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FeSi melting curve up to 70 GPa
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Earth and Planetary Science Letters 265 (2008) 743 – 747 www.elsevier.com/locate/epsl
FeSi melting curve up to 70 GPa D. Santamaría-Pérez ⁎, R. Boehler Max-Planck-Institute für Chemie, Postfach 3060, D-55020 Mainz, Germany Received 2 May 2007; received in revised form 7 November 2007; accepted 8 November 2007 Available online 17 November 2007 Editor: T. Spohn
Abstract The melting curve of iron monosilicide, FeSi, has been determined in a laser-heated diamond anvil cell from 6 up to 70 GPa by direct visual observation of the continuous laser-speckle motion in the liquid state. At 12 GPa and 1700 K, a discontinuous change in the slope of the melting curve indicates the first-order phase transition between the ɛ-FeSi (B20) and the CsCl-type FeSi structures (B2). During the phase transition the coordination number of both, Fe and Si atoms, increases from 7 to 8. Above this pressure, the melting curve rises steeply but shows significant flattening at higher pressures. A comparison with the melting curve of Fe shows that both curves cross at 32 ± 3 GPa, FeSi having higher melting temperatures (about 100 K) at high pressures. © 2007 Elsevier B.V. All rights reserved. Keywords: iron silicide; melting curve; phase transition; Earth's core
1. Introduction Iron silicides have been found to be materials of wide interest due to their interesting magnetic (Tajima et al., 1988; Songlin et al., 2002) and electronic properties (Takarabe et al., 2002), which make them promising candidates for optoelectronic applications (Leong et al., 1997). For Earth scientists, silicon is a candidate for a light element in the Earth's core, which mainly consists of iron (Lin et al., 2003a; Georg et al., 2007). Cosmochemical arguments and constraints from mantle composition or density suggest that the core could contain between 5 and 20 wt.% Si (Allègre et al., 1995; Sherman, 1997; Poirier, 1994). At high pressure, ⁎ Corresponding author. E-mail address: [email protected] (D. Santamaría-Pérez). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.11.008
experimental and theoretical studies on the compressibility of the silicides β-FeSi2 (Takarabe et al., 2002), Fe5Si3 (Santamaría-Pérez et al., 2004) and ɛ-FeSi (Moroni et al., 1999; Vočadlo et al., 1999; Guyot et al., 1997; Dobson et al., 2002; Dobson et al., 2003; Caracas and Wentzcovitch, 2004; Knittle and Williams, 1995; Lin et al., 2003b) have been reported. High-pressure structural studies on FeSi are controversial. At room conditions, it has a cubic structure (S.G. P213) in which both, Fe and Si, have a seven-fold coordination. Its structure can be described as intermediate between the NaCl-type and the CsCl-type in such a way that the small cubes (1/8 in the unit cell) present in the rock-salt structure are distorted along one diagonal. The evolution of its structural parameters was recorded experimentally by time-of-flight neutron powder diffraction up to 8.5 GPa (Wood et al., 1996) This study shows that the atomic coordinates remain
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D. Santamaría-Pérez, R. Boehler / Earth and Planetary Science Letters 265 (2008) 743–747
essentially unaltered by pressure and that the compression of the cell parameters is almost uniform. Knittle and Williams (1995) and Lin et al., (2003b) did not find any phase transition up to 50 GPa and 2000 K using the laser-heated diamond cell. However, Dobson et al. (2002) reported the high-pressure, high-temperature synthesis of a new phase of iron silicide with a CsCltype structure, after quenching from 24 GPa and 1950 K in a multi-anvil apparatus. Computational studies also show some discrepancies. Moroni et al. (1999) and Vočadlo et al. (1999) suggested that the CsCl-type phase should be thermodynamically stable above 13–15 GPa, whereas Caracas and Wentzcovitch (2004) stated that the transition takes place at 30 GPa using local-density approximation (LDA) and at 40 GPa using generalized-gradient approximation (GGA). Both structures, ɛ-FeSi and CsCl-type, are paramagnetic, the low-pressure phase being semiconducting and the highpressure phase metallic. Even though iron silicides may play a role in the liquid cores of planets, their melting properties have not yet been determined. Here, we present the first DAC melting data for FeSi in the pressure range between 6 and 70 GPa and the temperature range 1600–3000 K. The melting curve shows that the ɛ-FeSi → CsCl-type FeSi phase transition occurs at 12 GPa and 1700 K. 2. Experimental details Single