Crystal structure of post-perovskite-type CaIrO3 reinvestigated: new insights into atomic thermal vibration behaviors

Nakatsuka, A.; Sugiyama, K.; Yoneda, A.; Fujiwara, K.; Yoshiasa, A.

doi:10.1107/S2056989015015649

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Volume 71| Part 9| September 2015| Pages 1109-1113

https://doi.org/10.1107/S2056989015015649

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Crystal structure of post-perovskite-type CaIrO₃ reinvestigated: new insights into atomic thermal vibration behaviors

Akihiko Nakatsuka,^a ^* Kazumasa Sugiyama,^b Akira Yoneda,^c Keiko Fujiwara ^a and Akira Yoshiasa ^d

^aGraduate 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

Edited by M. Weil, Vienna University of Technology, Austria (Received 30 June 2015; accepted 20 August 2015; online 29 August 2015)

Single crystals of the title compound, the post-perovskite-type CaIrO₃ [calcium iridium(IV) trioxide], have been grown from a CaCl₂ flux at atmospheric pressure. The crystal structure 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 refinement 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 refinement 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 phase transition 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 crystal structure of the post-perovskite-type CaIrO₃ 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 crystal structure 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 crystal structure 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 2θ_max = 80° at 45 kV/40 mA for one crystal and 2θ_max = 55° at 50 kV/85 mA for the other crystal. These operating conditions with a low X-ray power and a relatively low 2θ_max value may be insufficient for the determination of reliable atomic displacement parameters (ADPs).

In the present study, the crystal structure of the post-perovskite-type CaIrO₃ 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 secondary extinction effects on ADPs was paid as far as possible during data reduction and structure refinement procedures, as will be described in Section 5. On the basis of the resulting structural parameters, the validity of the crystal structure proposed by Hirai et al. (2009) is examined and the detailed atomic thermal vibration behaviors are discussed.

2. Structural commentary

The post-perovskite-type phase of CaIrO₃ crystallizes in the space group Cmcm. The crystal structure 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 Wyckoff position 4c and O2 at Wyckoff position 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 crystal structure are provided in our previous paper (Sugahara et al., 2008).

Table 1
Selected bond lengths (Å)

Ca—O1ⁱ	2.333 (3)	Ir—O1	1.9722 (15)
Ca—O2ⁱ	2.460 (2)	Ir—O2ⁱⁱⁱ	2.0488 (18)
Ca—O2ⁱⁱ	2.506 (3)
Symmetry codes: (i) $[x-{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ ; (ii) x, -y+1, -z+1; (iii) $[-x-{\script{1\over 2}}, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Figure 1
Polyhedral representation of the CaIrO₃ post-perovskite-type structure, composed of the alternate stacking of IrO₆ octahedral layers and CaO₈ hendecahedral layers along [010].

Figure 2
Polyhedral view of an IrO₆ octahedral layer projected on (010). Symmetry codes: (i) x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , z; (ii) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z; (iii) x + 1, −y + 1, z − $[{1\over 2}]$ ; (iv) x + 1, y, −z + $[{1\over 2}]$ ; (v) x, −y + 1, z − $[{1\over 2}]$ ; (vi) x, y, −z + $[{1\over 2}]$ .

Figure 3
Polyhedral view of a CaO₈ hendecahedral layer projected on (010). Symmetry codes: (i) x, −y + 1, −z + 1; (ii) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (iii) x − $[{1\over 2}]$ , −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (iv) x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z + 1; (v) x, y, −z + $[{3\over 2}]$ ; (vi) x − $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z + 1.

3. Atomic thermal vibration behaviors

In the present structure refinement using a high-power X-ray source, the accuracy of the refined structural parameters was considerably improved compared with our previous report (Sugahara 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 refinement 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.

Figure 4
Unit cell of the CaIrO₃ post-perovskite with displacement ellipsoids drawn at the 80% probability level.

As 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 site symmetry, 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 flux. The crystals were isolated by dissolving the solidified CaCl₂ melt with distilled water.

