research communications
3 reinvestigated: new insights into atomic thermal vibration behaviors
of post-perovskite-type CaIrOaGraduate School of Science and Engineering, Yamaguchi University, Ube 755-8611, Japan, bInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan, cInstitute for Study of the Earth's Interior, Okayama University, Misasa 682-0193, Japan, and dGraduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan
*Correspondence e-mail: tuka@yamaguchi-u.ac.jp
Single crystals of the title compound, the post-perovskite-type CaIrO3 [calcium iridium(IV) trioxide], have been grown from a CaCl2 at atmospheric pressure. The consists of an alternate stacking of IrO6 octahedral layers and CaO8 hendecahedral layers along [010]. Chains formed by edge-sharing of IrO6 octahedra (point-group symmetry 2/m..) run along [100] and are interconnected along [001] by sharing apical O atoms to build up the IrO6 octahedral layers. Chains formed by face-sharing of CaO8 hendecahedra (point-group symmetry m2m) run along [100] and are interconnected along [001] by edge-sharing to build up the CaO8 hendecahedral layers. The IrO6 octahedral layers and CaO8 hendecahedral layers are interconnected by sharing edges. The present structure using a high-power X-ray source confirms the atomic positions determined by Hirai et al. (2009) [Z. Kristallogr. 224, 345–350], who had revised our previous report [Sugahara et al. (2008). Am. Mineral. 93, 1148–1152]. However, the displacement ellipsoids of the Ir and Ca atoms based on the present can be approximated as uniaxial ellipsoids elongating along [100], unlike those reported by Hirai et al. (2009). This suggests that the thermal vibrations of the Ir and Ca atoms are mutually suppressed towards the Ir⋯Ca direction across the shared edge because of the dominant repulsion between the two atoms.
Keywords: crystal structure; redetermination; calcium iridium(IV) trioxide; post-perovskite; thermal vibration.
CCDC reference: 1419830
1. Chemical context
The orthorhombic perovskite-type MgSiO3, the dominant constituent in the Earth's lower mantle, is now believed to undergo the to the so-called `post-perovskite-type structure', associated with the D′′ seismic discontinuity, at 125 GPa and 2500 K (Murakami et al., 2004; Tsuchiya et al., 2004; Oganov & Ono, 2004; Iitaka et al., 2004; Mao et al., 2004; Ono & Oganov, 2005; Wentzcovitch et al., 2006; Shieh et al., 2006). Since the discovery of the post-perovskite-type MgSiO3, several orthorhombic A2+B4+O3 perovskite-type compounds have been found to transform into the post-perovskite-type structure under high pressure and high temperature (Kojitani et al., 2007; Yamaura et al., 2009; Tateno et al., 2010). Meanwhile, CaIrO3 is known to be one of the few post-perovskite-type compounds stable at ambient conditions (Rodi & Babel, 1965; McDaniel & Schneider, 1972). The post-perovskite-type CaIrO3 has attracted much attention in the field of Earth science as an excellent low-pressure analogue of the post-perovskite-type MgSiO3 (see, for example, Niwa et al., 2007; Tsuchiya & Tsuchiya, 2007; Yoneda et al., 2014).
The 3 was first proposed by Rodi & Babel (1965) on the basis of a single-crystal X-ray diffraction experiment, but incorrect atomic positions were reported. Recently, we have successfully grown single crystals of the post-perovskite-type CaIrO3 and refined the of this compound on the basis of single-crystal X-ray diffraction data measured using a sealed X-ray tube (40 kV, 30 mA) as the radiation source (Sugahara et al., 2008). However, the measured intensity data were rather weak and their accuracy was rather low, because thin needle-like crystals were obtained and the selected crystal for the intensity measurements had a poor grade of crystallinity. This resulted in rather large reliability indices [R(F) = 0.064, wR(F) = 0.065 for 377 reflections] and in structural parameters with rather large uncertainties. In particular, the resulting displacement ellipsoids were unusually elongated or flattened. Subsequently, Hirai et al. (2009) reinvestigated the of the post-perovskite-type CaIrO3 by single-crystal X-ray diffraction and conducted structure refinements for two different crystals using two different types of diffractometers. The two independent refinements showed convergent results with much better reliability indices [R(F2) = 0.013, wR(F2) = 0.031 for 365 reflections; R(F2) = 0.007, wR(F2) = 0.008 for 149 reflections] and structural parameters with reasonably smaller uncertainties. Consequently, Hirai et al. (2009) concluded that the displacement ellipsoids had no significant anisotropies in contradiction to our previous report (Sugahara et al., 2008), but provided no further details of the atomic thermal vibration behaviors. Their X-ray diffraction experiments were conducted under the operating conditions of 2θmax = 80° at 45 kV/40 mA for one crystal and 2θmax = 55° at 50 kV/85 mA for the other crystal. These operating conditions with a low X-ray power and a relatively low 2θmax value may be insufficient for the determination of reliable atomic displacement parameters (ADPs).
