Phys Chem Minerals (2008) 35:189–195 DOI 10.1007/s00269-007-0211-5
ORIGINAL PAPER
CaPtO3 as a novel post-perovskite oxide Kenya Ohgushi Æ Yoshitaka Matsushita Æ Nobuyoshi Miyajima Æ Yoshio Katsuya Æ Masahiko Tanaka Æ Fujio Izumi Æ Hirotada Gotou Æ Yutaka Ueda Æ Takehiko Yagi
Received: 15 September 2007 / Accepted: 6 December 2007 / Published online: 4 January 2008 Ó Springer-Verlag 2007
Abstract Structural, morphological, magnetic, and thermal properties have been investigated for a novel postperovskite oxide CaPtO3 synthesized under high pressure. By comparing obtained structural parameters with those for known post-perovskite compounds, we argue that the chemical bond has a strong covalent character. Precise measurements of the Langevin susceptibility v0 = -9.6 9 10-5 emu/mol and Debye temperature h * 470 K provide a good opportunity to confirm the reliability of first-principle calculations on predicting physical properties of the Earth’s D’’ layer. Keywords Post-perovskite CaPtO3 High-pressure synthesis Rietveld analysis Morphology
K. Ohgushi (&) H. Gotou Y. Ueda T. Yagi Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan e-mail:
[email protected] Y. Matsushita F. Izumi Quantum Beam Center, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan N. Miyajima Bayerisches Geoinstitut, Universita¨t Bayreuth, 95440 Bayreuth, Germany Y. Katsuya SPring-8 Service Co. Ltd, Sayo 679-5198, Japan M. Tanaka BL15XU/SPring8, National Institute for Materials Science (NIMS), Sayo 679-5198, Japan
Introduction The perovskite MgSiO3 is transformed into a denser polymorph with the layered structure (inset of Fig. 1), which is now termed the post-perovskite structure, at 125 GPa (Murakami et al. 2004; Oganov and Ono 2004). This phase is most likely to be a main constitute of the D’’ layer at the lowermost mantle. In contrast to extensive numerical calculations (Tsuchiya et al. 2004a, b, 2005; Iitaka et al. 2004), experimental information on the postperovskite MgSiO3 is severely limited because of the high transition pressure and the instability at ambient pressure. Thus, physical properties have been widely investigated for several model compounds. CaIrO3 had been the only oxide that crystallized in the post-perovskite structure until 2004 (Hyde et al. 1979; Rodi and Babel 1965). It has been demonstrated that the Clapeyron slope of the perovskite/post-perovskite phase boundary has a large positive value in CaIrO3 by means of the phase identification under various synthesis conditions (Hirose and Fujita 2005a) and high-temperature drop calorimetry experiments (Kojitani et al. 2007a). A transmission electron microscope (TEM) analysis of dislocations in the synthetic post-perovskite CaIrO3 has revealed the dominant slip plane of (010) with [100] Burgers vector (Miyajima et al. 2006); afterward, this is confirmed by electron backscattered diffraction and TEM experiments for deformed CaIrO3 samples (Yamazaki et al. 2006 and Walte et al. 2007). The lattice preferred orientation expected from the (010) slip plane, together with the anisotropic nature of elastic tensors, well explains the observed seismic anisotropy in the D’’ layer (Yamazaki et al. 2006 and Walte et al. 2007; Niwa et al. 2007). Despite these successes, there still remains a question whether CaIrO3 is really a good analog to MgSiO3. For example, an Ir4+ ion with the t2g5 electronic
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Phys Chem Minerals (2008) 35:189–195
1.5x106
Intensity (counts)
CaPtO3
λ = 0.63598 Å
1.0
ba
0.5
c 0.0
10
20
30
40 2 θ (deg)
50
60
70
Fig. 1 The synchrotron X-ray diffraction profile for CaPtO3 at room ˚ . The dots temperature. The incident X-ray wavelength is 0.63598 A and solid line represent the observed and calculated intensities, respectively, and the solid line at the bottom of the panel indicates the difference in between. The tick marks at the middle of the two solid lines stand for positions of Bragg reflections expected from the Cmcm orthorhombic symmetry with the unit cell of a = 3.12607(1), b = 9.91983(4), and c = 7.35059(3). The inset displays the postperovskite structure, in which PtO6 octahedra share their edges and corners along the a and c axes, respectively
configuration is Jahn-Teller active, thus making a manner of plastic deformation complicated. In this respect, the postperovskite NaMgF3 without Jahn-Teller ions is more promising. The pressure applied to the perovskite NaMgF3 gradually enhances the orthorhombic distortion through compression of NaF12 polyhedra, culminating in the perovskite/post-perovskite phase transition around 30 GPa (Liu et al. 2005; Martin et al. 2006a). It has been argued that the degree of the orthorhombic distortion is a useful indicator of the perovskite/post-perovskite phase transition (Martin et al. 2006b). The purpose of the present paper is to report the synthesis, crystal structure, morphology, and magnetic and thermal properties for a novel post-perovskite CaPtO3. This compound is the second example of quenchable postperovskite oxides, and takes an advantage of containing no Jahn-Teller ions over CaIrO3.