grains of synthetic commercial ɛ-FeSi iron silicide (Alpha Aesar Chemical Company) with 99.9% purity, and dimensions of about 40 μm in diameter and 10 μm thickness were loaded into a diamond anvil cell. FeSi samples were placed in the center of the pressure transmitting medium in a 130 μm diameter hole of a tungsten gasket, preindented to a thickness of 40 μm. All starting materials were dried under vacuum at 120 °C and subsequently flushed with dry argon, prior to pressurizing. Polycrystalline KBr, KCl and CsCl were used as pressure transmitting media to thermally insulate the sample from the diamonds. Melting curves of alkali halides cross that of FeSi at 4 GPa, their melting temperatures being around 1000 K higher than of the sample above about 10 GPa (Boehler et al., 1996). Pressures were measured by the ruby fluorescence method (Mao et al., 1986), both before and after heating, with an uncertainty of about ±1 GPa. This uncertainty was estimated from pressure measurements (after heating) of several ruby chips, finding differences of about 1 GPa among those placed closer, and those more distant to the sample. These differences are most likely due to stress relaxation in the pressure medium in the close vicinity of the heated sample. Thermal pressure increase for
the present configuration of the laser-heated DAC was calculated to be 0.8 ±0.2 GPa (Chudinovskikh and Boehler, 2004). Nevertheless, the thermal pressure can not be fully evaluated and the present data would represent the minimum pressure of melting at a given temperature. The samples were heated with a Nd:YVO4 infrared laser (1.064 μm wavelength), the defocused beam creating a hotspot on the sample of about 25 μm in diameter. Temperatures were determined by fitting temperature and emissivity using the Planck equation (Boehler et al., 1990). Melting was detected with the laser speckle created by the IR-YAG laser or an argon laser described previously (Boehler et al., 1997). When the surface of the ɛ-FeSi reached the melting temperature, motion in the laser speckle pattern was observed. Melting temperatures correspond to the mean value of a minimum of five experiments (melting–freezing cycles). The maximum deviation from these values was 50 K. No optical change was observed at the surface of the sample after heating. 3. Experimental results and discussion Fig. 1 shows the melting curve of iron silicide. The data using different pressure media are consistent, indicating that the observed melting temperatures are independent of the medium employed. Initially, the melting curve is flat up to 12 GPa, with temperatures around 1700 K, and then the slope increases drastically. This change in the slope indicates the presence of a triple point. This triple point must be due to the intersection of the melting curve with the phase boundary between ɛFeSi and the CsCl-type structure, the only stable highpressure, high-temperature phase known from previous measurements (Dobson et al., 2002). Dobson et al. (2002) observed experimentally that, at 15 GPa and 1873 K, ɛ-FeSi was structurally stable, in disagreement with the results reported here. This point falls within the stability field of the high-pressure phase. They found, however, the new CsCl-structured FeSi at 24 GPa and 1950 K. An explanation for Dobson's higher transition pressures may be related to kinetics (Vočadlo et al., 1999; Dobson et al., 2002), but this is unlikely, because run durations in Dobson's experiments were similar to those in the present experiment. An alternative explanation could be that Dobson et al. synthesised an iron deficient FeSi alloy at high pressure. An experimental study on RuSi (Buschinger et al., 1997), compound with the same structure and the same phase transition as FeSi, shows that this silicide can accommodate several percent solid solution, maintaining the ɛ-FeSi structure when a Si-rich bulk composition is present. In this way, the persistence of ɛ-FeSi in Dobson's
D. Santamaría-Pérez, R. Boehler / Earth and Planetary Science Letters 265 (2008) 743–747
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Fig. 1. Melting curve of FeSi. The present melting data are represented by solid symbols. The different pressure media are indicated in the figure. Room pressure melting point was taken from the Alpha Aesar catalogue. 0 K static transition pressures from ab initio simulations are represented as vertical marks (Moroni et al., 1999 (15 GPa); Vočadlo et al. 1999 (13 GPa); Caracas and Wentzcovitch, 2004 (30-40 GPa)). These pressures are connected with the triple point with straight dashed lines to see the different stability fields of both phases.
