Phase transition in CaSiO3 perovskite

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Abstract

A phase transition in pure CaSiO3 perovskite was investigated at 27 to 72 GPa and 300 to 819 K by in-situ X-ray diffraction experiments in an externally-heated diamond-anvil cell. The results show that CaSiO3 perovskite takes a tetragonal form at 300 K and undergoes phase transition to a cubic structure above 490–580 K in a pressure range studied here. The transition boundary is strongly temperature-dependent with a slightly positive dT / dP slope of 1.1 (± 1.3) K/GPa. It is known that the transition temperature depends on Al2O3 content dissolved in CaSiO3 perovskite [Kurashina et al., Phys. Earth Planet. Inter. 145 (2004) 67–74]. The phase transition in CaSiO3(+ 3 wt.% Al2O3) perovskite therefore could occur in a cold subducted mid-oceanic ridge basalt (MORB) crust at about 1200 K in the upper- to mid-lower mantle. This phase transition is possibly ferroelastic-type and may cause large seismic anomalies in a wide depth range.

Introduction

CaSiO3-rich perovskite (Ca-perovskite) is an important mineral in both peridotitic mantle and basaltic compositions in the Earth's transition zone and lower mantle. Previous high-pressure experimental studies have shown that the peridotitic mantle and subducted MORB include about 6% and 23% Ca-perovskite, respectively, in the lower mantle conditions (e.g., Wood, 2000, Hirose, 2002, Hirose et al., 2005, Murakami et al., 2005). Therefore, its crystal structure and possible phase transition at mantle P–T conditions is of importance for understanding the mantle mineralogy and interpretations of seismic observations.

Ca-perovskite had long been believed to adopt a cubic structure with a space group Pm3m (Liu and Ringwood, 1975, Mao et al., 1989, Wentzcovitch et al., 1995). Stixrude et al. (1996) first proposed, on the basis of theoretical calculations, that a slightly distorted tetragonal structure with a space group I4/mcm is stable at 0 K. Shim et al. (2002) experimentally observed that CaSiO3 perovskite has indeed the tetragonal structure at 300 K above 20 GPa, but the space group I4/mcm is not consistent with their experimental results. Recent first-principles calculations (Caracas et al., 2005, Jung and Oganov, 2005, Adams and Oganov, 2006) again showed that the I4/mcm tetragonal structure is stable, at least to 160 GPa at 0 K. Adams and Oganov (2006) also predicted that a pseudotetragonal structure Imma explains the X-ray diffraction patterns observed in the experiments and further suggested that this structure can be stable at room temperature. On the other hand, Li et al. (2006) suggested from ab initio molecular dynamics calculations that an orthorhombic structure is stable at 0 K and it transforms to a tetragonal structure at around room temperature. In contrast to the I4/mcm tetragonal structure, the tetragonal structure proposed by Li et al. (2006) explains the experimental X-ray diffraction pattern although the space group was not given.

Stixrude et al. (1996) further suggested that CaSiO3 perovskite undergoes phase transition to a cubic perovskite structure above 2200 K at 80 GPa. In contrast, an experimental study by Kurashina et al. (2004) observed this phase transition at much lower temperature of about 580 K and 50 GPa. However, temperature measurements in Kurashina et al. (2004) included significantly large uncertainty. They used laser-heated diamond-anvil cell (DAC) techniques, and therefore temperature gradient in the sample was large. More importantly, since such low temperatures could not be measured by spectroradiometric method, temperatures were estimated from the relationship between laser-output power and sample temperature obtained at much higher temperatures (> 1500 K). The phase transition in Ca-perovskite was also observed in a multi-component peridotite system (KLB-1 peridotite) using laser-heated DAC with synchrotron X-rays (Ono et al., 2004, Murakami et al., 2005). They demonstrated that Ca-perovskite took a cubic structure on heating above 2000 K while it had tetragonal structure at room temperature. However, these studies did not put tight constraint on the P–T locations of the transition boundary.

This phase transition in CaSiO3 perovskite is possibly ferroelastic-type, as is observed in many distorted perovskites at the phase transitions to higher-symmetric structures with increasing temperature Carpenter and Salje, 1998, Carpenter, 2006). If true, a shear modulus is significantly reduced near the phase transition, causing a large drop in S-wave velocities. This may be one of the origins of seismic heterogeneities observed in the lower mantle (e.g., Kaneshima and Helffrich, 2003).

Here we determined the phase transition in pure CaSiO3 perovskite by a combination of externally-heated diamond-anvil cell (EHDAC) and in-situ synchrotron X-ray diffraction measurements. Contrary to the laser-heated DAC techniques, the uncertainty in temperature is very small (about 5 K) in the present EHDAC experiments.