Experimental procedure Polycrystalline CaPtO3 was synthesized by utilizing a cubic-anvil-type high-pressure apparatus. The CaO and PtO2 powders with the molar ratio of 1.1:1 were mixed and packed into a platinum capsule of 4 mm length and 5.5 mm diameter. We placed a 10 mg of disk-shaped KClO4 pellet as an oxidizer at both ends of the capsule so that reactants contain no platinum low-valence phase, CaPt2O4. The capsule in a BN container was loaded into a
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pyrophyllite cube of 21 mm edge length. The temperature was raised to 1,073 K by a carbon heater at 4 GPa, held for 30 min, and then quenched to room temperature before release of pressure. The recovered dark-yellow powders were characterized by a powder X-ray diffraction using Cu Ka radiation, a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) spectrometer (JEOL JSM-5600), and TEM operating at 200 kV (Philips CM20 FEG). In order to determine structural parameters precisely, we have carried out synchrotron X-ray powder diffraction experiments at SPring-8 BL15XU, where a Debye-Scherrer camera is installed. The incident X-ray wavelength estimated from diffraction data for a CeO2 standard was ˚ . We used a Lindenmann glass capillary of 0.63598 A 0.2 mm diameter to pack fine powders of CaPtO3. The data were recorded on an imaging plate during the exposure time of 2 s. Thus obtained diffraction profiles were analyzed by the Rietveld method with the help of RIETAN-FP software (Izumi and Ikeda 2000). We chose a split pseudoVoigt function and a sum of Legendre polynomials for a profile function and a background function, respectively. Throughout the refinement procedure, atomic displacement parameters were set to the fixed values: 0.1 for Pt, and 0.3 for Ca and O. We checked that the discussion made in the present paper is not seriously affected by the choice of these values. Magnetic data were collected using a superconducting quantum interference device magnetometer (Quantum Design MPMS). We measured the specific heat for an 8.15 mg thin plate by utilizing a commercial setup (Quantum Design PPMS), where the relaxation method is in use (Dachs and Bertoldi 2005).
Results and interpretations Figure 1 shows the synchrotron X-ray diffraction profile for CaPtO3 at room temperature. All the peaks except for tiny contributions from an unknown impurity phase, which ˚ , 2.00 A ˚ , 2.23 A ˚, is characterized by the d values of 1.82 A ˚ ˚ 2.83 A, and 3.16 A, can be indexed in the Cmcm orthorhombic symmetry. When performing the Rietveld analysis, we assumed full occupancy for all the sites, because the EDX analysis indicates that the atomic ratio of Ca to Pt is unity within an experimental accuracy. Fitting results shown in Fig. 1 exhibit a high fit quality with a i.P nhP o1=2 2 2 reliability index Rwp ¼ w ð y f Þ w y i i i i i i i of 4.0%, where yi is the observed intensity, fi is the calculated intensity, and wi (= 1/ yi) is the statistical weight. We list refined structural parameters together with those for CaIrO3 (Martin et al. 2007) in Table 1.