experimental study, at 15 GPa and 1873 K, could be due to a small iron deficiency in its composition. At the synthesis conditions where the CsCl-type FeSi structure was synthesised, Dobson et al. (2002) also recovered ɛ-FeSi. At 24 GPa both phases are present: The small region corresponding to the furnace hot-spot contained the CsCl-type structure and in the low-temperature region the predominant phase was ɛ-FeSi. This fact, together with the location of the triple point at 12 GPa and 1700 K, suggests that the solid–solid boundary has a strongly negative dT/dP Clapeyron slope. These experi-
mental data (Dobson et al., 2002) are in good agreement with the 0 K predicted transition pressures by Caracas and Wentzcovitch (2004) (30–40 GPa) from ab initio simulations but, in disagreement with those predicted by Moroni et al. (1999) and Vočadlo et al. (1999) (15 and 13 GPa, respectively). Computational 0 K transition pressures and the different solid–solid boundaries after connection to our triple point are also represented in Fig. 1. A comparison with Fe melting curve (Boehler, 1993; Shen et al., 1998; Shen et al., 2004) shows that both curves cross at 32 ± 3 GPa (see Fig. 2). At pressures
Fig. 2. FeSi melting curve in comparison with those of elemental Fe up to 120 GPa. The melting curves are represented by solid lines in their measured pressure range and the extrapolation of the FeSi melting curve to higher pressures is represented by a dashed line.
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between 32 and 70 GPa, both curves show significant flattening, the difference between them being almost constant, of about 100 K. At higher pressures the slope of the Fe melting curve increases significantly as a consequence of the transition between fcc-and hcp-Fe. (Boehler, 1996) (see Fig. 2). Thus, assuming that there is no other phase transition in FeSi at high pressures, both curves would cross again at about 80 GPa and 2950 K. As we will discuss later, this fact is relevant to consider Si as a possible light component of the Earth's core. The present melting study of FeSi is important because of the probable presence of this compound in the core– mantle boundary (CBM) (Dobson et al., 2003). The mantle at the CMB has a chemical and thermal boundary layer, denoted as the D´´ layer. Within this layer an influx of iron from the molten core and a reaction with the solid (Mg, Fe)SiO3, with either perovskite (Guyot et al., 1997; Goarant et al., 1992) or post-perovskite structure may occur. As can be seen in the following reaction proposed by Knittle and Jeanloz (1989), one of the products should be iron silicide: 3Fein metal þ ðMg; FeÞSiO3 ↔2FeO þ ðMg; FeÞO þ FeSi Iron silicide FeSi is denser than the mantle but less dense than the outer core. If so, even with a presence of a few percent in volume, the CsCl-type phase of FeSi will contribute to the observed decrease of both, isotropic compressional (vp) and shear (vs) wave velocities in this layer (Caracas and Wentzcovitch, 2004). Moreover, the metallic character of this high-pressure B2 phase may play an important role in the D´´ layer, being part of a possible electromagnetic coupling with the iron of the core (Caracas and Wentzcovitch, 2004). However, in order to consider Si as a possible light component of the earth's core, the core composition must be on the ironrich side of the eutectic system, or, for a solid solution system, the melting point of iron must be higher than that of its alloys. Kuwayama and Hirose (2004) found that the eutectic point in the binary system Fe–FeSi at 21 GPa is placed at 26 wt.