5. Refinement

A total of 2593 intensity data up to 2θ_max = 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 F_o ≤ 3σ(F_o) were eliminated. Even if independent reflections had intensities of F_o > 3σ(F_o) after averaging, those averaged from a data set of equivalent reflections including reflection(s) with F_o ≤ 3σ(F_o) 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 secondary extinction 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 refinement. Several correction models for the secondary extinction effects were attempted during the refinement, 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 refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	CaIrO₃
M_r	280.30
Crystal system, space group	Orthorhombic, Cmcm
Temperature (K)	298
a, b, c (Å)	3.1466 (5), 9.8690 (16), 7.3019 (5)
V (Å³)	226.75 (6)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	61.02
Crystal size (mm)	0.20 × 0.01 × 0.01

Data collection
Diffractometer	Rigaku AFC7R
Absorption correction	ψ scan (North et al., 1968)
T_min, T_max	0.486, 0.543
No. of measured, independent and observed [F > 3.0σ(F)] reflections	2593, 692, 438
R_int	0.019
(sin θ/λ)_max (Å⁻¹)	1.078

Refinement
R[F > 3σ(F)], wR(F), S	0.011, 0.010, 1.56
No. of reflections	412
No. of parameters	20
Δρ_max, Δρ_min (e Å⁻³)	1.21, −1.89
Computer programs: WinAFC (Rigaku, 1999 ), RADY (Sasaki, 1987 ), ATOMS for Windows (Dowty, 2000 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1419830

Crystal structure: contains datablocks General, I. DOI: https://doi.org/10.1107/S2056989015015649/wm5181sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989015015649/wm5181Isup2.hkl

checkCIF report

Computing details top

Data collection: WinAFC (Rigaku, 1999); cell refinement: WinAFC (Rigaku, 1999); data reduction: RADY (Sasaki, 1987); program(s) used to solve structure: coordinates taken from a previous refinement; 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).

Calcium iridium(IV) trioxide top

Crystal data top

CaIrO₃	F(000) = 484
M_r = 280.30	D_x = 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

Data collection top

Rigaku AFC7R diffractometer	R_int = 0.019
ω–2θ scans	θ_max = 50.0°
Absorption correction: ψ scan (North et al., 1968)	h = 0→6
T_min = 0.486, T_max = 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 top

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	Δρ_max = 1.21 e Å⁻³
S = 1.56	Δρ_min = −1.89 e Å⁻³
412 reflections	Extinction correction: isotropic Type II (Becker & Coppens, 1974a,b)
20 parameters	Extinction coefficient: 1.50E4 (5)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
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)

Atomic displacement parameters (Å²) top

	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)

Geometric parameters (Å, º) top

Ca—O1ⁱ	2.333 (3)	Ir—Ir^xxv	3.6510 (3)
Ca—O1ⁱⁱ	2.333 (3)	O1—Ir	1.9722 (15)
Ca—O2ⁱ	2.460 (2)	O1—Ir^xxv	1.9722 (15)
Ca—O2ⁱⁱ	2.460 (2)	O1—Ca^xxiii	2.333 (3)
Ca—O2ⁱⁱⁱ	2.460 (2)	O1—Ca^xxiv	2.333 (3)
Ca—O2^iv	2.460 (2)	O1—O1^xii	3.1466 (5)
Ca—O2^v	2.506 (3)	O1—O1^xiii	3.1466 (5)
Ca—O2^vi	2.506 (3)	O1—O1^vii	3.944 (3)
Ir—O1	1.9722 (15)	O1—O1^xxv	3.944 (3)
Ir—O1^vii	1.9722 (15)	O1—O2^viii	2.748 (2)
Ir—O2^viii	2.0488 (18)	O1—O2^ix	2.748 (2)
Ir—O2^ix	2.0488 (18)	O1—O2^xxvi	2.748 (2)
Ir—O2^x	2.0488 (18)	O1—O2^xxvii	2.748 (2)
Ir—O2^xi	2.0488 (18)	O1—O2^xxiii	2.936 (3)
Ca—Ca^xii	3.1466 (5)	O1—O2^xxiv	2.936 (3)
Ca—Ca^xiii	3.1466 (5)	O1—O2^x	2.936 (3)
Ca—Ca^xiv	3.9755 (3)	O1—O2^xi	2.936 (3)
Ca—Ca^xv	3.9755 (3)	O1—O2^xxviii	3.271 (4)
Ca—Ca^xvi	3.9755 (3)	O1—O2	3.271 (4)
Ca—Ca^xvii	3.9755 (3)	O2—Ir^xx	2.0488 (18)
Ca—Ir^xviii	3.0678 (9)	O2—Ir^xxi	2.0488 (18)
Ca—Ir^xix	3.0678 (9)	O2—Ca^xxiii	2.460 (2)
Ca—Irⁱ	3.4501 (8)	O2—Ca^xxiv	2.460 (2)
Ca—Irⁱⁱ	3.4501 (8)	O2—Ca^xix	2.506 (3)
Ca—Ir^xx	3.4501 (8)	O2—O1^xx	2.748 (2)
Ca—Ir^xxi	3.4501 (8)	O2—O1^xxi	2.748 (2)
Ir—Ca^xxii	3.0678 (9)	O2—O1ⁱ	2.936 (3)
Ir—Ca^vi	3.0678 (9)	O2—O1ⁱⁱ	2.936 (3)
Ir—Ca^xxiii	3.4501 (8)	O2—O1	3.271 (4)
Ir—Ca^xxiv	3.4501 (8)	O2—O2^v	2.625 (5)
Ir—Ca^viii	3.4501 (8)	O2—O2^xxviii	2.913 (5)
Ir—Ca^ix	3.4501 (8)	O2—O2^xxvi	2.976 (5)
Ir—Ir^xii	3.1466 (5)	O2—O2^xxvii	2.976 (5)
Ir—Ir^xiii	3.1466 (5)	O2—O2^xii	3.1466 (5)
Ir—Ir^vii	3.6510 (3)	O2—O2^xiii	3.1466 (5)

