American Mineralogist, Volume 93, pages 1654–1658, 2008

0003-004X/08/0010–1654$05.00/DOI: 10.2138/am.2008.2938 1654

Raman spectroscopy of CaIrO3 postperovskite up to 30 GPa

Justin Hustoft,1,* sang-Heon sHim,1,† atsusHi Kubo,2 and norimasa nisHiyama2,‡

1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.2Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.

abstract We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO3 up to 30 GPa to

constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm–1, which is in contrast with the Raman spectra of Mn2O3 and MgGeO3-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIrO3, which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO3. The spectroscopic Grüneisen parameter, γsp,0 = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γth,0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO3-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γsp constrained by Raman measurements provides a reasonable estimate on the γth of the PPv phase in CaIrO3.

Keywords: Postperovskite, CaIrO3, Gruneisen parameter, Raman sepctroscopy, thermal equation of state

introduction

Recent studies have shown that the dominant lower-mantle phase, (Mg,Fe)SiO3 perovskite (Pv), undergoes a phase transition at the pressure (P)-temperature (T) conditions relevant to the D″

discontinuity, the postperovskite (PPv) transition (Murakami et al. 2004; Oganov and Ono 2004; Shim et al. 2004). It has been suggested that some enigmatic seismic observations at the core-mantle boundary (CMB) region can be explained by the proper-ties of the PPv phase (e.g., Helmberger et al. 2005; Wookey et al. 2005; Wentzcovitch et al. 2006; Monnereau and Yuen 2007; Shieh et al. 2006; Merkel et al. 2007—see Hirose 2006; Shim 2008, for review). Theoretical studies have suggested that the new phase is isotypic to CaIrO3 (Murakami et al. 2004; Oganov and Ono 2004; Tsuchiya et al. 2004) and this has been further supported by a recent Rietveld study of (Mg,Fe)SiO3-PPv at P-T conditions relevant to the CMB (Shim et al. 2008). Because CaIrO3-PPv is stable at ambient conditions, it has been recently studied extensively: phase equilibria (Hirose and Fujita 2005; Kojitani et al. 2007), thermoelastic properties (Ballaran et al. 2007; Lindsay-Scott et al. 2007; Martin et al. 2007; Tsuchiya and Tsuchiya 2007), lattice preferred orientation (Walte et al. 2007; Miyajima et al. 2006; Niwa et al. 2007), and kinetics (Yoshino and Yamazaki 2007).

The thermal equation of state (EOS) of mantle silicate PPv is particularly important for understanding both the dynamic stabil-ity of the PPv lenses documented in recent seismic studies (Lay et al. 2006; van der Hilst et al. 2007) and mantle flow at the CMB

region (Buffett 2007). Although the EOS of mantle silicate PPv has been studied at high P and room T (Shieh et al. 2006; Mao et al. 2006; Guignot et al. 2007; Shim et al. 2008), the EOS fit for the volumes (V) of PPv measured at high P-T suffers larger uncertainties due to unconstrained parameters (e.g., volume at ambient conditions, V0), pressure scale issues, and other technical difficulties (e.g., Guignot et al. 2007; Komabayashi et al. 2008). Vibrational spectroscopy, i.e., Raman and infrared spectroscopy, provides an alternative means of constraining thermoelastic parameters (Gillet et al. 1998).

Recent Raman spectroscopy measurements by Shim et al. (2007) have suggested that the Grüneisen parameter (γ) decreases by 25 ± 10% across the PPv transition in MgGeO3. In an X-ray diffraction (XRD) study of (Mg,Fe)SiO3, a similar magnitude of decrease in γ has been documented (Shim et al. 2008).

Yet it should be mentioned that estimates of γ using vi-brational spectroscopy (hereafter γsp or spectroscopic γ) do not include the effect from dispersion and often use a smaller number of normal modes than expected from the group theory. Nevertheless, previous studies have shown that γsp still provides a reasonable estimate for the thermodynamic Grüneisen parameter (γth) for many mantle minerals: for forsterite γsp = 1.09 and γth = 1.17 where only 19 Raman modes out of 84 total modes were used (Chopelas 1990), and for Mg2SiO4-spinel γsp = 1.10 and γth = 1.25 where only five Raman modes out of 42 total modes were used (Chopelas et al. 1994).

However, agreement between γsp and γth has not been con-firmed for PPv structured materials. CaIrO3 provides an op-portunity to examine the agreement between γsp and γth, as some of the key parameters for calculating its γth are available in the literature (e.g., Ballaran et al. 2007; Lindsay-Scott et al. 2007; Martin et al. 2007).

In this study, we measure the Raman spectra of CaIrO3-PPv up to 30 GPa. This data set enables us to constrain the γsp of PPv

* Present address: Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, U.S.A.† E-mail: sangshim@mit.edu‡ Present address: Geodynamics Research Center, Ehime Uni-versity, Japan.

