Post-perovskite phase transition in CaRuO3

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Physics of the Earth and Planetary Interiors 165 (2007) 127–134

Letter to the Editor Abstract High-pressure phase relations in CaRuO3 were examined at 13–27 GPa and 900–1200 ◦ C using a multi-anvil apparatus. At about 21–25 GPa at 900–1200 ◦ C, CaRuO3 with orthorhombic perovskite structure transformed to CaIrO3 -type post-perovskite structure. The phase boundary of the post-perovskite transition in CaRuO3 was determined as P (GPa) = 0.010 × T (◦ C) + 12.4. The post-perovskite phase of CaRuO3 was quenchable to ambient conditions like that of CaIrO3 . Rietveld refinement confirmed that CaRuO3 post-perovskite and perovskite have the structures of CaIrO3 -type post-perovskite (space group Cmcm) and GdFeO3 -type orthorhombic perovskite (Pbnm), respectively. Lattice parameters and unit cell volume of CaRuO3 post-perovskite were determined ˚ b = 9.8268(1) A, ˚ c = 7.2963(1) A ˚ and V = 223.34(1) A ˚ 3 , and those of CaRuO3 perovskite a = 5.3635(2) A, ˚ to be a = 3.1150(1) A, 3 ˚ ˚ ˚ b = 5.5261(2) A, c = 7.6668(2) A and V = 227.24(1) A . The structural features of CaRuO3 post-perovskite and perovskite are similar to those of the polymorphs of CaIrO3 and MgSiO3 . The post-perovskite transition in CaRuO3 is consistent with the general tendency that orthorhombic perovskites with relatively large tilting of the octahedral framework transform to post-perovskite structure at high pressure. CaRuO3 would be a low-pressure, quenchable analogue material suitable for investigation on the post-perovskite phase transition of MgSiO3 . © 2007 Elsevier B.V. All rights reserved. Keywords: CaRuO3 ; Post-perovskite; Perovskite; High-pressure; Rietveld analysis; Lower mantle

Post-perovskite phase transition in CaRuO3 1. Introduction Recent studies on the post-perovskite phase transition in MgSiO3 have significantly improved our understanding of the lowermost part of the Earth’s mantle called the D layer (Murakami et al., 2004; Oganov and Ono, 2004; Tsuchiya et al., 2004). MgSiO3 with GdFeO3 -type orthorhombic perovskite structure (space group Pbnm) transforms to the post-perovskite structure (space group Cmcm) at about 120 GPa and 2100 ◦ C (Hirose et al., 2006). The post-perovskite structure of MgSiO3 is identical to that of a CaIrO3 phase stable at 1 atm below about 1100 ◦ C (McDaniel and Schneider, 1972). Investigation on structure and physical properties of MgSiO3 -rich post-perovskite phase is crucial to clarify the nature and dynamics of the D layer of the mantle. The post-perovskite transition in MgSiO3 necessitates a very high pressure over 100 GPa. Furthermore, the MgSiO3 post-perovskite phase is unquenchable to ambient pressure. These facts make it rather difficult to investigate in detail structural features and physical properties such as elasticity, rheology and thermodynam0031-9201/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2007.09.003

ics of MgSiO3 post-perovskite. For these investigations, analogue materials such as germanates are particularly useful, because they transform at lower pressures than silicate. In addition to MgSiO3 , the transition from perovskite to post-perovskite has been found in MgGeO3 , MnGeO3 , CaIrO3 and NaMgF3 . These analogue materials exhibit much lower transition pressures than that in MgSiO3 , i.e. ∼60 GPa for both of MgGeO3 (Hirose et al., 2005) and MnGeO3 (Tateno et al., 2006), ∼20 GPa for NaMgF3 (Liu et al., 2005), and ∼2 GPa for CaIrO3 (Hirose and Fujita, 2005; Kojitani et al., 2007a). However, the post-perovskite phases of MgGeO3 , MnGeO3 and NaMgF3 are unquenchable at ambient conditions. Therefore, in ABX3 (X = O, F) compounds studied so far, CaIrO3 is currently the only known material in which the perovskite phase transforms directly to the postperovskite phase and both of the two phases can be quenched to ambient conditions. To investigate the post-perovskite phase transition in more detail, particularly in the views of crystal chemistry, elasticity, thermochemistry and rheology, further analogue materials will be needed in which the postperovskite transition occurs at relatively low pressure and the post-perovskite phase is quenchable at ambi-