% Si and 2093 K, 400 K lower than the melting point of elemental Fe. This eutectic melt is more Si-rich than that at ambient pressure, silicon increasing its solubility with pressure. On the Ferich side of the eutectic point, the compositional difference between the coexisting solid and liquid is small, indicating that a large amount of Si could also be present in the solid inner core. For a solid solution system, the above-mentioned cross between FeSi and Fe melting curves at high pressure becomes important. In this way, assuming that there is no other phase transition in FeSi at high pressures, this compound would have
lower melting temperatures than Fe at the Earth's outer core conditions and could be considered as a candidate for a light element. Acknowledgments We wish to thank G. Mukherjee, B. Schwager and R. Ditz for constructive and helpful discussions. References Allègre, C.J., Poirier, J.P., Humler, E., Hofmann, A.W., 1995. The chemical composition of the Earth. Earth Planet. Sci. Lett. 134, 515–526. Boehler, R., 1996. Melting temperature of the Earth's mantle and core: Earth's thermal structure. Annu. Rev. Earth Planet. Sci. 24, 15–40. Boehler, R., 1993. Temperature in the Earth's core from melting-point measurements of iron at high static pressures. Nature 363, 534–536. Boehler, R., Bargen, N.V., Chopelas, A., 1990. Melting, thermal expansion and phase transitions of iron at high-pressures. J. Geophys. 95, 21731–21736. Boehler, R., Ross, M., Boercker, D.B., 1996. High-pressure melting curves of alkali halides. Phys. Rev., B 53, 556–563. Boehler, R., Ross, M., Boercker, D.B., 1997. Melting of LiF and NaCl to 1 Mbar: systematics of ionic solids at extreme conditions. Phys. Rev. Lett. 78, 4589–4592. Buschinger, B., Geibel, C., Diehl, J., Guth, W., Weiden, M., Wildbrett, A., Horn, S., Steglich, F., 1997. Preparation and low-temperature properties of FeSi-type RuSi. J. Alloys. Compds. 256, 57–60. Caracas, R., Wentzcovitch, R., 2004. Equation of state and elasticity of FeSi. Geophys. Res. Lett. 31, L20603. Chudinovskikh, L., Boehler, R., 2004. MgSiO3 phase boundaries measured in the laser-heated diamond cell. Earth Planet. Sci. Lett. 219 (3–4), 285–296. Dobson, D.P., Vočadlo, L., Wood, I.G., 2002. A new high-pressure phase of FeSi. Am. Mineral. 87, 784–787. Dobson, D.P., Crichton, W.A., Bouvier, P., Vočadlo, L., Wood, I.G., 2003. The equation of state of CsCl-structured FeSi to 40 GPa: implications for silicon in the Earth's core. Geophys. Res. Lett. 30, 1014. Georg, R.B., Halliday, A.N., Schauble, E.A., Reynolds, B.C., 2007. Silicon in the Earth's core. Nature 447, 1102–1106. Goarant, F., Guyot, F., Peyronneau, J., Poirier, J.P., 1992. Highpressure and high-temperature reactions between silicates and liquid-iron alloys in the diamond anvil cell studied by analytical electron microscopy. J. Geophys. Res. 97, 4477–4487. Guyot, F., Zhang, J., Martinez, I., Matas, J., Ricard, Y., Javoy, M., 1997. P-V-T measurements on iron silicide (ɛ-FeSi). Implications for silicate-metal interactions in the early Earth. Eur. J. Mineral. 9, 277–285. Knittle, E., Jeanloz, R., 1989. Simulating the core–mantle boundary. An experimental study of high-pressure reactions between silicates and liquid iron. Geophys. Res. Lett. 16, 609–612. Knittle, E., Williams, Q., 1995. Static compression of ɛ-FeSi and an evaluation of reduced silicon as a deep Earth constituent. Geophys. Res. Lett. 22, 445–448. Kuwayama, Y., Hirose, K., 2004. Phase relations in the system Fe–FeSi at 21 GPa. Amer. Mineral. 89, 273–276. Leong, D., Harry, M., Reeson, K.J., Homewood, K.P., 1997. A silicon/ iron-disilicide light emitting diode operating at a wavelength of 1.5 mu m. Nature 387, 686–688.