HUSTOFT ET AL.: RAMAN SPECTROSCOPY OF CaIrO3 POSTPEROVSKITE UP TO 30 GPA 1655

from the P-induced shift of mode frequencies. Using this result, we compare the γsp and γth of CaIrO3-PPv.

experimental metHodsTo synthesize CaIrO3, we mixed CaCO3 and IrO2 with a molar ratio of 1:1.02.

This mixture was then heated in air at 1173 K for 2 h and at 1253 K for 3 h. Diffrac-tion measurements using a laboratory X-ray diffractometer showed that the product is a CaIrO3-Pv + Ca2IrO4 + IrO2 mixture. This mixture was then vacuum-sealed in a silica tube and heated to 1253 K at ambient pressure for 68 h. The product was a CaIrO3-Pv + CaIrO3-PPv mixture with small amounts of Ca2IrO4 and IrO2. This mixture was then compressed to 7.7 GPa and heated at 1370 K for 4 h in a large-volume press at beamline 13IDD of the GSECARS sector at the Advanced Photon Source (APS). The product was a pre-sintered polycrystalline aggregate of CaIrO3-PPv (space group Cmcm) as confirmed by synchrotron powder XRD.

The aggregate was crushed to produce powder. Platelets of CaIrO3 with 10 µm thickness were prepared by pressing the powder between moissenite anvils. A platelet was loaded into a stainless steel gasket. A hole 150 µm in diameter was drilled in the indentation of the gasket. Three different pressure media were used in four separate high-P runs: Ar, a methanol-ethanol (4:1) mixture, and NaCl. Small grains of ruby were loaded together with the sample to measure pressure (Mao et al. 1986). A platelet was loaded such that the sample is in contact with one of the diamond anvils. Because diamond is an excellent heat conductor, this sample setup reduces the heating of the sample by a laser beam. No sign of chemical reaction or laser radiation damage was recognized throughout our runs except for one run with a methanol-ethanol medium. In this run, we used a relatively high-intensity laser beam and this perhaps generated sufficient heat to induce chemical reaction on the surface of the dark-colored CaIrO3. Therefore, we have not used this data set. In a separate run with a methanol-ethanol mixture, we reduced laser power sufficiently low and did not find any evidence of chemical reaction. This latter data set is used for data analysis.

The sample in the diamond-anvil cell was illuminated with an Ar/Kr mixed ion laser at 514.5 nm, with a spot size of 2 µm at the sample surface. Raman scattering was measured for 10–30 min using a CCD detector at the end of a 0.5 m single spectrometer. A Ne lamp was used to calibrate the positions of the Raman modes. The precision of our measurements is ~1 cm–1.

To examine the possible existence of modes at a spectral range between 10 and 200 cm–1, we used a triple spectrometer setup in a subtractive mode for the sample in the diamond-anvil cell at 1 GPa (shown in the inset of Fig. 1). In this measurement, we used a 488 nm line of an Ar/Kr mixed ion laser.

result and discussion

A total of four intense peaks are observed between 200 and 650 cm–1 up to 30 GPa (Fig. 1). A weak mode (ν4 in Fig. 1) is found in some of our measurements. This mode is more read-ily observed during decompression than compression. Hirose and Fujita (2005) reported a Raman spectrum of CaIrO3-PPv at ambient conditions. Mode frequencies in our measurements are in good agreement with the result by Hirose and Fujita (2005). Yet, ν1 and ν4 are more intense and less intense, respectively, in our study.

No Raman modes are detected between 10 and 200 cm–1 (in

the inset of Fig. 1, a sharp spike at 30 cm–1 is not from sample). Although the throughput of a triple spectrometer setup used for the low-frequency measurement is lower than that of a single spectrometer setup by 1 order of magnitude, the detection of the ν1 mode in the triple setup documents that no Raman mode with significant intensity exists at the low-frequency region.

More than 9 modes have been detected in the PPv phases of Mn2O3 and MgGeO3 (Shim et al. 2007). Furthermore, strong resemblance in the Raman spectra between these two compounds has been recognized, as also shown in Figures 2b and 2c. From group theory for space group Cmcm, 12 modes among a total of 30 modes are Raman active (Caracas and Cohen 2006). There-fore, only about 1/3 of the Raman-active modes are detected

for CaIrO3 in this study, whereas almost all the Raman-active modes have been observed in Mn2O3 and MgGeO3. Raman intensities are determined by polarizability, which is related to the deformability of the electronic clouds of the atoms in the crystal lattice (Fadini and Schnepel 1989). Therefore, the smaller number of observed modes in CaIrO3-PPv compared with other PPv phases is perhaps related to different bonding characteristics in CaIrO3.