Phys Chem Minerals (2008) 35:189–195
191
Table 1 Structural parameters of the post-perovskite CaPtO3 deduced from the Rietveld analysis of the synchrotron X-ray powder diffraction profile at room temperature. The crystal lattice holds the Cmcm orthorhombic symmetry. Corresponding data for the postperovskite CaIrO3 are also listed according to Martin et al. (2007) Compounds
CaPtO3
CaIrO3 (Martin et al.)
˚) a (A ˚) b (A
3.12607(1)
3.1468
9.91983(4)
9.8662
˚) c (A ˚ 3) V (A
7.35059(3)
7.2994
227.942(2)
226.63
x
0
0
y
0.2500(2)
0.2517
z
¼
¼
x y
0 0
0 0
z
0
0
x
½
½
y
0.4253(6)
0.4253
z
¼
¼
Ca (4c)
Pt or Ir (4a)
O1 (4c)
O2 (8f) x
½
½
y
0.1230(3)
0.1258
z
0.0451(5)
0.0477
The ionic radius of Pt4+ ions is slightly smaller than that of Ir4+ ions owing to incomplete screening of the nuclear charge by an excess electron; therefore, if each ion behaves as a hard sphere, all the lattice parameters of CaPtO3 become smaller than those of CaIrO3. In contrast to this naive idea, our experimental results show that the replacement of Ir atoms by Pt atoms expands a unit cell in an anisotropic way; the a value decreases by 0.7%, whereas the b and c values increase by 0.5 and 0.7%, respectively. Similar feature is also discernible in Ca4MO6 and Sr4MO6 with M = Ir and Pt (Segal et al. 1996; Wong-Ng et al. 1999), indicating that a simple ionic model cannot be applicable to less localized 5d electron systems. It is most likely that Ir-O and Pt-O bonds have strong covalent character in these systems owing to a large electronegativity of Ir and Pt atoms. This consideration well accounts for the lattice parameter difference between CaIrO3 and CaPtO3; Pauling’s covalent radius of Pt is larger than that of Ir (Pauling 1960), and directional bonds in covalent crystals lead to an anisotropic lattice parameter change. There are two types of Pt-O bonds in one PtO6 octa˚ ) participating in hedron: two shorter Pt-O1 bonds (1.981 A a corner-sharing octahedral network, and four longer Pt-O2
˚ ) participating in an edge-sharing octahebonds (2.010 A dral network (inset of Fig. 1). The striking aspect stressed here is that, compared to CaIrO3, CaPtO3 has rather regular PtO6 octahedra. This is exemplified by the ratio of a shorter-bond distance (ds) to a longer-bond distance (dl), ds/ dl = 0.985 for CaPtO3 and ds/dl = 0.967 for CaIrO3 (Martin et al. 2007). Although the orbital degeneracy among the t2g manifold cannot be lifted by a compressed-octahedral crystal field in the point charge model (Ohgushi et al. 2006), the origin of more distorted octahedra in CaIrO3 likely resides in the Jahn-Teller instability associated with Ir4+ ions. It is impossible to compare these ds/dl values with that of the post-perovskite MgSiO3, because there is a disparity between the reported values: ds/dl = 0.987 (Murakami et al. 2004) and 0.969 (Oganov and Ono 2004). Figure 2a, b is TEM images for CaPtO3 polycrystals from two different zone axes. The electron diffraction pattern (insets of Fig. 2a, b) revealed that each small crystal has a needle-like shape oriented along the a axis. A similar crystal habit is also discernible in the isostructural ThMnSe3 and UMnSe3 grown by a solid-state reaction (Ijjali et al. 2004) as well as CaIrO3 grown under high pressure (Miyajima et al. 2006), hinting that the needle-like shape is not a growth form but the equilibrium form, in which the surface free energy is minimized. Since the surface free energy for a certain crystal face is equal to one half of the energy expended in cutting the crystal into two pieces along the specific crystal plane, we can roughly estimate the surface free energy by counting a number of bonds associated with the cutting procedure. For example, we need to break 16 Ca-O bonds and 8 Pt-O bonds per bc area for cutting a crystal within the a plane, while we only need to break 6 Ca-O bonds and 2 Pt-O bonds per ab area for cutting a crystal within the c plane; therefore, the surface free energy per unit area of the a plane is about twice that of the c plane. Following similar steps for other crystal surfaces, we conclude that the a plane has the largest surface free energy. This leads to the morphology characterized by a needle-like shape oriented along the a axis. We have observed some dislocations having a Burgers vector with the [00w] component; however, low dislocation density precludes accurate identification of the Burgers vector. Figure 3a presents the temperature (T) dependence of the magnetic susceptibility (v) for CaPtO3. The data exhibit diamagnetic signals due to the Langevin susceptibility in a wide temperature range, indicating that Pt4+ ions have the low-spin electronic configuration (t2g6). In the low temperature range (T \ 50 K), there is a slight upturn originating from a small amount of impurity spins. The fitting of these data with the function v(T) = v0 + A/T yields the Langevin susceptibility v0 = -9.6 9 10-5 emu/ mol and the A value corresponding to 0.2% impurity spins (S = 1/2).