D. Santamaría-Pérez, R. Boehler / Earth and Planetary Science Letters 265 (2008) 743–747 Lin, J.F., Campbell, A.J., Heinz, D.L., Shen, G.Y., 2003a. Static compression of iron-silicon alloys: implications for silicon in the Earth's core. J. Geophys. Res. Solid Earth 108, 2045. Lin, J.F., Struzhkin, V.V., Sturhahn, W., Huang, E., Zhao, J., Hu, M.Y., Alp, E.E., Mao, H.K., Boctor, N., Hemley, R.J., 2003b. Sound velocities of iron–nickel and iron–silicon alloys at high pressures. Geophys. Res. Lett. 30, 2112. Mao, H.K., Xu, J., Bell, P.M., 1986. Calibration of the ruby pressure gauge to 800 kbar under quasi-hydrostatic conditions. J. Geophys. Res. 91, 4673–4676. Moroni, E.G., Wolf, W., Hafner, J., Podloucky, R., 1999. Cohesive, structural and electronic properties of Fe–Si compounds. Phys. Rev., B 59, 12860–12871. Poirier, J.P., 1994. Light elements in the Earth's outer core. A critical review. Phys. Earth Planet. Inter. 85, 319–337. Santamaría-Pérez, D., Nuss, J., Haines, J., Jansen, M., Vegas, A., 2004. Iron silicides and their corresponding oxides: a high-pressure study of Fe5Si3. Solid State Sci. 6, 673–678. Shen, G., Mao, H.K., Hemley, R.J., Duffy, T.S., Rivers, M.L., 1998. Melting and crystal structure of iron at high-pressures and temperatures. Geophys. Res. Lett. 25, 373–376.
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Shen, G., Prakapenka, V.B., Rivers, M.L., Sutton, S.R., 2004. Structure of liquid iron at pressures up to 58 GPa. Phys. Rev. Lett. 92, 185701. Sherman, D.M., 1997. The composition of the Earth's core. Constrains on S and Si vs. temperature. Earth Planet. Sci. Lett. 153, 149–155. Songlin, D., Tegus, O., Bruck, E., Klaase, J.C.P., De Boer, F.R., Buschow, K.H.J., 2002. Magnetic phase transition and magnetocaloric effect in Mn5−xFexSi3. J. Alloys Compd. 334, 249–252. Tajima, K., Endoh, Y., Fischer, J.E., Shirane, G., 1988. Spin fluctuations in the temperature-induced paramagnet FeSi. Phys. Rev., B 38, 6954–6960. Takarabe, K., Teranishi, R., Oinuma, J., Mori, Y., 2002. Electronic properties of beta-FeSi2 under pressure. J. Phys., Condens. Matter 14, 11007. Vočadlo, L., Price, G.G., Wood, I.G., 1999. Crystal structure, compressibility and possible phase transitions in the ɛ-FeSi studied by first-principles pseudopotential calculations. Acta Crystallogr., B 55, 484–493. Wood, I.G., David, W.I.F., Hull, S., Price, G.D., 1996. A high-pressure study of epsilon-FeSi between 0 and 8.5 GPa by time-of-flight neutron powder diffraction. J. Appl. Crystallogr. 29, 215–218.