The ν2 and ν3 peaks have asymmetric shapes at lower P. Bechstedt and Peuker (1975) and Balkanski et al. (1975) found that electron-phonon coupling can result in asymmetric peak shapes of Raman modes observed in semiconductors and transi-tion metal oxides (e.g., Manlief and Fan 1972; Sakita et al. 2001). Therefore, the observed asymmetric shapes of phonon modes in CaIrO3 may be due to electron-phonon coupling. Asymmetric peak shapes in Raman scattering can also result from anharmo-nicity (Foster et al. 1993).

Up to 30 GPa we do not identify any significant changes in the Raman spectra of CaIrO3-PPv (Fig. 1). Furthermore, no significant discontinuous changes are detected in the P-induced mode shifts (Fig. 3). These observations indicate that CaIrO3-PPv does not undergo any phase transition up to 30 GPa at room temperature.

Mode Grüneisen parameters (γi) can be obtained from the

ν

ν ν

ν

ν

ν

figure 1. Raman spectra of CaIrO3-PPv at high pressures. The inset shows a Raman spectrum measured at a lower-frequency region using a triple spectrometer set-up at 1 GPa in the diamond-anvil cell. The modes are labeled in the order of their frequencies (ν1–ν5).

HUSTOFT ET AL.: RAMAN SPECTROSCOPY OF CaIrO3 POSTPEROVSKITE UP TO 30 GPA1656

P-induced shifts of phonon mode frequencies (Born and Huang 1954; Wallace 1972). Briefly, we combine the following two relations:

γω

=−∂∂

lnln

iV

(1)

and

γ γ=

0

0

VV

q (2)

where ωi is the frequency of mode i; q is the logarithmic volume derivative of γ, which is assumed to be a constant; V is the volume and subscript 0 denotes the reference conditions, which are 1 bar and 300 K in our study (detailed information on this analytical method can be found in Shim et al. 2007). The volume at dif-ferent pressures can be obtained from the EOS of CaIrO3-PPv (Martin et al. 2007). The difference in volume estimated from the EOS by Martin et al. (2007) and Ballaran et al. (2007) does not exceed 1% up to 30 GPa.

The widths of peaks are systematically larger in a run with a NaCl medium than in runs with either a methanol-ethanol mixture or Ar medium (Fig. 1). Also the mode frequencies of CaIrO3 measured in a NaCl medium are systematically lower. This is perhaps due to higher deviatoric stresses in a NaCl me-dium. Therefore, we do not use the data points measured in a NaCl medium for the calculation of γi. The data points measured during compression agree well with those during decompression in runs with either an Ar or methanol-ethanol mixture medium (Fig. 3). Therefore, both compression and decompression data are included in the fitting.

By fixing ωi,0 to measured values at 1 bar and 300 K, we obtain γi,0 and qi (Table 1). The P-induced shifts of ν1–ν3 are strongly non-linear, whereas the behavior of ν5 is almost linear (Fig. 3). The strong non-linearities result in high qi values (Table 1) and also rapid decreases in γi at high P for the corresponding modes.

We do not use ν4 because of its weak intensity and limited observation. Furthermore, its mode shift behavior is very dif-

figure 3. Frequencies of Raman modes of CaIrO3-PPv at high pressure. The solid and open circles are measured during compression and decompression, respectively. The solid curves are predicted mode shifts calculated from fitted γi,0 and qi.

Pressure (GPa)

200

300

400

500

600

700

Ram

an s

hift

(cm

-1)

0 10 20 30

ν1

ν2

ν3ν4

ν5compressiondecompression

figure 2. Comparison of Raman spectra of the PPv phases in (a) CaIrO3, (b) Mn2O3, and (c) MgGeO3.

Table 1. The Raman mode frequencies (ωi,0), mode Grüneisen pa-rameters (γi,0), and their logarithmic volume derivatives (qi) of CaIrO3-PPv

ωi,0 (cm–1) γi,0 qν1 226 2.17(4) 4.5(4)ν2 312 1.86(3) 5.0(3)ν3 450 1.50(2) 3.3(3)ν5 563 1.115(3) 0

Average Grüneisen parameter γ0 1.72(8) <γi,0> 1.66(8) Notes: We also present average Grüneisen parameters calculated from two different methods (see text for detail). The numbers in the parentheses are 1σ uncertainties. Because of small non-linearity of ν5, q5 is fixed to 0.

HUSTOFT ET AL.: RAMAN SPECTROSCOPY OF CaIrO3 POSTPEROVSKITE UP TO 30 GPA 1657

ferent from other CaIrO3 modes. Therefore, we cannot rule out the possibility of this peak being associated with either a minor impurity in the sample or a reaction product from long exposure to the laser beam. We note that the ν4 mode becomes more visible during decompression, after at least 6 h of laser illumination on the surface of the sample.