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Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134

ent conditions. Because the crystal structure of CaRuO3 is the GdFeO3 -type orthorhombic perovskite at ambient pressure (Bensh et al., 1990) and also an ionic ˚ is very close to that of VI Ir4+ radius of VI Ru4+ (0.62 A) ˚ (Shannon and Prewitt, 1969), it is worth exam(0.63 A) ining whether or not CaRuO3 perovskite transforms to post-perovskite at high pressure. In this study, we have examined high-pressure high-temperature transition of CaRuO3 . Our study has indicated that CaRuO3 perovskite transforms to post-perovskite at around 23 GPa which is accessible by multi-anvil high-pressure experiment, and that the post-perovskite phase is quenchable to ambient conditions. We have determined the phase relations and have refined crystal structures of both the perovskite and post-perovskite phases in CaRuO3 using Rietveld analysis. Obtained results are compared with the post-perovskite phase transition in MgSiO3 and CaIrO3 , and are used to discuss on the post-perovskite transition in A2+ B4+ O3 compounds. 2. Experimental methods 2.1. High-pressure experiments High-pressure high-temperature experiments were performed using a Kawai-type 6–8 multi-anvil apparatus at Gakushuin University. Tungsten carbide anvils with truncated edge lengths of 1.5, 2.5 and 5.0 mm were used. Pressure was calibrated against press load at room temperature using phase transitions of Bi I–II (2.55 GPa), Ba I–II (5.5 GPa), Bi III–V (7.7 GPa), Ba II–III (12.3 GPa), ZnS (15.5 GPa), GaAs (18.3 GPa), and GaP (23 GPa). The pressure was corrected at 1200 ◦ C using phase transitions of SiO2 coesite-stishovite (Zhang et al., 1996), Mg2 SiO4 forsterite-wadsleyite (Morishima et al., 1994), Mg2 SiO4 wadsleyite-ringwoodite (Suzuki et al., 2000), and MgSiO3 ilmenite-perovskite (Ito and Takahashi, 1989; Ono et al., 2001). A semi-sintered MgO octahedron was used as a pressure medium. A cylindrical Pt heater was put in the octahedron together with a LaCrO3 sleeve for thermal insulation. Powdered starting material was put directly into the Pt heater. LaCrO3 plugs were stuffed at both ends of the furnace. Thin Pt discs separating the sample from the LaCrO3 plugs were inserted to prevent any reaction between them. Temperature was measured by a Pt/Pt–13%Rh thermocouple, the hot junction of which was positioned at the central part of the outer surface of the heater. No correction was made on the emf of the thermocouple. Experimental precisions of pressure and temperature were estimated as ±0.3 GPa and ±20 ◦ C, respectively. Starting material of CaRuO3 perovskite was synthesized by the following procedure.

