of temperatures below 500– 600 K. This interesting coexistence has been discussed in a previous report by C.M.B. Henderson et al. [9], in which this is explained as behavior similar to those of martensitic transformations in metals. That is, the strain energy arising from the transformation acts as the additional degree of freedom in the phase rule, which enables the two-phase coexistence. The cell parameter trends are shown in Fig. 3. Dis-

T s1 decreases with decreasing x and is 1050 K for x = 0.9 and 775 K for x = 0.8, as marked by filled triangles in Figs. 2(b) and (c), respectively. For x ≤ 0.7, no √ superlattice peaks of the P63 ( 3ap ) phase are observed P21 cm

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Figure 3: Temperature variations of lattice parameters and cell volumes for Ba1−x Srx Al2 O4 . (a) x = 1.0, (b) x = 0.9, (c) x = 0.8, (d) x = 0.7, (e) x = 0.6. (f) Axial angles β of the monoclinic P21 structure of x = 0.7–1.0. Error bars are usually smaller than symbols.

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Figure 5: Phase Diagram of Ba1−x Srx Al2 O4 (x ≥ 0.6). T s1 (filled triangles) and T s2 (filled circles) are suppressed by Ba substitution. Solid lines are guides for the eyes. T s1 reported by ref.[17] is also plotted (open diamond).

Figure 4: Electron diffraction pattern for x = 0.7 obtained at 773 K with [001]p incidence. The indices are based on the P63 22 parent phase. Symmetry points in the Brillouin zone of P63 22 are also indicated in the figure. Triangles indicate the diffuse scatterings near the M- and K-points.

structural phase transition of the Sr-rich Ba1−x Sr x Al2 O4 system is driven by the tilting of AlO4 -blocks along the c-axis associated with RUMs, which leads to the volume contraction at the P21 phase boundary. Figure 5 summarizes the T s1 and T s2 determined as the temperatures at which the superlattice reflections appear and the P21 peaks develop, respectively. The previously reported value of T s1 = 1153 K for x = 1.0 [17] is also shown. The coexistence of P21 in the P63 22 matrix is observed in a compositional window of x = 0.54–0.60 and over a wide range of temperatures. No systematic dependence on x was observed for the temperature at which the coexistence appears. In high-resolution transmission electron microscopy (HRTEM) observations of the P21 phase for x ≥ 0.6 samples, we found peculiar behavior of a twin structure. Fig. 6(a) displays the HRTEM image of x = 0.8 obtained at 293 K with [001]p incidence. There are twins in Fig. 6(a) associated with lowering the symmetry from hexagonal to monoclinic. The monoclinic b-axes (bm ) are shown in each twin domain. Fig. 6(b) shows the HRTEM image obtained at 373 K for the same position as Fig. 6(a). Because twins are generally pinned in crystals, the positions of twin boundaries are usually conserved. However, the domain shapes and the twin walls observed at 373 K are apparently different from those at 293 K. The monoclinic b-axes rotate within the same crystallite, and new twin boundaries are generated. Another example of the rotation of the monoclinic b-axis is shown in Figs. 6(c) and (d), which are the HRTEM images for x = 0.9 obtained at 293 K and 693 K, re-

√ continuities associated with the P21 to P63 ( 3ap ) firstorder transition are observed for x = 1.0–0.8. The discontinuity is also observed at the transition temperature from P21 to P63 22 for x = 0.7. At T s1 , the cell parameters increase continuously. It is reasonable that the axial angle β systematically decreases as x increases because the monoclinic distortion is relieved as the P63 22 phase approaches. We should note here the volume contraction at T s2 . The main origin of this contraction is the remarkable shrinkage that occurs along the hexagonal c-axis, which is parallel to the AlO4 -tetrahedral pillars. Because this transition is associated with tilting of the AlO4 -block along the c-axis, this negative thermal expansion (NTE) can be understood by a similar mechanism such as the Rigid Unit Mode (RUM) model. RUMs are the lowenergy phonon modes and are observed in several compounds with corner-sharing polyhedra [19, 20, 21]. Because of the cooperative rocking of the linked polyhedra, the cell volume can shrink as temperature increases [22]. The NTEs observed in the MW2 O8 (M = Zr, Hf) family and related materials are explained by this model [23, 24, 25]. BaAl2 O4 also possesses RUMs, which act as soft modes, namely, the M- and K-point soft modes, although BaAl2 O4 itself does not exhibit NTE during heating. Fig. 4 shows the electron diffraction (ED) pattern for the x = 0.7 sample obtained at 773 K. Strong diffuse scatterings are observed near the M- and Kpoints, as marked by triangles, indicating the presence of RUMs also in the Sr-rich compounds. That is, the 4

spectively. The crystal orientation rotates by 120◦ after heating, as shown in the figures. This behavior is very similar to the anti-phase boundaries (APBs) observed in BaAl2 O4 [12]. In the TEM observations by Abakumov et al., the shape and configuration of the APBs are mobile within a few tens of seconds. According to the ab initio calculations and distortion-mode analyses by Perez-Mato et al. [6], there are several latent instabilities other than the strongest primary distortion mode, an antiferrodistortive M2 mode with RUM nature, both in BaAl2 O4 and SrAl2 O4 . That is, there are other much weaker instabilities in both compounds despite the determinant distortion mode. The observed unstable twin walls and the mobile APBs in BaAl2 O4 [12] can be ascribed to these latent instabilities.

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4. Conclusions The structural phase transition and the domain structure of Ba1−x Sr x Al2 O4 (x ≥ 0.6) were investigated. Lattice parameters change discontinuously at T s2 , indicating a first-order phase transition. Small negative thermal expansion is also observed at T s2 , which can be explained by the tilting of the AlO4 -blocks associated with the RUMs of Ba1−x Sr x Al2 O4 . The boundary region near x = 0.6 exhibits the coexistence region of P21 in P63 22 matrix. In the P21 phase, the shapes of the twin domains change during heating due to the in-plane rotation of the monoclinic b-axis in the crystals. This singular behavior can be associated with the latent instabilities in this system. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) from Japan Society for the Promotion of Science (JSPS), and grants from The Murata Science Foundation. The synchrotron radiation experiments were performed at BL02B2 in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016A1349). References [1] Hk. M¨uller-Buschbaum, J. Alloy Compd. 349 (2003) 49–104. [2] C.-N. Xu, H. Yamada, X. Wang, and X.-G. Zheng, Appl. Phys. Lett. 84 (2004) 3040–3042. [3] Y. Liu and C.-N. Xu, Appl. Phys. Lett. 84 (2004) 5016–5018.

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Figure 6: HRTEM images obtained on the same crystallite with [001]p incidence. (a) x = 0.8 at 293 K and (b) at 373 K. (c) x = 0.9 at 293 K and (d) at 693 K. The insets show the fast-Fourier transform patterns of each domain. White arrows indicate the monoclinic b-axis.

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