Section snippets

Experimental procedures

A starting material was a pure CaSiO3 glass mixed with platinum powder, which was used as a pressure maker and a laser absorber. The sample mixture was sandwiched by an MgO pressure medium that served as a thermal insulator for laser-heating. This assembly is the same as that used in Kurashina et al. (2004). The sample was compressed to a pressure of interest at room temperature by 300-μm culet diamond-anvils together with Re gasket. CaSiO3 perovskite was then synthesized in the DAC by heating

Results

Two separate sets of experiments were conducted at different pressure ranges, 63.3–72.4 GPa and 26.8–33.4 GPa. The experimental conditions are summarized in Table 1. In the first run, Ca-perovskite was formed by laser-heating to 2000 K for 10 min at 70 GPa. Typical X-ray diffraction spectra of the sample are shown in Fig. 1. The temperature-quenched sample after laser-heating showed a tetragonal perovskite structure at 300 K. A splitting of the cubic (200) peak was clearly observed at about 14°

Phase transition in Ca-perovskite in the lower mantle

This study demonstrates that the phase transition in Ca-perovskite between tetragonal and cubic structures occurs at 490–580 K and 30–70 GPa, consistent with the previous estimates based on the laser-heated DAC experiments (about 580 K at 50 GPa) by Kurashina et al. (2004). Such temperatures are too low for the lower mantle, even in a cold subducting slab, indicating the structural transition in pure CaSiO3 perovskite is not relevant to the lower mantle.

However, the chemical compositions of

Implications for seismic anomalies in the mid-lower mantle

This phase transition in Ca-perovskite from tetragonal to cubic is a second-order structural phase transition without any volume change since the structure of low-temperature phase (tetragonal) merges continuously with that of the high-temperature phase (cubic) with increasing temperature (Fig. 2, Fig. 4). On the other hand, it possibly causes drastic changes in physical properties, in particular significant reduction in shear modulus, if it is accompanied with an elastic softening (Wang et

Acknowledgements

In-situ X-ray diffraction experiments were conducted at SPring-8 (proposal no. 2005A5013-LD2-np and 2005B6013-LD2-np). An anonymous reviewer is acknowledged for constructive comments. T.K. was supported by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.

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      The result shows that CaSiO3 at T = 300 K with P = 0 GPa, rSi-Si = 3.12 Å, rSi-O = 1.6 Å, rO-O = 2.6 Å, rSi-Ca = 3.56 Å, rO-Ca = 2.36 Å, rCa-Ca = 3.6 Å, this is consistent with the simulation result rSi-O = 1.61 Å[38], 1.7 Å [37]; rO-O = 2.6 Å [38]; rO-Ca = 2.48 Å [38], the experimental result rSi-Si = 3.21 Å[31]; rSi-O = 1.62 Å [31]; rO-O = 2.66 Å [31]; rO-Ca = 2.36 Å [31], RDF height: gSi-Si = 4.75, gSi-O = 29.65, gO-O = 4.03, gSi-Ca = 3.03, gO-Ca = 6.47, gCa-Ca = 2.14, the coordinate number of the bonds Si-Si, Si-O, O-O, Si-Ca, O-Ca, Ca-Ca is 2, 4, 3, 5, 2, 6, the angle of the structural units SiO4, SiO5, SiO6, CaO3, CaO4, CaO5, CaO6, CaO7, CaO8, CaO9 are 105°, 90°, -, -, 85°, 80°, 80°, 80°, 55°, 50°. When having increased the pressure (P) from P = 0 GPa to P = 50, 150, 250, 400 GPa, then the size (l) increases from l = 4.24 nm to l = 6.17 nm, the total energy of the system increases from Etot = −70306 eV to Etot = −58214 eV, r decreases respectively: rSi-Si = 3.12 Å to rSi-Si = 2.44 Å; rSi-O decreases from rSi-O = 1.60 Å to rSi-O = 1.54 Å; rO-O decreases from rO-O = 2.6 Å to rO-O = 2.06 Å; rSi-Ca decreases from rSi-Ca = 3.56 Å to rSi-Ca = 2.66 Å; rO-Ca decreases from rO-Ca = 2.36 Å to rO-Ca = 2.04 Å; rCa-Ca decreases from rCa-Ca = 3.60 Å to rCa-Ca = 2.16 Å (Fig. 3a1), this result is consistent with the report [35,36,38–50] at P < 100 GPa, result given that at P = 5 MPa then the length of the links has varies: rSi-O = 1.62 Å, rCa-O = 2.34 Å, rO-O = 2.66 Å and rSi-Si = 3.18 Å [35]; at P = 9 GPa, 503 GPa, 1019 GPa, the length of the links rSi-O is constant: 1.62 Å and rCa-O = 2.4 Å, 2.38 Å, 2.36 Å respectively, and rO-O is also constant: 2.64 Å and rSi-Si = 3.14 Å, 3.18 Å, 3.14 Å respectively [36]; rSi-O = 1.7 Å, and the number of structural units of SiO4 and SiO7 [37] is: rSi-O = 1.61 Å, rO-O = 2.6 Å, rCa-O = 2.48 Å [38] that shows at high temperatures, CaO and Ca tend to move to cold areas [46] while SiO2 tends to move to hot areas [47,48]. The results obtained corresponding to g(r) changes (Fig. 4a1); the coordination number of the links Si-Si increases from 2 to 6, Si-O increases from 4 to 7, O-O increases from 3 to 20, spikes at P = 50 GPa, Si-Ca increases from 5 to 12, O-Ca increased from 2 to 4, Ca-Ca increased from 6 to 12, huge increase at P = 150 GPa; The bonding angle of the structural units changes and appears the disappearance of SiO4 at P > 250 GPa, and SiO6 appears at P > 50 GPa, CaO4 disappears at P > 50 GPa, CaO5 disappears at P > 50 GPa, CaO6 disappears at P > 150 GPa, CaO7 disappears at P > 250 GPa, CaO10, CaO11, CaO12 appears at P > 50 GPa respectively.

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