Reagent grade CaCO3 and RuO2 were mixed (mol ratio of 1:1) in an agate mortar for 1 h, and then the mixture was pressed into a pellet. The pellet was heated at 1150 ◦ C for 14 h in air. Powder X-ray diffraction (XRD) analysis confirmed that the synthesized sample was a single phase of CaRuO3 perovskite, using a simulated XRD profile based on crystallographic data by Rama Rao et al. (2001). Single phase of CaRuO3 post-perovskite was also used as starting material for reverse experiments. The starting materials were held at 13–27 GPa and 900–1200 ◦ C for 30–300 min, and recovered to ambient conditions after quenching under pressure. The highpressure experiments indicated that starting perovskite completely transformed to post-perovskite at 900 ◦ C for 60 min. Therefore, in most of the experiments, we chose the run duration of 60 min at 900–1040 ◦ C and of 30 min at 1200 ◦ C. The recovered samples were examined by micro-focus and powder XRD using a Rigaku RINT2500V diffractometer with monochromatized Cr K␣ radiation (45 kV, 250 mA) at Gakushuin University. The phase relations were determined on the basis of micro-focus X-ray diffraction patterns in the central part of the sample using a collimated X-ray beam of 50 ␮m in diameter. 2.2. Rietveld analysis The XRD profiles of CaRuO3 perovskite synthesized at 1 atm and of the recovered CaRuO3 post-perovskite were measured using the Rigaku RINT2500V diffractometer in the 2θ range of 20–140◦ with 0.02◦ steps, and were used for Rietveld analysis. Powder sample of the perovskite was put on a non-reflective quartz plate with acetone. For the post-perovskite phase, the recovered polycrystalline sample was crushed into powder, and scattered on a thin layer of spray glue (Sumitomo 3M Inc.) on the quartz plate to reduce the effect of preferred orientation. The Rietveld analysis was made using the RIETAN2000 program (Izumi and Ikeda, 2000). Peak profiles were fitted with the pseudo-Voigt function. The effect of preferred orientation was corrected with the March–Dollase function (Dollase, 1986). The XRD profiles of the post-perovskite and perovskite phases were refined using the crystal structure models of CaIrO3 post-perovskite (Rodi and Babel, 1965) and of GdFeO3 perovskite (Marezio et al., 1970), respectively. All of isotropic atomic displacement factors of oxygen for both the CaRuO3 phases were fixed at 1.0. Although the isotropic atomic displacement factor of oxygen was varied in the range from 0.5 to 1.2, which was typical in Rietveld analysis, it hardly affected the other refined

Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134

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parameters. When isotropic atomic displacement factor of ruthenium of the CaRuO3 perovskite was refined, the result showed a negative value. Therefore, it was fixed at 0.2 which was a typical value for the smaller-sized B4+ cation in A2+ B4+ O3 orthorhombic perovskite. 3. Results and discussion 3.1. Phase relations in CaRuO3 Results of high-pressure experiments for determination of phase relations are summarized in Table 1, and are shown in Fig. 1. The results indicated that at about 21–25 GPa and 900–1200 ◦ C CaRuO3 perovskite transforms to a new high-pressure phase whose XRD pattern is very similar to that of CaIrO3 postperovskite. As shown in the next section, Rietveld refinement of the XRD pattern confirmed that the structure of the new CaRuO3 phase is identical to that of CaIrO3 post-perovskite. Fig. 1 indicates that CaRuO3 perovskite transforms directly to post-perovskite phase, like MgSiO3 , MgGeO3 , MnGeO3 and CaIrO3 . As shown in Fig. 1, pressure for the post-perovskite phase transition in CaRuO3 was tightly constrained by normal and reverse runs as about 23 GPa at 1040–1070 ◦ C. At 1200 ◦ C, normal runs indicate that transition pressure is 24–25 GPa, while reverse runs exhibit the pressure of 23–24 GPa. The reverse run at 24 GPa and 1200 ◦ C in Table 1 Results of high-pressure experiments of CaRuO3 Run no.

Starting material

P (GPa)

T (◦ C)

Time (min)

Phase

21 1 16 4 8 9 11 20 6 14 19 3 17 5 7 18 15 13 12 10 2

Pv Pv Pv Pv Pv Pv Pv P-Pv Pv Pv Pv Pv Pv Pv Pv P-Pv Pv P-Pv Pv Pv Pv

20 27 23.5 13 21 22 22.5 22.5 23 23 23.5 13 16 21 23 23 24 24 25 26 27

900 900 950 1000 1040 1040 1040 1040 1040 1070 170 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200

60 60 300 60 60 60 60 120 60 120 90 30 30 30 30 30 30 60 30 30 30

Pv P-Pv P-Pv Pv Pv Pv Pv Pv + P-Pv P-Pv Pv P-Pv Pv Pv Pv Pv Pv Pv P-Pv Pv + P-Pv P-Pv P-Pv

Pv, CaRuO3 perovskite; P-Pv, CaRuO3 post-perovskite.