of the post-perovskite-type CaIrOIn the present study, the 3 was reinvestigated on the basis of single-crystal X-ray diffraction data measured over a much wider 2θ range using a high-power rotating-anode X-ray generator (60 kV, 250 mA). Special attention to exclude the influence of multiple scattering effects and effects on ADPs was paid as far as possible during data reduction and structure procedures, as will be described in Section 5. On the basis of the resulting structural parameters, the validity of the proposed by Hirai et al. (2009) is examined and the detailed atomic thermal vibration behaviors are discussed.
of the post-perovskite-type CaIrO2. Structural commentary
The post-perovskite-type phase of CaIrO3 crystallizes in the Cmcm. The consists of IrO6 octahedral layers and CaO8 hendecahedral layers stacked alternately along [010] (Fig. 1). The Ca and Ir atoms occupy Wyckoff positions 4c and 4a, respectively. The O atoms occupy two non-equivalent sites: O1 at 4c and O2 at 8f. The site symmetries are m2m for Ca, 2/m.. for Ir, m2m for O1 and m.. for O2. Ca—O and Ir—O bond lengths are listed in Table 1. In the IrO6 octahedral layers (Fig. 2), chains of IrO6 octahedra along [100] are formed by sharing O2⋯O2 edges, and these chains are interconnected along [001] by sharing the apical O1 atoms. In the CaO8 hendecahedral layers (Fig. 3), chains of CaO8 hendecahedra along [100] are formed by sharing O2⋯O1⋯O2 faces, and these chains are interconnected along [001] by sharing O2⋯O2 edges. The alternate stacking of IrO6 octahedral layers and CaO8 hendecahedral layers along [010] results from sharing O1⋯O2 and O2⋯O2 edges between both layers. Further details of the general description of the are provided in our previous paper (Sugahara et al., 2008).
3. Atomic thermal vibration behaviors
In the present structure et al., 2008), being comparable to those reported by Hirai et al. (2009). The resulting positional parameters also show excellent consistency with those reported by Hirai et al. (2009). On the other hand, the present displacement ellipsoids (Fig. 4) are different from those given by Hirai et al. (2009). They considered that the thermal vibrations of the Ir and Ca atoms exhibited no significant anisotropies, but in fact the reported displacement ellipsoids of both atoms were somewhat elongated parallel to the (100) plane. In contrast, the mean-square displacements (MSDs) of both atoms obtained from the present are as follows: Ir, 0.00316 (5) Å2 along the shortest ellipsoid axis, 0.00319 (5) Å2 along the intermediate one and 0.00387 (6) Å2 along the longest one; Ca, 0.0055 (3) Å2 along the shortest ellipsoid axis, 0.0058 (3) Å2 along the intermediate one and 0.0065 (3) Å2 along the longest one. Here, in both atoms, the longest ellipsoid axes are just in the [100] direction and the intermediate and the shortest ones are within the (100) plane. The present results indicate that the MSDs of both atoms are significantly the largest in the [100] direction, in contradiction to the report of Hirai et al. (2009), although the thermal vibrations of both atoms only exhibit small anisotropies.
