Phase transition boundary between B1 and B8 structures of FeO up to 210 GPa
Introduction
FeO is an end-member of ferropericlase, which is thought to be the second most dominant phase in the lower mantle. Since molten iron coexisting with mantle silicates and oxides shows a high solubility of oxygen (Takafuji et al., 2005, Asahara et al., 2007, Ozawa et al., 2008), FeO may be an important component in the core as well. The behavior of FeO at high pressure and temperature (P–T) may thus provide essential information for understanding the mineralogy and chemistry of the Earth's interior.
FeO crystallizes in the B1 structure under ambient conditions and undergoes a phase transition into a distorted B1 structure with a rhombohedral cell above 9 GPa at 300 K (Zou et al., 1980, Shu et al., 1998). While static compression experiments have demonstrated that the rhombohedral structure remains stable up to 120 GPa at room temperature (Yagi et al., 1985), shock compression experiments have revealed that the density of FeO increases by approximately 4% above 70 GPa and high temperature (Jeanloz and Ahrens, 1980, Yagi et al., 1988). The shock-wave study by Knittle et al. (1986) also reported the metallization of FeO above 70 GPa at 1200–3600 K. Subsequently, Fei and Mao (1994) observed the phase transition from rhombohedral to NiAs-type (B8) structure above 90 GPa at 600 K using an externally resistance-heated DAC, suggesting that such density jump and metallization are caused by the phase transition from B1 to B8 structure around 70 GPa at high temperature. More recently, the phase transition boundary between B1 and B8 structures has been examined by Murakami et al. (2004) and Kondo et al. (2004). However, the phase transition was observed only in the very narrow pressure range in both the studies, and thus the boundary was not well constrained.
While substantial amount of oxygen dissolves into liquid iron at high P–T (Takafuji et al., 2005, Sakai et al., 2006, Asahara et al., 2007, Ozawa et al., 2008), it is still not clear whether solid iron incorporates oxygen. It is known that the solid solution between Fe and FeO is quite limited, at least up to 25 GPa based on the experiments using multi-anvil apparatus (Kato and Ringwood, 1989, Ringwood and Hibberson, 1990). Nevertheless, Knittle and Jeanloz (1991) reported on the basis of laser-heated DAC experiments that the melting temperature of Fe + FeO mixture was intermediate between the melting points of Fe and FeO. Hence, Knittle and Jeanloz concluded that Fe–FeO behaves like a solid solution rather than as an eutectic system above 80 GPa. Boehler (1993) also argued that the melting relation in Fe–FeO may change from eutectic to solid solution system based on the disappearance of depression of melting point above 60 GPa. In contrast, Seagle et al. (2008) reported that the Fe–FeO system remains eutectic at least to 93 GPa. This is supported by theory. The first principle calculations by Alfé et al. (2002) predicted that the solubility of oxygen in solid iron is very small at high P–T corresponding to the inner core boundary. The partitioning of oxygen between the liquid and solid core is one of the key points to identify the light element in the core.
Crystal structure of pure iron at the core P–T has been a matter of extensive debate. While the recent experimental study by Kuwayama et al. (2008) revealed a wide stability of hcp-iron up to 301 GPa and 2000 K, theory suggested a body-centered cubic (bcc) phase of pure iron at pressures above 100 GPa and temperatures over 3200 K (Belonoshko et al., 2003). Furthermore, the face-centered cubic (fcc) phase of pure Fe has been proposed to appear above 160 GPa and 3700 K on the basis of theoretical and experimental examinations (Mikhaylushkin et al., 2007).
Here we examined the stable phases in the system Fe–FeO up to 208 GPa and 3800 K based on the in situ XRD measurements. The phase transition between the B1 and B8 structures of FeO was observed, and its boundary was determined by both forward and backward experiments. In addition, the chemical compositions of the recovered samples were analyzed with the field-emission-type electron probe microanalyzer (FE-EPMA) in order to assess the solubility of oxygen in hcp-iron at such high P–T conditions.
Section snippets
Experimental techniques
The high P–T conditions were generated in a laser-heated DAC using the beveled diamond anvils with 90-μm culet. Starting materials were mixed fine powders of Fe and FeO. The FeO powder was obtained commercially from Kojundo Chemical Laboratory Co. Ltd. Its lattice parameter is a = 4.313(1) Å at ambient condition, corresponding to Fe0.955±0.002O (McCammon and Liu, 1984). The mixing ratio was 73:27 for runs #1 and #3, 33:67 for run #2 in weight. Such sample mixture was loaded into a hole drilled in
Phase transitions of FeO
Three separate sets of experiments were conducted at pressures between 170 and 208 GPa and temperatures up to 3800 K (Table 1). In the first set of experiments, the starting material was initially compressed to 180 GPa at room temperature. The broad diffraction peaks including those from hcp-iron and rhenium were observed before heating (Fig. 3(a)). The sample was subsequently heated to 2140 K at 201 GPa. The B8 phase was immediately formed upon heating (Fig. 3(b)). When the temperature was
Phase boundary between B1 and B8 structures of FeO
Both the phase transition boundaries between rhombohedral and B1 and between rhombohedral and B8 structures were previously determined by Fei and Mao (1994) (Fig. 7). Their results suggested that the B1–rhombohedral–B8 triple point is located around 65 GPa and 1000 K.
Here we introduce the phase transition boundary between B1 and B8 structures from the present new data obtained between 180 and 210 GPa, adopting the location of the triple point reported by Fei and Mao (1994) (Fig. 7). The Clapeyron
Acknowledgements
The manuscript was improved by comments from two anonymous reviewers. We thank D.C. Rubie for editorial handling. E. Sugimura kindly corrected the English of the manuscript. X-ray diffraction measurements were conducted at SPring-8 (proposal no. 2008A0099). HO was supported by the 21st century COE program.
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