Fig. 1. Phase relations in CaRuO3 . Solid and open squares indicate that CaRuO3 perovskite and CaRuO3 post-perovskite, respectively, were observed in normal reaction experiments. Solid and open reverse triangles show results of reverse reaction runs in the same manner as the squares. Half-filled symbols represent mixture of the two phases. A solid line shows the phase transition boundary.

Fig. 1 may appear inconsistent with other runs, but could be explained, when we consider pressure uncertainty of ±0.3 GPa. Thus, the transition pressure at 1200 ◦ C was estimated as 24.4 ± 0.6 GPa. Therefore, a Clapeyron slope of the transition boundary is calculated to be 10 ± 4 MPa/◦ C. The slope is close to 11.5 MPa/◦ C of the same transition in MgSiO3 based on Speziale et al.’s (2001) MgO pressure scale (Hirose et al., 2006), but is considerably smaller than about 40 ± 20 MPa/◦ C in CaIrO3 (Hirose and Fujita, 2005; Kojitani et al., 2007a; Stølen and Trønnes, 2007). Because the volume decrease for the transition in CaRuO3 is similar to that in CaIrO3 as shown in the next section, the difference of boundary slope may be caused mostly by difference in entropy of transition. Further study is necessary to clarify the difference in thermodynamic properties between CaRuO3 and CaIrO3 . 3.2. Rietveld refinement The XRD profiles of the new phase and perovskite of CaRuO3 were analyzed by Rietveld method. The results of the new phase and perovskite of CaRuO3 are shown in Fig. 2(a and b), respectively. Obtained lattice parameters and atomic coordinates are tabulated in Table 2. R factors of refinement for the two phases show reasonable values. In Table 3, observed d-spacings of the new phase are compared with those calculated values, together with observed diffraction intensities. All of the calculated dvalues are in very good agreement with those of the observed ones. Based on these results, we conclude that the new phase of CaRuO3 has the same structure as that of CaIrO3 post-perovskite. All of the lattice parame-

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Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134 Table 2 Lattice parameters and atomic coordinates determined by Rietveld refinement of post-perovskite and perovskite of CaRuO3

Fig. 2. Results of Rietveld refinement of (a) CaRuO3 post-perovskite and (b) CaRuO3 perovskite. X-ray diffraction profiles were taken at 1 atm and room temperature. Crosses and solid lines are the observed and calculated X-ray diffraction profiles, respectively. Vertical bars under the diffraction peaks show peak positions of each phase. The plots under the bars represent residues. In the profile of CaRuO3 postperovskite, a bump of the background in the 2θ range lower than 40◦ is due to spray glue used to decrease the effect of preferred orientation.

ters of CaRuO3 post-perovskite are smaller than those of CaIrO3 post-perovskite. This is consistent with the ˚ than that slightly smaller ionic radius of Ru4+ (0.62 A) 4+ ˚ of Ir (0.63 A) (Shannon and Prewitt, 1969). Structural studies on CaRuO3 perovskite have been performed by several previous investigators, including Bensh et al. (1990), Kobayashi et al. (1994), Cuffini et al. (1996), Kiyama et al. (1996), and Rama Rao et al. (2001). In this study, lattice parameters of CaRuO3 perovskite were more accurately determined than the previous data. Our lattice parameters of CaRuO3 perovskite are the same as those determined by Cuffini et al. (1996) within the uncertainties. Atomic coordinates in this study are consistent with those reported by the previous studies. The volume change associated with the transition from perovskite to post-perovskite is −1.7% at 1 atm and room temperature, which is close to −1.4% in CaIrO3 (Kojitani et al., 2007a). Interatomic distances and bond angles of both the post-perovskite and perovskite phases of CaRuO3 are

Post-perovskite

Perovskite

Space group ˚ a (A) ˚ b (A) ˚ c (A) ˚ 3) V (A Z Vm (cm3 /mol)

Cmcm 3.1150(1) 9.8268(1) 7.2963(1) 223.34(1) 4 33.62(1)

Pbnm 5.3635(2) 5.5261(2) 7.6668(2) 227.24(1) 4 34.21(1)