Earth and Planetary Science Letters 265 (2008) 743 – 747 www.elsevier.com/locate/epsl
FeSi melting curve up to 70 GPa D. Santamaría-Pérez ⁎, R. Boehler Max-Planck-Institute für Chemie, Postfach 3060, D-55020 Mainz, Germany Received 2 May 2007; received in revised form 7 November 2007; accepted 8 November 2007 Available online 17 November 2007 Editor: T. Spohn
Abstract The melting curve of iron monosilicide, FeSi, has been determined in a laser-heated diamond anvil cell from 6 up to 70 GPa by direct visual observation of the continuous laser-speckle motion in the liquid state. At 12 GPa and 1700 K, a discontinuous change in the slope of the melting curve indicates the first-order phase transition between the ɛ-FeSi (B20) and the CsCl-type FeSi structures (B2). During the phase transition the coordination number of both, Fe and Si atoms, increases from 7 to 8. Above this pressure, the melting curve rises steeply but shows significant flattening at higher pressures. A comparison with the melting curve of Fe shows that both curves cross at 32 ± 3 GPa, FeSi having higher melting temperatures (about 100 K) at high pressures. © 2007 Elsevier B.V. All rights reserved. Keywords: iron silicide; melting curve; phase transition; Earth's core
1. Introduction Iron silicides have been found to be materials of wide interest due to their interesting magnetic (Tajima et al., 1988; Songlin et al., 2002) and electronic properties (Takarabe et al., 2002), which make them promising candidates for optoelectronic applications (Leong et al., 1997). For Earth scientists, silicon is a candidate for a light element in the Earth's core, which mainly consists of iron (Lin et al., 2003a; Georg et al., 2007). Cosmochemical arguments and constraints from mantle composition or density suggest that the core could contain between 5 and 20 wt.% Si (Allègre et al., 1995; Sherman, 1997; Poirier, 1994). At high pressure, ⁎ Corresponding author. E-mail address: [email protected] (D. Santamaría-Pérez). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.11.008
experimental and theoretical studies on the compressibility of the silicides β-FeSi2 (Takarabe et al., 2002), Fe5Si3 (Santamaría-Pérez et al., 2004) and ɛ-FeSi (Moroni et al., 1999; Vočadlo et al., 1999; Guyot et al., 1997; Dobson et al., 2002; Dobson et al., 2003; Caracas and Wentzcovitch, 2004; Knittle and Williams, 1995; Lin et al., 2003b) have been reported. High-pressure structural studies on FeSi are controversial. At room conditions, it has a cubic structure (S.G. P213) in which both, Fe and Si, have a seven-fold coordination. Its structure can be described as intermediate between the NaCl-type and the CsCl-type in such a way that the small cubes (1/8 in the unit cell) present in the rock-salt structure are distorted along one diagonal. The evolution of its structural parameters was recorded experimentally by time-of-flight neutron powder diffraction up to 8.5 GPa (Wood et al., 1996) This study shows that the atomic coordinates remain
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D. Santamaría-Pérez, R. Boehler / Earth and Planetary Science Letters 265 (2008) 743–747
essentially unaltered by pressure and that the compression of the cell parameters is almost uniform. Knittle and Williams (1995) and Lin et al., (2003b) did not find any phase transition up to 50 GPa and 2000 K using the laser-heated diamond cell. However, Dobson et al. (2002) reported the high-pressure, high-temperature synthesis of a new phase of iron silicide with a CsCltype structure, after quenching from 24 GPa and 1950 K in a multi-anvil apparatus. Computational studies also show some discrepancies. Moroni et al. (1999) and Vočadlo et al. (1999) suggested that the CsCl-type phase should be thermodynamically stable above 13–15 GPa, whereas Caracas and Wentzcovitch (2004) stated that the transition takes place at 30 GPa using local-density approximation (LDA) and at 40 GPa using generalized-gradient approximation (GGA). Both structures, ɛ-FeSi and CsCl-type, are paramagnetic, the low-pressure phase being semiconducting and the highpressure phase metallic. Even though iron silicides may play a role in the liquid cores of planets, their melting properties have not yet been determined. Here, we present the first DAC melting data for FeSi in the pressure range between 6 and 70 GPa and the temperature range 1600–3000 K. The melting curve shows that the ɛ-FeSi → CsCl-type FeSi phase transition occurs at 12 GPa and 1700 K. 2. Experimental details Single grains of synthetic commercial ɛ-FeSi iron silicide (Alpha Aesar Chemical