Static compression of ε‐FeSi and an evaluation of reduced silicon as a deep Earth constituent

Abstract

The volume of ε‐FeSi has been measured to pressures of 50 GPa. No high pressure transformations are observed in this system, including after laser‐heating at high pressures. The bulk modulus of this phase is 209 (±6) GPa, with a pressure derivative of 3.5 (±0.4). This high bulk modulus will slightly elevate the thermochemically‐inferred pressure conditions at which chemical reactions between iron and mantle silicates should commence at mid‐mantle depths. Moreover, because of the relatively low pressure derivatives of the bulk moduli of FeSi alloys manifested both in our data and in previous shock results, silicon as the sole light alloying component of the outer core is unlikely to produce a sufficiently large change in the bulk sound speed of the outer core relative to pure iron liquid. Therefore, silicon is not likely to be the primary alloying component in the outer core, unless its effect on the elasticity of the outer core is fortuitously offset by other light alloying constituents: its role is probably that of a subsidiary constituent. Thus, silicon is unique (to date) among proposed major outer core alloying constituents in being potentially precludable on purely geophysical grounds.

Crystal structure, compressibility and possible phase transitions in \boldvarepsilon-FeSi studied by first-principles pseudopotential calculations

“…It was found that the structure was essentially invariant over the accessible pressure range (0±9 GPa), with no detectable tendency either towards, or away from, the ideal sevenfoldcoordinated structure, a result which was con®rmed by single-crystal X-ray diffraction using a diamond-anvil cell (Ross, 1996). Lattice-parameter measurements to much higher pressures (50 GPa) by powder X-ray diffraction (Knittle & Williams, 1995) did not suggest the presence of any structural phase transitions, even after laser-heating to about 1500 K at 49 GPa (the published data do, however, contain one point, at 36 GPa, which lies well away from the PV curve on which the rest of the data lie). Similarly, powder neutron diffraction studies both above and below room temperature (Watanabe et al, 1963) suggested that there was little change in the structure between 79 and 573 K. Recently, however, experiments on thin ®lms of FeSi grown on silicon substrates (von Ka È nel et al, 1992;Kafader et al, 1993;von Ka È nel et al, 1994;Dekoster et al, 1997) have indicated the formation of a CsCl-type structure, stable for layers thinner than 890 A Ê , and it is known that RuSi undergoes a transition from an FeSi-type to a CsCl-type structure at 1578 (AE15) K (Buschinger et al, 1997).…”

Structural investigations of -FeGe at high pressure and low temperature

“…The resulting fitting parameters are given in Table 4. The bulk modulus of -FeGe is considerably smaller than that of -FeSi which is ranging from B 0 ¼ 160ð1Þ GPa (B 0 ¼ 4:0) [11] to 209(6) GPa (B 0 ¼ 5:3) [12]. A simple comparison of the unit-cell volume and using the compressibility of -FeGe shows that -FeGe attains the same unit-cell volume as -FeSi at about 25 GPa.…”

“…Theoretical studies on -FeSi predict a phase transformation to the B2 structure (CsCl-type) at about 13 GPa [13] or in the range of 30-40 GPa [14]. On the other hand, no phase transformation in -FeSi has been found up to 50 GPa [12]. Thus, more structural information at pressures well above 30 GPa is needed to resolve the stability of -FeGe.…”

Heavy-impurity resonance, hybridization, and phonon spectral functions inFe1−xMxSi (M=Ir, Delaire1, Al-Qasir2, May3 et al. 2015Phys. Rev. B412190View full textAdd to dashboardCite

“…Possibly strong electronic correlations have also be noted 34,35 , and it has been proposed that that FeSi could be an unusual d−electron Kondo insulator, although this point remains controversial 32,34,[36][37][38][39] . Interest in FeSi also arises from geophysics, as it is a possible reaction product between molten iron and mantle silicates at the core-mantle boundary 14,[40][41][42] .…”