Ca x y z ˚ 2) B (A

0 0.2512(2) 1/4 0.15(4)

0.9829(7) 0.0555(4) 1/4 0.75(6)

Ru x y z ˚ 2) B (A

0 0 0 0.12(3)

1/2 0 0 0.2

O1 x y z ˚ 2) B (A

1/2 0.4308(5) 1/4 1.0

0.0893(13) 0.4792(11) 1/4 1.0

O2 x y z ˚ 2) B (A

1/2 0.1281(4) 0.0528(5) 1.0

0.7032(10) 0.2987(8) 0.0451(7) 1.0

Rwp (%) RI (%) RF (%) Re (%)

8.1 3.8 2.6 6.1

14.0 3.7 2.7 9.9

 i

Rwp =

wi [yi (o) − yi (c)]2



w [y (o)]2 i i i



|[I (o)]1/2 − [Ik (c)]1/2 | k k

 k

RI =



|Ik (o) − Ik (c)|



I (o) k k

N −P  Re = [Ik (o)]1/2 w [y (o)]2 i i i The yi (o) and yi (c) are observed and calculated intensities at profile point i, respectively. The wi is a weight for each step i. Ik (o) and Ik (c) are observed and calculated integrated intensities, respectively. N and P show numbers of data and of refined parameters, respectively. RF =



k

shown in Table 4 together with those of post-perovskite phases of CaIrO3 and MgSiO3 . As three different Ru O distances of RuO6 octahedra in CaRuO3 perovskite are very close to each other and O Ru O bond angles are almost 90◦ , the RuO6 octahedra are regarded as regular octahedra. Tilt angles of the RuO6 octahedra in the Ru–O1 and Ru–O2 directions are 151.2◦ and 150.6◦ , respectively, which are close to those of MgSiO3 perovskite (146.8◦ and 147.2◦ , respectively) calculated on

Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134 Table 3 Observed and calculated d-values and observed diffraction intensities of CaRuO3 post-perovskite hkl

˚ dobs (A)

˚ dcalc (A)

Iobs

020 002 110 022 111 040 112 130 023 131 042 132 113 004 024 150 133 060 151 200 114 152 062 220 044 202 134 222 240 115 170 223 171 242 080 135

4.9143 3.6477 2.9690 2.9286 2.7498 2.4565 2.3026 2.2570 2.1794 2.1562 2.0375 1.9193 1.8813 1.8239 1.7098 1.6619 1.6543 1.6377 1.6204 1.5575 1.5542 1.5125 1.4941 1.4848 1.4645 1.4325 1.4187 1.3752 1.3153 1.3095 1.2796 1.2671 1.2605 1.2374 1.2285 1.2255

4.9134 3.6482 2.9694 2.9291 2.7504 2.4567 2.3030 2.2573 2.1797 2.1565 2.0377 1.9196 1.8815 1.8241 1.7100 1.6622 1.6545 1.6378 1.6207 1.5575 1.5543 1.5126 1.4941 1.4847 1.4645 1.4324 1.4188 1.3752 1.3154 1.3097 1.2799 1.2672 1.2606 1.2375 1.2284 1.2255

39 24 46 100 6 20 24 44 4 18 3 36 7 18 6 11 3 1 2 9 15 19 15 3 10 2 13 14 5 <1 3 1 1 2 11 3

the basis of the data of Dobson and Jacobsen (2004). On the other hand, the RuO6 octahedra in CaRuO3 post-perovskite are considerably deformed, as shown in the O2–Ru–O2 bond angle of 80.4◦ . The bond angle agrees very well with corresponding O2–Ir–O2 bond angle (80.9◦ ) in CaIrO3 post-perovskite (Table 4). The RuO6 octahedra extend to a-axis direction in which they are connected with edge-sharing each other. Therefore, the deformation of RuO6 octahedra in CaRuO3 post-perovskite can be explained by repulsion between neighboring Ru4+ ions. Since the O2–Si–O2 angle in MgSiO3 post-perovskite at 121 GPa and room temperature is larger than O2–Ru–O2 angle in CaRuO3 post-perovskite, it is shown that the deformation of octahedra by repulsion between neighboring Si4+ cations in MgSiO3 post-perovskite at high pressure is smaller,