Company) with 99.9% purity, and dimensions of about 40 μm in diameter and 10 μm thickness were loaded into a diamond anvil cell. FeSi samples were placed in the center of the pressure transmitting medium in a 130 μm diameter hole of a tungsten gasket, preindented to a thickness of 40 μm. All starting materials were dried under vacuum at 120 °C and subsequently flushed with dry argon, prior to pressurizing. Polycrystalline KBr, KCl and CsCl were used as pressure transmitting media to thermally insulate the sample from the diamonds. Melting curves of alkali halides cross that of FeSi at 4 GPa, their melting temperatures being around 1000 K higher than of the sample above about 10 GPa (Boehler et al., 1996). Pressures were measured by the ruby fluorescence method (Mao et al., 1986), both before and after heating, with an uncertainty of about ±1 GPa. This uncertainty was estimated from pressure measurements (after heating) of several ruby chips, finding differences of about 1 GPa among those placed closer, and those more distant to the sample. These differences are most likely due to stress relaxation in the pressure medium in the close vicinity of the heated sample. Thermal pressure increase for
the present configuration of the laser-heated DAC was calculated to be 0.8 ±0.2 GPa (Chudinovskikh and Boehler, 2004). Nevertheless, the thermal pressure can not be fully evaluated and the present data would represent the minimum pressure of melting at a given temperature. The samples were heated with a Nd:YVO4 infrared laser (1.064 μm wavelength), the defocused beam creating a hotspot on the sample of about 25 μm in diameter. Temperatures were determined by fitting temperature and emissivity using the Planck equation (Boehler et al., 1990). Melting was detected with the laser speckle created by the IR-YAG laser or an argon laser described previously (Boehler et al., 1997). When the surface of the ɛ-FeSi reached the melting temperature, motion in the laser speckle pattern was observed. Melting temperatures correspond to the mean value of a minimum of five experiments (melting–freezing cycles). The maximum deviation from these values was 50 K. No optical change was observed at the surface of the sample after heating. 3. Experimental results and discussion Fig. 1 shows the melting curve of iron silicide. The data using different pressure media are consistent, indicating that the observed melting temperatures are independent of the medium employed. Initially, the melting curve is flat up to 12 GPa, with temperatures around 1700 K, and then the slope increases drastically. This change in the slope indicates the presence of a triple point. This triple point must be due to the intersection of the melting curve with the phase boundary between ɛFeSi and the CsCl-type structure, the only stable highpressure, high-temperature phase known from previous measurements (Dobson et al., 2002). Dobson et al. (2002) observed experimentally that, at 15 GPa and 1873 K, ɛ-FeSi was structurally stable, in disagreement with the results reported here. This point falls within the stability field of the high-pressure phase. They found, however, the new CsCl-structured FeSi at 24 GPa and 1950 K. An explanation for Dobson's higher transition pressures may be related to kinetics (Vočadlo et al., 1999; Dobson et al., 2002), but this is unlikely, because run durations in Dobson's experiments were similar to those in the present experiment. An alternative explanation could be that Dobson et al. synthesised an iron deficient FeSi alloy at high pressure. An experimental study on RuSi (Buschinger et al., 1997), compound with the same structure and the same phase transition as FeSi, shows that this silicide can accommodate several percent solid solution, maintaining the ɛ-FeSi structure when a Si-rich bulk composition is present. In this way, the persistence of ɛ-FeSi in Dobson's
D. Santamaría-Pérez, R. Boehler / Earth and Planetary Science Letters 265 (2008) 743–747
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Fig. 1. Melting curve of FeSi. The present melting data are represented by solid symbols. The different pressure media are indicated in the figure. Room pressure melting point was taken from the Alpha Aesar catalogue. 0 K static transition pressures from ab initio simulations are represented as vertical marks (Moroni et al., 1999 (15 GPa); Vočadlo et al. 1999 (13 GPa); Caracas and Wentzcovitch, 2004 (30-40 GPa)). These pressures are connected with the triple point with straight dashed lines to see the different stability fields of both phases.