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Phys Chem Minerals (2008) 35:189–195
0.5
(a) CaPtO3
c (emu/mol)
0.0 -0.5 -1.0 -1.5x10-4
(b)
15
2
C/T (mJ/mol K )
C (J/mol K)
100
10
50
5 0
0
50
2
100 2
T (K )
0 0
100
200
300
T (K) Fig. 3 a The magnetic susceptibility (v) as a function of temperature (T) for CaPtO3. The data indicated by dots are fitted with a twocomponent model comprised of the Langevin part and the impurity spin part. The solid line shows the result of fitting. b The temperature dependence of the specific heat (C) for CaPtO3. Experimental data (dots) are nicely fitted with the Debye model with a Debye temperature of h = 530 K (solid line). The inset shows an expanded view on the low temperature part; data in this range well obey the T3 law, giving rise to a Debye temperature of h = 410 K
Fig. 2 Bright field transmission electron microscope images for CaPtO3 polycrystals from (a) \ 1–10[ and (b) [100] zone axes. The insets show the selected area electron diffraction patterns for crystals indicated by arrows in the main images. The crystal in the image (a) exhibits a needle-like shape with the a axis as the needle direction, while the crystal in the image (b) shows a rectangular or elliptic shape corresponding to a cross section of a needle-like crystal
The closed-shell nature of the t2g manifold of Pt4+ ions enables us to estimate the Debye temperature (h) precisely from the specific heat (C) data listed in Table 2 and shown in Fig. 3b. As a fitting function, we adopted the conventional Debye model: 3 Z h=T T x4 ex C ðT Þ ¼ 9NR dx ; h ðex 1Þ2 0 where N is the number of atoms in a formula unit (N = 5), and R is the gas constant. Fitting the data for 1.8 B T B 300 K (the solid line in Fig. 3b) results in h = 530 K. We can also evaluate the h value by using the low-temperature form of the above model function, C (T) * (12p4/5) NR (T/h)3. The data
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below 10 K nicely obey this T3 low (the inset of Fig. 3b), giving rise to h = 410 K. We conclude that the Debye temperature of CaPtO3 is *470 K.