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compared with those in post-perovskites of CaRuO3 and CaIrO3 at 1 atm. The Ru–O1–Ru angle in CaRuO3 post-perovskite, which shows a degree of tilting of RuO6 octahedra, is close to Ir–O1–Ir angle in CaIrO3 post-perovskite. These two bond angles are larger than Si–O1–Si angle in MgSiO3 post-perovskite, as shown in Table 4. Ratios of average A–O distance to average B–O distance in A2+ B4+ O3 post-perovskites are 1.213, 1.208 and 1.183 for CaRuO3 , CaIrO3 and MgSiO3 postperovskites, respectively. These values suggest that the larger degree of tilting of SiO6 octahedra in MgSiO3 post-perovskite than those of RuO6 and IrO6 octahedra in CaRuO3 and CaIrO3 post-perovskites, respectively, is caused by accommodating the relatively small Mg2+ ion in the large eight-fold coordination site. Average Ca–O distances for both CaRuO3 perovskite and postperovskite indicate only a small difference in size of Ca sites between the perovskite and post-perovskite phases of CaRuO3 . 3.3. Post-perovskite transition in A2+ B4+ O3 Structural distortion from ideal cubic perovskite into distorted perovskite can be expressed by tilting of octahedral framework, when the octahedra are assumed to be regular. It has been proposed that, when degree of distortion of orthorhombic perovskite is enhanced with pressure, the perovskite tends to transform to the postperovskite structure (Liu et al., 2005; Tateno et al., 2006). This suggests that, if an orthorhombic perovskite phase shows a large structural distortion at 1 atm, it is likely that the perovskite transforms to post-perovskite at high pressure. In the GdFeO3 -type perovskite (space group Pbnm), the octahedral tilting is expressed in terms of three tilt angles, θ, φ and Φ, which represent rotations of octahedra about pseudo-cubic cell axes of [1 1 0], [0 0 1] and [1 1 1], respectively (Zhao et al., 1993; Mitchell, 2002). However, it is shown that the distortion of perovskite structure can be actually described by only one tilt angle Φ (O’Keeffe and Hyde, 1977; O’Keeffe et al., 1979). Assuming all the octahedra remain regular, the tilt angle Φ is expressed by: √  2 2a Φ = cos−1 bc where a, b and c are lattice parameters of GdFeO3 -type orthorhombic perovskite (space group Pbnm) (Zhao et al., 1993). Here, we adopt the tilt angle Φ to describe distortion of orthorhombic perovskite. Table 5 lists the lattice parameters and the tilt angle of various orthorhombic A2+ B4+ O3 perovskites. The table indi-

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Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134

Table 4 Interatomic distances and bond angles of perovskite and post-perovskite of CaRuO3 and post-perovskites of CaIrO3 and MgSiO3 CaRuO3 Pv ˚ Interatomic distance (A) Ru O1 × 2 Ru O2 × 2 Ru O2 × 2 Average Ca O1 Ca O2 × 2 Ca O1 Ca O2 × 2 Ca O2 × 2 Average Bond angle (◦ ) O1 Ru O2 O1 Ru O2 O2 Ru O2 Ru O1 Ru Ru O2 Ru

CaIrO3 P-Pva

CaRuO3 P-Pv

MgSiO3 P-Pvb

1.979(5) 1.973(2) 2.008(5) 1.987

Ru O1 × 2 Ru O2 × 4 Average

1.947(2) 2.039(2) 2.008

Ir O1 × 2 Ir O2 × 4 Average

1.940 2.066 2.024

Si O1 × 2 Si O2 × 4 Average

1.644 1.664 1.657

2.333(8) 2.341(5) 2.410(6) 2.554(6) 2.677(6) 2.486

Ca O1 × 2 Ca O2 × 4 Ca O2 × 2 Average

2.353(4) 2.441(3) 2.508(4) 2.436

Ca O1 × 2 Ca O2 × 4 Ca O2 × 2 Average

2.396 2.428 2.526 2.445

Mg O1 × 2 Mg O2 × 4 Mg O2 × 2 Average

1.838 1.938 2.129 1.961

90.4(3) 90.7(3) 90.3(1) 151.2(4) 150.6(3)