using a high-power X-ray source, the accuracy of the refined structural parameters was considerably improved compared with our previous report (SugaharaAs understood from the MSDs shown above, the displacement ellipsoid of the Ir atom is very close to a uniaxial ellipsoid elongating along [100]. The Ir⋯Ca direction across the O2⋯O2 shared edge between the IrO6 octahedron and CaO8 hendecahedron is parallel to the (100) plane; hence, this direction can be considered as the direction of nearly the smallest MSD of the Ir atom although it deviates by 10.1° from the direction of the shortest ellipsoid axis. The ellipsoid axes of the Ca atom are in the [100], [010] and [001] directions by requirements of its but its displacement ellipsoid can also be approximated as a uniaxial ellipsoid elongating along [100]. The Ir⋯Ca direction across the O2⋯O2 shared edge can thus be characterized also as the direction of nearly the smallest MSD of the Ca atom. These suggest that the dominant Ir⋯Ca repulsion across the O2⋯O2 shared edge suppresses the mutual thermal vibrations of both atoms towards the Ir⋯Ca direction. Indeed, the Ir⋯Ca distance [= 3.0678 (9) Å] is the shortest of the cation–cation distances [cf. 3.1466 (5) Å for the Ir⋯Ir distance across the O2⋯O2 shared edge between IrO6 octahedra; 3.1466 (5) Å for the Ca⋯Ca distance across the O2⋯O1⋯O2 shared face between CaO8 hendecahedra; 3.4501 (8) Å for the Ir⋯Ca distance across the O1⋯O2 shared edge between the IrO6 octahedron and CaO8 hendecahedron; 3.9755 (3) Å for the Ca⋯Ca distance across the O2⋯O2 shared edge between CaO8 hendecahedra].
4. Synthesis and crystallization
The reagents Ca(OH)2 and Ir were employed as the starting materials, and mixed together with CaCl2 in the molar ratio Ca(OH)2:Ir:CaCl2 = 1:1:10. The mixture was heated in air at 1100 K for 8 h, and then cooled gradually to 600 K at a rate of 10 K h−1. Dark reddish-brown crystals of the post-perovskite-type CaIrO3 with a thin needle shape were grown from the CaCl2 The crystals were isolated by dissolving the solidified CaCl2 melt with distilled water.
5. Refinement
A total of 2593 intensity data up to 2θmax = 100° were collected. After the intensity data were corrected for Lorentz-polarization factors and absorption effects (ψ-scan method; North et al., 1968), they were averaged in Laue symmetry mmm to give 692 independent reflections. Of these, independent reflections with Fo ≤ 3σ(Fo) were eliminated. Even if independent reflections had intensities of Fo > 3σ(Fo) after averaging, those averaged from a data set of equivalent reflections including reflection(s) with Fo ≤ 3σ(Fo) were also discarded since these reflections were potentially affected by multiple scattering. Moreover, independent reflections with (sin θ)/λ < 0.334 Å−1 were eliminated to reduce effects and to avoid dependence on atomic charge as far as possible in the choice of atomic scattering factors. Finally, 412 independent reflections were used in the present Several correction models for the effects were attempted during the and the isotropic correction of Type II (Becker & Coppens, 1974a,b) with a Gaussian particle-size-distribution model yielded the best fit. The reliability indices converged to R(F) = 0.0108 and wR(F) = 0.0104 for 412 reflections, comparable to those of Hirai et al. (2009), and were dramatically improved in comparison with those of our previous report (Sugahara et al., 2008). Crystal data, data collection and structure details are summarized in Table 2.