A weighted average Grüneisen parameter, γ, can be obtained from:

γγ

=∑∑

i i i

i i

CC

(3)

where Ci is the Einstein heat capacity. We obtain γ0 = 1.72 ± 0.08. A simple average, <γi,0>, yields 1.66 ± 0.08.

We did not include ν4 for the calculation of γ0 and <γi,0> due to the reasons mentioned above. In addition, unambiguous determination of γ4,0 is difficult because ν4 is not well resolved below 1 GPa and therefore ω4,0 is unconstrained. Furthermore, its limited P range makes it difficult to constrain q4. Therefore, we fix q4 = 0 and vary ω4,0 and γ4,0 in the fitting. This yields γ4,0 = 2.27. If we include γ4,0, then γ0 and <γi,0> increase to 1.80 and 1.77, respectively.

The asymmetry observed for the ν2 and ν3 peaks at lower P is not sufficiently high to make doublet peak fitting better than singlet fitting. Therefore, we treat them as singlets. As mentioned earlier, the asymmetry likely results from electron-phonon cou-pling or anharmonicity in CaIrO3. Even if they are due to peak overlaps of two different adjacent phonon modes, the assumption of singlets would not introduce much artifact in the γ calculations, because the P-induced frequency shifts (therefore γi) of individual modes in the doublets should be very similar with each other as peak shapes do not change much with P in our study.

Thermodynamic γ can be obtained from:

γα

thV

K VC

= T (4)

where α is the thermal expansion parameter, KT is the isothermal bulk modulus, V is the volume, and CV is the heat capacity. Two existing reports on KT show an excellent agreement: KT0 = 181 ± 3 GPa (Ballaran et al. 2007) and KT0 = 180 ± 3 GPa (Martin et al. 2007). The V0 is well constrained by several measurements. The CV of CaIrO3-PPv is unknown. Kojitani et al. (2007) estimated the CV of CaIrO3-PPv by adding the CP of CaO (Berman 1988) and the CP of IrO2 (Knacke et al. 1991). For enstatite (MgSiO3) and forsterite (Mg2SiO4), the error involved in this type of estimation does not exceed 0.5%. In addition, the difference between CP and CV is typically <1% (Navrotsky 1994).

However, there exists a large discrepancy between two exist-ing measurements of α: 2.84(3) × 10–5 K–1

(Martin et al. 2007)

and 2.42(3) × 10–5 K–1

(Lindsay-Scott et al. 2007) at 1 bar and

300 K. These two results yield: γth,0 = 1.75 ± 0.08 when the result by Martin et al. (2007) is used and γth,0 = 1.49 ± 0.08 when the result by Lindsay-Scott et al. (2007) is used. The γth,0 calculated from the result by Martin et al. (2007) is in much better agree-ment with our γsp,0.

It is difficult to determine the cause of the discrepancy in

α. However, it is notable that Martin et al. (2007) used CaIrO3-PPv synthesized at 6 GPa and 1223 K, which is similar to our synthesis conditions. However, Lindsay-Scott et al. (2007) used CaIrO3-PPv synthesized at ambient pressure. Together with our own experience, recent studies (e.g., Martin et al. 2007; Hirose and Fujita 2005) have shown that synthesis of CaIrO3-PPv at ambient pressure could result in many different by-products, such as Ca2IrO4, Ir, IrO2, and CaIrO3-Pv, as well as CaIrO3-PPv. In fact, Lindsay-Scott et al. (2007) observed Ir and IrO2 in their sample. Therefore, we conclude that it is more appropriate to make comparison between the results on the samples synthesized at similar P-T conditions.

We have shown that the spectroscopically constrained Grü-neisen parameter (γsp,0 = 1.66~1.80) from Raman spectroscopy is in agreement with the thermodynamic Grüneisen parameter (γth,0 = 1.75 ± 0.08) for CaIrO3-PPv synthesized at similar P-T conditions (Martin et al. 2007). Our study demonstrates that Ra-man spectroscopy is as a valuable tool to understand the physical properties of the PPv structured phases.

acKnowledgmentsConstruction of the laser Raman system was supported by NSF (EAR0337156).

Raman measurements were supported by NSF (EAR0337005). We thank T.S. Duffy, D.V. West, R.J. Cava, and Y. Wang for help synthesizing the sample. Discussions with two anonymous reviewers improve this paper. High-pressure sample synthesis was conducted at the GSECARS sector of Advanced Photon Source, which is supported by NSF and the State of Illinois.

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Manuscript received January 22, 2008Manuscript accepted april 18, 2008Manuscript handled by daniel neuville