Discussion An orthorhombic distortion in the perovskite material ABX3 is absolutely essential to the perovskite/postperovskite phase transition, because the cubic perovskite structure with twelvefold-coordinated A cations has no reason to be transformed into the post-perovskite structure with eightfold-coordinated A cations under pressure. This distortion is known to increase with applying pressure when the compressibility of A-X bonds exceeds that of B-X bonds (Zhao et al. 2004). The best-known measure of the orthorhombic distortion in the perovskite pffiffiffi structure is the tolerance factor t ¼ ðrA þ rX Þ 2 ðrB þ rX Þ; where rA, rB, and rX are ionic radii at ambient pressure of A, B, and X ions, respectively (Goldschmidt 1926); this value is evaluated by substituting ionic radii
Phys Chem Minerals (2008) 35:189–195 Table 2 Specific heat (C) data at the measured temperature (T) for the post-perovskite CaPtO3
193
T (K)
C (J/mol K)
T (K)
C (J/mol K)
T (K)
C (J/mol K)
1.826
0.00104
4.300
0.0116
11.20
0.198
28.69
3.29
1.831
0.00114
4.415
0.0125
11.25
0.201
29.15
3.39
1.833
0.00117
4.681
0.0148
11.92
0.240
30.42
3.85
1.928
0.00118
4.784
0.0158
12.45
0.273
32.43
4.50
1.939
0.00131
4.964
0.0176
12.64
0.286
36.07
5.89
2.034
0.00144
5.264
0.0208
13.39
0.342
40.09
7.52
2.055
0.00148
5.324
0.0215
13.84
0.378
44.59
9.54
2.180
0.00171
5.582
0.0249
14.20
0.409
49.61
12.0
2.265
0.00195
5.917
0.0297
15.05
0.487
55.18
14.9
2.306
0.00192
5.920
0.0298
15.39
0.521
61.38
18.3
2.446
0.00224
6.271
0.0352
15.96
0.581
68.27
22.0
2.520
0.00248
6.599
0.0406
16.92
0.700
75.91
26.5
2.596
0.00265
6.649
0.0415
17.12
0.726
84.44
31.3
2.751
0.00312
7.065
0.0497
17.94
0.837
93.92
36.7
2.804 2.921
0.00333 0.00372
7.318 7.467
0.0552 0.0586
19.01 19.04
0.995 1.00
104.5 116.2
42.4 48.7
3.100
0.00451
7.918
0.0698
20.15
1.18
129.3
55.2
3.127
0.00456
8.138
0.0754
21.17
1.36
143.8
62.2
3.291
0.00536
8.395
0.0828
21.37
1.40
159.9
69.7
3.479
0.00623
8.899
0.0988
22.66
1.68
177.9
77.2
3.488
0.00639
9.049
0.104
23.54
1.85
197.9
84.6
3.698
0.00761
9.441
0.117
24.04
1.99
220.1
92.2
3.866
0.00848
10.01
0.140
25.49
2.36
244.8
100
3.922
0.00897
10.07
0.143
26.19
2.52
272.3
106
4.161
0.0106
10.61
0.168
27.04
2.78
303.0
110
in the literature (Shannon 1976). For the post-perovskite pffiffiffi material ABX3, we introduce a factor t0 ¼ dAX 2dBX ; where dA-X (dB-X) is the average distance of eight A-X (six B-X) bonds deduced from an experimentally resolved crystal structure; this factor is useful for speculating the orthorombic distortion of the polymorphic perovskite material just before the perovskite/postperovskite phase transition under pressure, which dose not always exist in reality. We list here the t and t0 values for published post-perovskite compounds: t = 0.90, and t0 = 0.84 (Murakami et al. 2004) and 0.83 (Oganov and Ono 2004) for MgSiO3; t = 0.84, and t0 = 0.86 (Hirose et al. 2005b) and 0.80 (Kubo et al. 2006) for MgGeO3, t = 0.88 and t0 = 0.86 (Martin et al. 2007) for CaIrO3; t = 0.88 and t0 = 0.87 for CaPtO3; t = 0.87 and t0 = 0.81 (Martin et al. 2006b) for NaMgF3; t0 = 0.81 (Julien et al. 1978) for UScS3; t = 0.76 and t0 = 0.79 for ThMnSe3 (Ijjali et al. 2004); t = 0.78 and t0 = 0.80 for LaYbSe3 (Mitchell et al. 2004); and t = 0.81 and t0 = 0.83 (Schilling et al. 1992) for KTmI3. A wide variation in t0 (0.79 B t0 B 0.87), which overlaps with the stability condition of the perovskite structure 0.75 B t B 1, indicates that the orthorhombic distortion is not the
T (K)
C (J/mol K)