Supporting information
CCDC reference: 1419830
https://doi.org/10.1107/S2056989015015649/wm5181sup1.cif
contains datablocks General, I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989015015649/wm5181Isup2.hkl
Data collection: WinAFC (Rigaku, 1999); cell
WinAFC (Rigaku, 1999); data reduction: RADY (Sasaki, 1987); program(s) used to solve structure: coordinates taken from a previous program(s) used to refine structure: RADY (Sasaki, 1987); molecular graphics: ATOMS for Windows (Dowty, 2000); software used to prepare material for publication: publCIF (Westrip, 2010).CaIrO3 | F(000) = 484 |
Mr = 280.30 | Dx = 8.214 Mg m−3 |
Orthorhombic, Cmcm | Mo Kα radiation, λ = 0.71069 Å |
Hall symbol: -C 2c 2 | Cell parameters from 25 reflections |
a = 3.1466 (5) Å | θ = 22.5–25.0° |
b = 9.8690 (16) Å | µ = 61.02 mm−1 |
c = 7.3019 (5) Å | T = 298 K |
V = 226.75 (6) Å3 | Needle, dark reddish-brown |
Z = 4 | 0.20 × 0.01 × 0.01 mm |
Rigaku AFC7R diffractometer | Rint = 0.019 |
ω–2θ scans | θmax = 50.0° |
Absorption correction: ψ scan (North et al., 1968) | h = 0→6 |
Tmin = 0.486, Tmax = 0.543 | k = −21→21 |
2593 measured reflections | l = −15→15 |
692 independent reflections | 3 standard reflections every 150 reflections |
438 reflections with F > 3.0σ(F) | intensity decay: none |
Refinement on F | Weighting scheme based on measured s.u.'s w = 1/σ2(F) |
R[F2 > 2σ(F2)] = 0.019 | (Δ/σ)max = 0.0003 |
wR(F2) = 0.021 | Δρmax = 1.21 e Å−3 |
S = 1.56 | Δρmin = −1.89 e Å−3 |
412 reflections | Extinction correction: isotropic Type II (Becker & Coppens, 1974a,b) |
20 parameters | Extinction coefficient: 1.50E4 (5) |
x | y | z | Uiso*/Ueq | ||
Ca | 0.0000 | 0.7502 (1) | 0.2500 | 0.0060 (2) | |
Ir | 0.0000 | 0.0000 | 0.0000 | 0.00340 (5) | |
O1 | 0.0000 | 0.0756 (4) | 0.2500 | 0.0065 (11) | |
O2 | 0.0000 | 0.3724 (3) | 0.4495 (3) | 0.0059 (7) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Ca | 0.0065 (3) | 0.0055 (3) | 0.0058 (3) | 0.0000 | 0.0000 | 0.0000 |
Ir | 0.00387 (6) | 0.00317 (5) | 0.00317 (5) | 0.0000 | 0.0000 | 0.00001 (10) |
O1 | 0.0084 (13) | 0.0067 (12) | 0.0045 (10) | 0.0000 | 0.0000 | 0.0000 |
O2 | 0.0064 (9) | 0.0054 (8) | 0.0058 (7) | 0.0000 | 0.0000 | −0.0007 (6) |
Ca—O1i | 2.333 (3) | Ir—Irxxv | 3.6510 (3) |
Ca—O1ii | 2.333 (3) | O1—Ir | 1.9722 (15) |
Ca—O2i | 2.460 (2) | O1—Irxxv | 1.9722 (15) |
Ca—O2ii | 2.460 (2) | O1—Caxxiii | 2.333 (3) |
Ca—O2iii | 2.460 (2) | O1—Caxxiv | 2.333 (3) |
Ca—O2iv | 2.460 (2) | O1—O1xii | 3.1466 (5) |
Ca—O2v | 2.506 (3) | O1—O1xiii | 3.1466 (5) |
Ca—O2vi | 2.506 (3) | O1—O1vii | 3.944 (3) |
Ir—O1 | 1.9722 (15) | O1—O1xxv | 3.944 (3) |
Ir—O1vii | 1.9722 (15) | O1—O2viii | 2.748 (2) |
Ir—O2viii | 2.0488 (18) | O1—O2ix | 2.748 (2) |
Ir—O2ix | 2.0488 (18) | O1—O2xxvi | 2.748 (2) |
Ir—O2x | 2.0488 (18) | O1—O2xxvii | 2.748 (2) |
Ir—O2xi | 2.0488 (18) | O1—O2xxiii | 2.936 (3) |