O1 Ru O2 O2 Ru O2 Ru O1 Ru

92.2(2) 80.4(2) 139.0(3)

O1 Ir O2 O2 Ir O2 Ir O1 Ir

91.5 80.9 140.2

O1 Si O2 O2 Si O2 Si O1 Si

91.2 84.9 135.8

Pv, perovskite; P-Pv, post-perovskite. a The values were calculated using atomic coordinates by Rodi and Babel (1965) and lattice parameters by Kojitani et al. (2007a). b The values were calculated on the basis of crystallographic data observed at 121 GPa and room temperature by Murakami et al. (2004).

cates that orthorhombic perovskites that have been found to transform to the post-perovskite structure (CaIrO3 , CaRuO3 , MnGeO3 , MgGeO3 and MgSiO3 ) have the Φ values larger than about 16◦ . On the other hand, CaTiO3 , CaSnO3 , CdTiO3 and CdGeO3 perovskites that do not transform to post-perovskite at pressure up to 70–110 GPa (Tateno et al., 2006) have the Φ values smaller than 16◦ . This may support the above idea that orthorhombic perovskite having a large degree of octahedral tilting at 1 atm likely transforms to postperovskite at high pressure. From Table 5, we suggest

that MnSnO3 , MnTiO3 and CdSnO3 perovskites whose transition behaviors have not yet been experimentally examined in the pressure range to 70–150 GPa would be potential candidates that transform to the post-perovskite structure. Our study showed that CaRuO3 perovskite transforms to post-perovskite at about 21–25 GPa at 900–1200 ◦ C. The transition boundary has a Clapeyron slope of 10 ± 4 MPa/◦ C which is similar to that of MgSiO3 , but is placed at much lower pressure accessible by multi-anvil experiments using tungsten carbide anvils.

Table 5 Lattice parameters and tilt angles of A2+ B4+ O3 orthorhombic perovskites Perovskite

˚ a (A)

˚ b (A)

˚ c (A)

Φ

Ref.

CaIrO3 CaRuO3 MnGeO3 MgGeO3 (18 GPa) MgSiO3 CaSnO3 CaTiO3 CaGeO3 CdSnO3 CdTiO3 CdGeO3 MnSnO3 (7.4 GPa) MnTiO3 (4.5 GPa)

5.3478 5.3635 5.0764 4.832 4.7784 5.5142 5.3796 5.2607 5.4588 5.3053 5.209 5.301 5.1048

5.5935 5.5261 5.1956 5.031 4.9303 5.6634 5.4423 5.2688 5.5752 5.4215 5.253 5.445 5.3046

7.6757 7.6668 7.3065 7.022 6.8990 7.8816 7.6401 7.4452 7.8711 7.6176 7.434 7.690 7.4180

19.61 16.21 16.26 20.83 18.32 15.56 10.16 3.86 16.20 15.46 10.69 18.36 20.52

Kojitani et al. (2007a) This study Ito and Matsui (1979) Leinenweber et al. (1994) Kojitani et al. (2007b) Zhao et al. (2004) Sasaki et al. (1987) Sasaki et al. (1983) Mizoguchi et al. (2004) Sasaki et al. (1987) Susaki (1989) Leinenweber et al. (1991) Ross et al. (1989)

The values are at 1 atm and room temperature except for MgGeO3 , MnSnO3 and MnTiO3 . The data of the three perovskites at high pressure are given in parentheses.

Letter to the Editor / Physics of the Earth and Planetary Interiors 165 (2007) 127–134

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Hiroshi Kojitani Yuichi Shirako Masaki Akaogi ∗ Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan ∗ Corresponding

author. Tel.: +81 3 3986 0221; fax: +81 3 5992 1029. E-mail address: [email protected] (M. Akaogi) 29 June 2007