O1ⁱ—Ca—O1ⁱⁱ	84.82 (13)	O2^vi—Ca—O2ⁱⁱⁱ	73.63 (9)
O1ⁱ—Ca—O2ⁱ	86.02 (7)	O2^vi—Ca—O2^iv	73.63 (9)
O1ⁱ—Ca—O2ⁱⁱ	142.50 (6)	O2^vi—Ca—O2^v	122.28 (13)
O1ⁱ—Ca—O2^vi	69.12 (5)	O2ⁱⁱⁱ—Ca—O2^iv	79.52 (8)
O1ⁱ—Ca—O2ⁱⁱⁱ	86.02 (7)	O2ⁱⁱⁱ—Ca—O2^v	139.04 (5)
O1ⁱ—Ca—O2^iv	142.50 (6)	O2^iv—Ca—O2^v	139.04 (5)
O1ⁱ—Ca—O2^v	69.12 (5)	O1—Ir—O1^vii	180.00
O1ⁱⁱ—Ca—O2ⁱ	142.50 (6)	O1—Ir—O2^viii	86.22 (10)
O1ⁱⁱ—Ca—O2ⁱⁱ	86.02 (7)	O1—Ir—O2^ix	86.22 (10)
O1ⁱⁱ—Ca—O2^vi	69.12 (5)	O1—Ir—O2^x	93.78 (10)
O1ⁱⁱ—Ca—O2ⁱⁱⁱ	142.50 (6)	O1—Ir—O2^xi	93.78 (10)
O1ⁱⁱ—Ca—O2^iv	86.02 (7)	O1^vii—Ir—O2^viii	93.78 (10)
O1ⁱⁱ—Ca—O2^v	69.12 (5)	O1^vii—Ir—O2^ix	93.78 (10)
O2ⁱ—Ca—O2ⁱⁱ	79.52 (8)	O1^vii—Ir—O2^x	86.22 (10)
O2ⁱ—Ca—O2^vi	139.04 (5)	O1^vii—Ir—O2^xi	86.22 (10)
O2ⁱ—Ca—O2ⁱⁱⁱ	72.61 (10)	O2^viii—Ir—O2^ix	100.33 (12)
O2ⁱ—Ca—O2^iv	121.28 (13)	O2^viii—Ir—O2^x	79.67 (12)
O2ⁱ—Ca—O2^v	73.63 (9)	O2^viii—Ir—O2^xi	179.97
O2ⁱⁱ—Ca—O2^vi	139.04 (5)	O2^ix—Ir—O2^x	179.97
O2ⁱⁱ—Ca—O2ⁱⁱⁱ	121.28 (13)	O2^ix—Ir—O2^xi	79.67 (12)
O2ⁱⁱ—Ca—O2^iv	72.61 (10)	O2^x—Ir—O2^xi	100.33 (12)
O2ⁱⁱ—Ca—O2^v	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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