Ca—Caxii | 3.1466 (5) | O1—O2xxiv | 2.936 (3) |
Ca—Caxiii | 3.1466 (5) | O1—O2x | 2.936 (3) |
Ca—Caxiv | 3.9755 (3) | O1—O2xi | 2.936 (3) |
Ca—Caxv | 3.9755 (3) | O1—O2xxviii | 3.271 (4) |
Ca—Caxvi | 3.9755 (3) | O1—O2 | 3.271 (4) |
Ca—Caxvii | 3.9755 (3) | O2—Irxx | 2.0488 (18) |
Ca—Irxviii | 3.0678 (9) | O2—Irxxi | 2.0488 (18) |
Ca—Irxix | 3.0678 (9) | O2—Caxxiii | 2.460 (2) |
Ca—Iri | 3.4501 (8) | O2—Caxxiv | 2.460 (2) |
Ca—Irii | 3.4501 (8) | O2—Caxix | 2.506 (3) |
Ca—Irxx | 3.4501 (8) | O2—O1xx | 2.748 (2) |
Ca—Irxxi | 3.4501 (8) | O2—O1xxi | 2.748 (2) |
Ir—Caxxii | 3.0678 (9) | O2—O1i | 2.936 (3) |
Ir—Cavi | 3.0678 (9) | O2—O1ii | 2.936 (3) |
Ir—Caxxiii | 3.4501 (8) | O2—O1 | 3.271 (4) |
Ir—Caxxiv | 3.4501 (8) | O2—O2v | 2.625 (5) |
Ir—Caviii | 3.4501 (8) | O2—O2xxviii | 2.913 (5) |
Ir—Caix | 3.4501 (8) | O2—O2xxvi | 2.976 (5) |
Ir—Irxii | 3.1466 (5) | O2—O2xxvii | 2.976 (5) |
Ir—Irxiii | 3.1466 (5) | O2—O2xii | 3.1466 (5) |
Ir—Irvii | 3.6510 (3) | O2—O2xiii | 3.1466 (5) |
O1i—Ca—O1ii | 84.82 (13) | O2vi—Ca—O2iii | 73.63 (9) |
O1i—Ca—O2i | 86.02 (7) | O2vi—Ca—O2iv | 73.63 (9) |
O1i—Ca—O2ii | 142.50 (6) | O2vi—Ca—O2v | 122.28 (13) |
O1i—Ca—O2vi | 69.12 (5) | O2iii—Ca—O2iv | 79.52 (8) |
O1i—Ca—O2iii | 86.02 (7) | O2iii—Ca—O2v | 139.04 (5) |
O1i—Ca—O2iv | 142.50 (6) | O2iv—Ca—O2v | 139.04 (5) |
O1i—Ca—O2v | 69.12 (5) | O1—Ir—O1vii | 180.00 |
O1ii—Ca—O2i | 142.50 (6) | O1—Ir—O2viii | 86.22 (10) |
O1ii—Ca—O2ii | 86.02 (7) | O1—Ir—O2ix | 86.22 (10) |
O1ii—Ca—O2vi | 69.12 (5) | O1—Ir—O2x | 93.78 (10) |
O1ii—Ca—O2iii | 142.50 (6) | O1—Ir—O2xi | 93.78 (10) |
O1ii—Ca—O2iv | 86.02 (7) | O1vii—Ir—O2viii | 93.78 (10) |
O1ii—Ca—O2v | 69.12 (5) | O1vii—Ir—O2ix | 93.78 (10) |
O2i—Ca—O2ii | 79.52 (8) | O1vii—Ir—O2x | 86.22 (10) |
O2i—Ca—O2vi | 139.04 (5) | O1vii—Ir—O2xi | 86.22 (10) |
O2i—Ca—O2iii | 72.61 (10) | O2viii—Ir—O2ix | 100.33 (12) |
O2i—Ca—O2iv | 121.28 (13) | O2viii—Ir—O2x | 79.67 (12) |
O2i—Ca—O2v | 73.63 (9) | O2viii—Ir—O2xi | 179.97 |
O2ii—Ca—O2vi | 139.04 (5) | O2ix—Ir—O2x | 179.97 |
O2ii—Ca—O2iii | 121.28 (13) | O2ix—Ir—O2xi | 79.67 (12) |
O2ii—Ca—O2iv | 72.61 (10) | O2x—Ir—O2xi | 100.33 (12) |
O2ii—Ca—O2v | 73.63 (9) |
Symmetry codes: (i) x−1/2, y+1/2, z; (ii) x+1/2, y+1/2, z; (iii) −x−1/2, y+1/2, −z+1/2; (iv) −x+1/2, y+1/2, −z+1/2; (v) x, −y+1, −z+1; (vi) −x, −y+1, z−1/2; (vii) −x, −y, z−1/2; (viii) −x−1/2, −y+1/2, z−1/2; (ix) −x+1/2, −y+1/2, z−1/2; (x) −x−1/2, y−1/2, −z+1/2; (xi) −x+1/2, y−1/2, −z+1/2; (xii) x−1, y, z; (xiii) x+1, y, z; (xiv) −x−1/2, −y+3/2, z−1/2; (xv) −x−1/2, −y+3/2, z+1/2; (xvi) −x+1/2, −y+3/2, z−1/2; (xvii) −x+1/2, −y+3/2, z+1/2; (xviii) x, y+1, z; (xix) −x, −y+1, z+1/2; (xx) −x−1/2, −y+1/2, z+1/2; (xxi) −x+1/2, −y+1/2, z+1/2; (xxii) x, y−1, z; (xxiii) x−1/2, y−1/2, z; (xxiv) x+1/2, y−1/2, z; (xxv) −x, −y, z+1/2; (xxvi) x−1/2, −y+1/2, −z+1; (xxvii) x+1/2, −y+1/2, −z+1; (xxviii) −x, y, −z+1/2. |
Acknowledgements
This work was partly performed under the research project No. 2010G608 at the Photon Factory BL-10A, High Energy Accelerator Research Organization, Tsukuba, Japan. We thank F. Yachi and K. Fujii of Yamaguchi University for their experimental assistance.
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