experimental study, at 15 GPa and 1873 K, could be due to a small iron deficiency in its composition. At the synthesis conditions where the CsCl-type FeSi structure was synthesised, Dobson et al. (2002) also recovered ɛ-FeSi. At 24 GPa both phases are present: The small region corresponding to the furnace hot-spot contained the CsCl-type structure and in the low-temperature region the predominant phase was ɛ-FeSi. This fact, together with the location of the triple point at 12 GPa and 1700 K, suggests that the solid–solid boundary has a strongly negative dT/dP Clapeyron slope. These experi-
mental data (Dobson et al., 2002) are in good agreement with the 0 K predicted transition pressures by Caracas and Wentzcovitch (2004) (30–40 GPa) from ab initio simulations but, in disagreement with those predicted by Moroni et al. (1999) and Vočadlo et al. (1999) (15 and 13 GPa, respectively). Computational 0 K transition pressures and the different solid–solid boundaries after connection to our triple point are also represented in Fig. 1. A comparison with Fe melting curve (Boehler, 1993; Shen et al., 1998; Shen et al., 2004) shows that both curves cross at 32 ± 3 GPa (see Fig. 2). At pressures
Fig. 2. FeSi melting curve in comparison with those of elemental Fe up to 120 GPa. The melting curves are represented by solid lines in their measured pressure range and the extrapolation of the FeSi melting curve to higher pressures is represented by a dashed line.
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between 32 and 70 GPa, both curves show significant flattening, the difference between them being almost constant, of about 100 K. At higher pressures the slope of the Fe melting curve increases significantly as a consequence of the transition between fcc-and hcp-Fe. (Boehler, 1996) (see Fig. 2). Thus, assuming that there is no other phase transition in FeSi at high pressures, both curves would cross again at about 80 GPa and 2950 K. As we will discuss later, this fact is relevant to consider Si as a possible light component of the Earth's core. The present melting study of FeSi is important because of the probable presence of this compound in the core– mantle boundary (CBM) (Dobson et al., 2003). The mantle at the CMB has a chemical and thermal boundary layer, denoted as the D´´ layer. Within this layer an influx of iron from the molten core and a reaction with the solid (Mg, Fe)SiO3, with either perovskite (Guyot et al., 1997; Goarant et al., 1992) or post-perovskite structure may occur. As can be seen in the following reaction proposed by Knittle and Jeanloz (1989), one of the products should be iron silicide: 3Fein metal þ ðMg; FeÞSiO3 ↔2FeO þ ðMg; FeÞO þ FeSi Iron silicide FeSi is denser than the mantle but less dense than the outer core. If so, even with a presence of a few percent in volume, the CsCl-type phase of FeSi will contribute to the observed decrease of both, isotropic compressional (vp) and shear (vs) wave velocities in this layer (Caracas and Wentzcovitch, 2004). Moreover, the metallic character of this high-pressure B2 phase may play an important role in the D´´ layer, being part of a possible electromagnetic coupling with the iron of the core (Caracas and Wentzcovitch, 2004). However, in order to consider Si as a possible light component of the earth's core, the core composition must be on the ironrich side of the eutectic system, or, for a solid solution system, the melting point of iron must be higher than that of its alloys. Kuwayama and Hirose (2004) found that the eutectic point in the binary system Fe–FeSi at 21 GPa is placed at 26 wt.% Si and 2093 K, 400 K lower than the melting point of elemental Fe. This eutectic melt is more Si-rich than that at ambient pressure, silicon increasing its solubility with pressure. On the Ferich side of the eutectic point, the compositional difference between the coexisting solid and liquid is small, indicating that a large amount of Si could also be present in the solid inner core. For a solid solution system, the above-mentioned cross between FeSi and Fe melting curves at high pressure becomes important. In this way, assuming that there is no other phase transition in FeSi at high pressures, this compound would have
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