only control parameter of the perovskite/post-perovskite transition. One likely candidate for an additional control parameter is the covalency of B-X bonds; this turned out to be prominent in CaPtO3 in the present study. Corroborative evidence is given by a close inspection of the material characteristics. All the post-perovskite compounds that can be synthesized at ambient pressure contain X anions with relatively low Pauling’s electronegativity vP (Pauling 1960) such as S (vP = 2.58), Se (vP = 2.55), and I (vP = 2.66), or B cations with relatively high vP such as Ir (vP = 2.20). Hence, the electronegativity difference between X and B atoms DvP is small in these compounds, for example DvP = 1.24 in CaIrO3, DvP = 1.22 in UScS3, DvP = 1.00 in ThMnSe3, DvP = 1.45 in LaYbSe3, and DvP = 1.41 in KTmI3. The ionicity of B-X bonds fi estimated from the formula, fi = 1 - exp (-Dv2P/4), is less than 41% (Pauling 1960). This observation, together with the lattice parameter difference between CaIrO3 and CaPtO3 (see the discussion in the preceding section), persuasively evidences an importance of B-X bond covalency in stabilizing the postperovskite structure. It is unclear whether the covalency is prominent or not in the post-perovskite MgSiO3, because
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194
the electronegativity difference between O and Si atoms is DvP = 1.54 at ambient pressure. However, the covalent character of Si-O bonds has been revealed by depicting the electron density maps for many minerals (Gibbs et al. 1994). Since an electronic charge distribution in the postperovskite MgSiO3, which is densely packed at an extremely high pressure, is likely more extended than that in a conventional mineral, we can fully expect that the covalency of Si-O bonds plays an important role in the D’’ layer. Magnetic susceptibility and specific heat data for CaPtO3 presented here should be highly modified before applying them to the post-perovskite MgSiO3. Since the Langevin susceptibility is roughly proportional to a sum of the atomic number in a unit cell (Ashcroft and Mermin 1976), the post-perovskite MgSiO3 likely shows a diamagnetic response with less than half magnitude of the magnetic susceptibility of CaPtO3. The Debye temperature of MgSiO3 is expected to be higher than that of CaPtO3, because the decrease in the atomic mass generally leads to an upward energy shift of the phonon spectrum. In fact, a first-principle calculation has predicted the Debye temperature of 1,100 K for the post-perovskite MgSiO3 (Tsuchiya et al. 2004b). It is highly desirable to compute the magnetic susceptibility and specific heat for CaPtO3 from first principles and compare them with experimental values. In this sense, our results offer a good testing ground for checking how first-principle calculations work well in predicting physical properties of the D’’ layer.
Conclusions By employing a high-pressure apparatus, we have successfully synthesized the post-perovskite CaPtO3, which is quenchable to ambient conditions. This compound has no Jahn-Teller ions; thus being a good analog to the isostructural MgSiO3. The crystals exhibit a needle-like habit with the a axis as the needle direction. Detailed crystal structure analysis revealed that not only the ionic-radii ratio among three ions but also the covalency of the Pt-O bonds plays an important role in stabilizing the post-perovskite structure; this is helpful in searching a new post-perovskite compound. We deduced the Langevin susceptibility v0 = -9.6 9 10-5 emu/mol and Debye temperature h * 470 K; these values are expected to be in comparison with theoretical results. Note added in proof Recently, we became aware that CaRuO3 exhibits the post-perovskite phase transition at high pressure (Kojitani et al. 2007b). Acknowledgments We wish to thank H. Schulze (Bayreuth) for his assistance with TEM sample preparation, A. Yamamoto (Tsukuba) for his cooperation in the structural analysis, K. Niwa (Kashiwa),
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Phys Chem Minerals (2008) 35:189–195 T. Okada (Kashiwa), A. Yamamoto (Wako), and P. Cordier (Lille) for enlightening discussions, and Y. Kiuchi (Kashiwa) for her technical assistance in SEM experiments. We also acknowledge two anonymous reviewers for useful comments. One of the authors (K. O.) is supported by the Japan Society for the Promotion of Science for Young Scientists.
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